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Smithsonian Contributions to Knowledge
Volume 27 Number 3
LANGLEY MEMOIR ON MECHANICAL FLIGHT
PART I. 1887 TO 1896
by
SAMUEL PIERPONT LANGLEY
Edited by Charles M. Manly
PART II. 1897 TO 1903
by
CHARLES M. MANLY
Assistant in Charge of Experiments
[Illustration]
(Publication 1948)
City of Washington
Published by the Smithsonian Institution
1911
Commission to whom this Memoir
has been referred:
Otto Hilgard Tittman,
George Owen Squier,
Albert Francis Zahm.
The Lord Baltimore Press
Baltimore, MD., U. S. A.
67009
[page iii]
ADVERTISEMENT
The present work, entitled “Langley Memoir on Mechanical Flight,” as
planned by the late Secretary Samuel Pierpont Langley, follows his
publications on “Experiments in Aerodynamics” and “The Internal Work
of the Wind” printed in 1891 and 1893, respectively, as parts of
Volume 27 of the Smithsonian Contributions to Knowledge.
This Memoir was in preparation at the time of Mr. Langley’s death
in 1906, and Part I, recording experiments from 1887 to 1896, was
written by him. Part II, on experiments from 1897 to 1903, has been
written by Mr. Charles M. Manly, who became Mr. Langley’s Chief
Assistant in June, 1898. The sources of information for this Part
were the original carefully recorded accounts of the experiments
described.
It is expected later to publish a third part of the present memoir,
to consist largely of the extensive technical data of tests of the
working of various types of curved surfaces, propellers, and other
apparatus.
It is of interest here to note that experiments with the Langley type
of aerodrome[1] did not actually cease in December, 1903, when he
made his last trial with the man-carrying machine, but as recently
as August 6, 1907, a French aviator made a flight of nearly 500 feet
with an aerodrome of essentially the same design. (See Appendix.)
In accordance with the established custom of referring to experts
in the subject treated, all manuscripts intended for publication in
the Smithsonian Contributions to Knowledge, this work was examined
and recommended by a Commission consisting of Mr. O. H. Tittman,
Superintendent of the United States Coast and Geodetic Survey, who
witnessed some of the field trials, George O. Squier, Ph. D. (Johns
Hopkins), Major, Signal Corps, U. S. Army, and Albert Francis Zahm,
Ph. D., of Washington City.
CHARLES D. WALCOTT,
‹Secretary of the Smithsonian Institution›.
FOOTNOTES:
[1] The name “aerodrome” was given by Secretary Langley to the flying
machine in 1893, from ἀεροδρομέω (to traverse the air) and ἀεροδρόμος
air runner.--Internal Work of the Wind, p. 5.
PREFACE
The present volume on Mechanical Flight consists, as the title-page
indicates, of two parts. The first, dealing with the long and
notable series of early experiments with small models, was written
almost entirely by Secretary Langley with the assistance of Mr. E.
C. Huffaker and Mr. G. L. Fowler in 1897. Such chapters as were not
complete have been finished by the writer and are easily noted as
they are written in the third person. It has been subjected only to
such revision as it would have received had Mr. Langley lived to
supervise this publication, and has therefore the highest value as an
historical record. The composition of the second part, dealing with
the later experiments with the original and also new models and the
construction of the larger aerodrome, has necessarily devolved upon
me. This is in entire accordance with the plan formed by Mr. Langley
when I began to work with him in 1898, but it is to me a matter of
sincere regret that the manuscript in its final form has not had the
advantage of his criticism and suggestions. If the reader should feel
that any of the descriptions or statements in this part of the volume
leave something to be desired in fullness of detail, it is hoped that
some allowance may be made for the fact that it has been written in
the scanty and scattered moments that could be snatched from work
in other lines which made heavy demands upon the writer’s time and
strength. It is believed, however, that sufficient data are given to
enable any competent engineer to understand thoroughly even the most
complicated phases of the work.
Persons who care only for the accomplished fact may be inclined to
underrate the interest and value of this record. But even they may
be reminded that but for such patient and unremitting devotion as is
here enregistered, the now accomplished fact of mechanical flight
would still remain the wild unrealized dream which it was for so many
centuries.
To such men as Mr. Langley an unsuccessful experiment is not a
failure but a means of instruction, a necessary and often an
invaluable stepping-stone to the desired end. The trials of the
large aerodrome in the autumn of 1903, to which the curiosity of the
public and the sensationalism of the newspapers gave a character of
finality never desired by Mr. Langley, were to him merely members of
a long series of experiments, as much so as any trial of one of the
small aerodromes or even of one of the earliest rubber-driven models.
Had his health and strength been spared, he would have gone on with
his experiments undiscouraged by these accidents in launching and
undeterred by criticism and misunderstanding.
Moreover, it is to be borne in mind that Mr. Langley’s contribution
to the solution of the problem is not to be measured solely by
what he himself accomplished, important as that is. He began his
investigations at a time when not only the general public but even
the most progressive men of science thought of mechanical flight
only as a subject for ridicule, and both by his epoch-making
investigations in aerodynamics and by his own devotion to the subject
of flight itself he helped to transform into a field of scientific
inquiry what had before been almost entirely in the possession of
visionaries.
The original plans for this publication provided for a third part
covering the experimental data obtained in tests of curved surfaces
and propellers. Owing to the pressure of other matters on the writer,
the preparation of this third part is not yet complete and is
reserved for later publication.
CHARLES M. MANLY.
NEW YORK CITY.
CONTENTS
PART I
I. Introductory … 1
II. Preliminary … 5
Experiments with small models … 6
Abbreviations and symbols employed … 14
Experiments with Aerodromes Nos. 30 and 31 … 16
III. Available motors … 21
India rubber … 21
Steam engine … 24
Gunpowder, hot-water, compressed air … 25
Gas, electricity … 26
Carbonic-acid gas … 28
IV. Early steam motors and other models … 30
V. On sustaining surfaces … 41
Experiments in the open wind … 42
Relation of air to weight and power … 43
VI. Balancing the aerodrome … 45
Lateral and longitudinal stability … 45
VII. History of construction of frame and engines of
aerodromes … 53
1893 … 53
1894 … 64
1895 … 75
1896 … 79
VIII. History of construction of sustaining and guiding
surfaces of aerodromes 4, 5, and 6 … 80
Introduction … 80
1893, 1894 … 81
1895 … 85
1896 … 89
IX. History of launching apparatus and field trials of
aerodromes 4, 5, and 6 … 92
1892 … 92
1893 … 93
Field trials … 93
1894 … 96
1895 … 101
1896 … 106
X. Description of the launching apparatus of aerodromes
Nos. 5 and 6 … 110
Description of Aerodrome No. 5 … 111
Description of Aerodrome No. 6 … 120
PART II
I. Introductory … 123
II. General considerations … 128
III. Experiments with models … 133
Condensed record of flights of aerodromes Nos. 5 and 6,
from June 7 to August 3, 1899 … 135
June 7--aerodrome No. 6 … 135
June 13--aerodrome No. 6 … 137
June 22--aerodrome No. 6 … 139
June 23--aerodrome No. 6 … 140
June 27--aerodrome No. 5 … 140
June 30--aerodrome No. 5 … 141
July 1 to July 8 … 142
July 11 to July 14--aerodrome No. 5 … 143
July 19--aerodrome No. 5 … 144
July 27--aerodrome No. 6 … 145
July 28--aerodrome No. 6 … 146
July 29--aerodrome No. 5 … 147
August 1--aerodrome No. 5 … 148
August 3--aerodrome No. 5 … 149
IV. House-boat and launching apparatus … 156
V. Construction of frame of large aerodrome … 164
Transverse frame … 174
Propellers … 178
Aviator’s car … 185
VI. Construction of supporting surfaces … 188
VII. Equilibrium and control … 207
VIII. The experimental engine … 218
IX. The quarter-size model aerodrome … 226
X. Construction and tests of the large engine … 234
XI. Shop tests of the aerodrome … 251
XII. Field trials in 1903 … 255
Statement made by Mr. Manly to associated press … 266
Report of War Department, January, 1904 … 276
Langley aerodrome, Official Report of Board of Ordnance,
October, 1904 … 278
Statement to the Press … 280
Present status of the work … 281
Blériot Machine of 1907 on Langley type … 283
Appendix. Study of American Buzzard and “John Crow” … 285
Instructions to assistants … 294
Data sheets 1 to 12 … 297
Index … 309
LIST OF PLATES
1. Rubber-motor model aerodromes Nos. 11, 13, 14, 15, 26,
30, 31 … 16
2. Rubber-motor model aerodromes Nos. 11, 13, 14 … 16
3. Rubber-motor model aerodromes Nos. 15, 24 … 16
4. Rubber-motor model aerodrome No. 26 … 16
5. Rubber-pull model aerodrome … 24
6. Rubber-pull model aerodrome … 24
7. Rubber-pull model aerodrome … 24
8. Rubber-pull model aerodrome … 24
9. Rubber-pull model aerodrome … 24
10. Steel frames of aerodromes Nos. 0, 1, 2, 3, 1891 and
1892 … 33
11. Steel frames of aerodromes Nos. 4, 5, 6, 1893, 1895,
and 1896 … 53
12. Burners, aeolipiles, and separators … 56
13. Boilers of aerodromes … 56
14. Aerodrome No. 5, December 3, 1895. Plan view. Rudder
removed … 78
15. Aerodrome No. 5, December 3, 1895. Side view … 78
16. Early types of wings and systems of guying … 81
17. Aerodrome No. 5. Plan of wings and system of guying …
89
18. House-boat with overhead launching apparatus, 1896 …
106
19. Paths of aerodrome flights, May 6 and November 28,
1896, near Quantico, Va., on the Potomac River … 108
20. Instantaneous photograph of the aerodrome at the
moment after launching in its flight at Quantico on the
Potomac River, May 6, 1896. Enlarged ten times … 108
21. Instantaneous photograph of the aerodrome at a
distance in the air during its flight at Quantico on the
Potomac River, May 6, 1896. Enlarged ten times … 108
22. Instantaneous photograph of the aerodrome at a
distance in the air during its flight at Quantico on the
Potomac River, May 6, 1896. Enlarged ten times … 108
23. Overhead launching apparatus … 108
24. Overhead launching apparatus … 108
25. Side view of steel frame of aerodrome No. 5 suspended
from launching-car, October 24, 1896 … 112
26A. Dimensioned drawing of boiler coils, burners, pump,
needle valve, and thrust bearing … 116
26B. Dimensioned drawing of engine No. 5 … 116
27A. Side and end elevations of aerodrome No. 5, May 11,
1896 … 116
27B. Aerodrome No. 5. Plan view. October 24, 1896 … 116
28. Steel frame of aerodrome No. 6 on launching car … 120
29A. Plan view of aerodrome No. 6. October 23, 1896 … 122
29B. Side elevation of aerodrome No. 6. October 23, 1896 …
122
30. Plan view of steel frames and power plants of
aerodromes Nos. 5 and 6 … 122
31. Details of aerodrome No. 5 … 122
32. Drawings of proposed man-carrying aerodrome, 1898 … 130
33. Path of flight of aerodrome No. 6, June 7, 1899 … 136
34. Paths of flight of aerodrome No. 6, June 13 and 23,
1899 … 140
35. Aerodrome No. 5 on launching-ways … 142
36. Paths of flights of aerodrome No. 5, July 29, 1899 …
148
37. Experimental forms of superposed surfaces, 1898, 1899.
(See also plates 64 and 65.) … 153
38. House-boat and launching apparatus, 1899 … 156
39. Method of attaching guy-wires to guy-posts to relieve
torsional strain … 158
40. General plan and details of launching-car … 160
41. Aerodrome on launching-car … 160
42. Details of clutch-post for launching-car … 160
43. Front end of track just preparatory to launching
aerodrome … 160
44. Resistance of wires at given velocities … 166
45. Frame of aerodrome A, January 31, 1900 … 168
46. Frame of aerodrome A, January 31, 1900 … 168
47. Frame of aerodrome A, February 1, 1900 … 168
48. Frame of aerodrome A, February 1, 1900 … 168
49. Guy-wire system, July 10, 1902 … 170
50. Guy-wire system, July 10, 1902 … 170
51. Guy-wire system, July 10, 1902 … 170
52. Scale drawing of aerodrome A. End elevation … 170
53. Scale drawing of aerodrome A. Side elevation … 170
54. Scale drawing of aerodrome A. Plan … 170
55. Frame fittings and guy-wire attachments, etc. … 174
56. Frame fittings and guy-wire attachments, etc. … 174
57. Frame fittings and guy-wire attachments, etc. … 174
58. Bed plate gears, etc. … 176
59. Wing clamps … 183
60. Hoisting aerodrome to launching-track … 184
61. Aerodrome on launching-car; front wings in place,
guy-wires adjusted … 184
62. Details of guy-posts … 184
63. Guy-post and pin on launching-car … 184
64. Experimental type of superposed wings, March 2, 1899 …
192
65. Experimental type of superposed wings, March 2, 1899 …
192
66. Details of ribs and fittings for wings … 200
67. Cross-section of ribs … 201
68. Automatic equilibrium devices … 212
69. Mechanism of control … 212
70. Plan view of quarter-size model aerodrome, June 1,
1900 … 232
71. Plan view of quarter-size model aerodrome … 232
72. End, side, and three-quarter elevations of
quarter-size model aerodrome … 232
73. Launching-car with floats … 232
74. Launching-car with floats … 232
75. Quarter-size model aerodrome equipped with superposed
surfaces, June 11, 1901. Side view … 232
76. Quarter-size model aerodrome equipped with superposed
surfaces, June 11, 1901. End view … 232
77. Cylinders of engine of quarter-size model aerodrome …
233
78. Engine of aerodrome A. Section through cylinder and
drum … 236
79. Engine of aerodrome A. End elevation, port side … 236
80. Engine of aerodrome A. Top plan … 236
81. Engine of aerodrome A. Elevation starboard bed plate,
sparking mechanism … 236
82. Dynamometer tests of large engine … 248
83. Dynamometer tests of large engine … 248
84. Dynamometer tests of large engine … 248
85. Location of house-boat in center of Potomac River,
July 14, 1903 … 256
86. Quarter-size model aerodrome mounted on launching-car
… 260
87. Quarter-size model aerodrome in flight, August 8, 1903
… 260
88. Quarter-size model aerodrome in flight, August 8, 1903
… 260
89. Quarter-size model aerodrome in flight, August 8, 1903
… 260
90. Quarter-size model aerodrome in flight, August 8, 1903
… 260
91. Quarter-size model aerodrome in flight, August 8, 1903
… 260
92. Quarter-size model aerodrome in flight, August 8, 1903
… 260
93. Quarter-size model aerodrome at end of flight, August
8, 1903 … 260
94. Hoisting wing of full-size aerodrome … 263
95. Flight of large aerodrome, October 7, 1903 … 266
96. Flight of large aerodrome, October 7, 1903 … 266
97. Aerodrome being recovered, October 7, 1903 … 270
98. Aerodrome in water, October 7, 1903 … 270
99. Aerodrome in water, October 7, 1903 … 270
100. Aerodrome in water, October 7, 1903 … 270
101. Attempted launching of aerodrome, December 8, 1903 …
274
REFERENCES
Aerodynamics. ‹See› Langley, S. P. Experiments in
Aerodynamics.
L’Aéronaute. ‹See› Pénaud, A.
Aeronautical Society. ‹See› Harting, P.
Aéroplane automoteur. ‹See› Pénaud, A.
Balancing of Engines, Steam, Gas, and Petrol. ‹See› Sharp,
A.
Century Magazine. ‹See› Maxim, H. S.
Comptes Rendus, of the Sessions of the Academy of
Sciences. ‹See› Langley, S. P. Description du vol
méchanique.
Encyclopædia Britannica. ‹See› Flight and Flying Machines.
Experiments in Aerodynamics. ‹See› Langley, S. P.
Flight and Flying Machines. Encyclopædia Britannica, 9th
ed., Vol. 9, 1879, Edinburgh, pp. 308–322, figs. 1–45.
Harting, P. Observations of the relative size of the wings
and the weight of the pectoral muscles in the vertebrated
flying animals. Annual Report of the Aeronautical Society
of Great Britain, No. 5, 1870, Greenwich, pp. 66–77.
Il nuovo aeroplano Blériot, Bollettino della Società
Aeronautica Italiana, Anno IV, N. 8, Agosto 1907, Roma,
pp. 279–282, figs. 4.
Langley, Bollettino della Società Aeronautica Italiana,
Anno II, Num. 11–12, Nov.–Dic. 1905, Roma, pp. 187–188,
figs. 10.
Langley, S. P. Description du vol méchanique. Comptes
Rendus de l’Académie des Sciences, T. 122, Mai 26, 1896,
Paris, pp. 1177–1178.
Langley, S. P. Experiments in Aerodynamics. Smithsonian
Contributions to Knowledge, Vol. 27, 1891, Washington, D.
C., pp. 115, pl. 10.
Langley, S. P. Le travail intérieur du vent. Revue de
l’Aéronautique, 6e année, 3e livraison, 1893, Paris, pp.
37–68.
Langley, S. P. The Flying Machine. McClures Magazine, Vol.
9, No. 2, June, 1897, N. Y., pp. 647–660.
Langley, S. P. The Internal Work of the Wind. Smithsonian
Contributions to Knowledge, Vol. 27, 1893, Washington, D.
C., pp. 23, pl. 6.
McClure’s Magazine. ‹See› Langley, S. P. The Flying
Machine.
Maxim, H. S. Aerial navigation. The power required.
Century Magazine, Vol. 42, No. 6, Oct., 1891, N. Y., pp.
829–836, figs. 1–6.
Pénaud, A. Aeroplane automoteur. L’Aéronaute, 5e année,
No. 1, Jan., 1872, Paris, pp. 2–9, figs. 1–4.
Revue de l’Aéronautique. ‹See› Langley, S. P. Le travail
intérieur du vent.
Sharp, A. Balancing of Engines, Steam, Gas, and Petrol.
New York, Longmans, Green & Co., 1907.
Wellner, Georg. Versuche ueber den Luftwiderstand
gewölbter Flächen im Winde und auf Eisenbahnen.
Zeitschrift für Luftschiffahrt, Bd. 12, Beilage, 1893,
Berlin, pp. 1–48.
Zeitschrift für Luftschiffahrt. ‹See› Wellner, Georg.
[p001]
LANGLEY MEMOIR ON MECHANICAL FLIGHT
PART I. 1887 TO 1896
BY S. P. LANGLEY
EDITED BY CHARLES M. MANLY
CHAPTER I
INTRODUCTORY
I[2] announced in 1891,[3] as the result of experiments carried on by
me through previous years, that it was possible to construct machines
which would give such a velocity to inclined surfaces that bodies
indefinitely heavier than the air could be sustained upon it, and
moved through it with great velocity. In particular, it was stated
that a plane surface in the form of a parallelogram of 76.2 cm.×12.2
cm. (30×4.8 inches), weighing 500 grammes (1.1 lbs.), could be driven
through the air with a velocity of 20 metres (65.6 feet) per second
in absolutely ‹horizontal› flight, with an expenditure of 1/200
horse-power, or, in other terms, that 1 horse-power would propel and
sustain in horizontal flight, at such a velocity (that is, about 40
miles an hour), a little over 200 pounds weight of such surface,
where the specific gravity of the plane was a matter of secondary
importance, the support being derived from the elasticity and inertia
of the air upon which the body is made to run rapidly.
It was further specifically remarked that it was not asserted that
planes of any kind were the best forms to be used in mechanical
flight, nor was it asserted, without restrictions, that mechanical
flight was absolutely possible, since this depended upon our ability
to get ‹horizontal› flight during transport, and to leave the earth
and to return to it in safety. Our ability actually to do this, it
was added, would result from the practice of some unexplored art or
science which might be termed Aerodromics, but on which I was not
then prepared to enter.
I had at that time, however, made certain preliminary experiments
with flying models, which have been continued up to the present
year,[4] and at the same time I have continued experiments distinct
from these, with the small whirling-table established at Washington.
The results obtained from the latter being supplemental to those
published in “Experiments in Aerodynamics,” and [p002] being more or
less imperfect, were at first intended not for publication, but for
my own information on matters where even an incomplete knowledge was
better than the absence of any.
It is to be remembered that the mechanical difficulties of artificial
flight have been so great that, so far as is known, never at any
time in the history of the world previous to my experiment of May,
1896, had any such mechanism, however actuated, sustained itself in
the air for more than a few seconds—never, for instance, a single
half-minute—and those models which had sustained themselves for
these few seconds, had been in almost every case actuated by rubber
springs, and had been of such a size that they should hardly be
described as more than toys. This refers to actual flights in free
air, unguided by any track or arm, for, since the most economical
flight must always be a horizontal one in a straight line,[5] the
fact that a machine has lifted itself while pressed upward against
an overhead track which compels the aerodrome to move horizontally
and at the proper angle for equilibrium, is no proof at all of real
“flight.”
I desire to ask the reader’s consideration of the fact that even ten
years ago,[6] the whole subject of mechanical flight was so far from
having attracted the general attention of physicists or engineers,
that it was generally considered to be a field fitted rather for
the pursuits of the charlatan than for those of the man of science.
Consequently, he who was bold enough to enter it, found almost
none of those experimental data which are ready to hand in every
recognized and reputable field of scientific labor. Let me reiterate
the statement, which even now seems strange, that such disrepute
attached so lately to the attempt to make a “flying-machine,” that
hardly any scientific men of position had made even preliminary
investigations, and that almost every experiment to be made was
made for the first time. To cover so vast a field as that which
aerodromics is now seen to open, no lifetime would have sufficed.
The preliminary experiments on the primary question of equilibrium
and the intimately associated problems of the resistance of the
sustaining surfaces, the power of the engines, the method of their
application, the framing of the hull structure which held these, the
construction of the propellers, the putting of the whole in initial
motion, were all to be made, and could not be conducted with the
exactness which would render them final models of accuracy.
I beg the reader, therefore, to recall as he reads, that everything
here has been done with a view to putting a trial aerodrome
successfully in flight within a few years, and thus giving an early
demonstration of the only kind which is conclusive in the eyes of the
scientific man, as well as of the general public—a demonstration
that mechanical flight is possible-—by actually flying.
All that has been done, has been with an eye principally to this
immediate [p003] result, and all the experiments given in this book
are to be considered only as approximations to exact truth. All were
made with a view, not to some remote future, but to an arrival within
the compass of a few years at some result in actual flight that could
not be gainsaid or mistaken.
Although many experimenters have addressed themselves to the problem
within the last few years--and these have included men of education
and skill--the general failure to arrive at any actual flight has
seemed to throw a doubt over the conclusions which I had announced as
theoretically possible.
When, therefore, I was able to state that on May 6, 1896, such a
degree of success had been attained that an aerodrome, built chiefly
of steel, and driven by a steam engine, had indeed flown for over
half a mile--that this machine had alighted with safety, and had
performed a second flight on the same day, it was felt that an
advance had been made, so great as to constitute the long desired
experimental demonstration of the possibility of mechanical flight.
These results were communicated to the French Academy in the note
given below.[7]
Independently of the preliminary experiments in aerodynamics already
published, I had been engaged for seven years in the development of
flying models. Although the work was discouraging and often resulted
in failure, success was finally reached under the conditions just
referred to, which obviously admitted of its being reached again, and
on a larger scale, if desired. [p004]
In view of the great importance of these experiments, as
demonstrating beyond question the practicability of the art of
mechanical flight, and also in view of the yet inchoate state of this
art, I have thought it worth while to publish an account of them
somewhat in detail, even though they involve an account of failures;
since it is from them, that those to whom it may fall to continue
such constructions, will learn what to avoid, as well as the ‹raison
d’etre› of the construction of the machines which have actually flown.
In an established art or science, this description of the essays
and failures which preceded full knowledge would have chiefly an
historical interest. Here almost nothing is yet established beyond
the fact that mechanical flight has actually been attained. The
history of failure is in this case, then, if I do not mistake, most
necessary to an understanding of the road to future success, to which
it led, and this has been my motive in presenting what I have next to
say so largely in narrative form.
[p005]
CHAPTER II
PRELIMINARY
Part I of the present work is intended to include an account of
the experiments with actual flying models, made chiefly at or
near Washington, from the earliest with rubber motors up to the
construction of the steam aerodromes that performed the flights of
May 6 and November 28, 1896.
An account of some observations conducted at Washington, with the
whirling table, on the reaction of various surfaces upon the air, is
relegated to a later part.
The experiments with working models, which led to the successful
flights, were commenced in 1887, and it has seemed to me preferable
to put them at first in chronological order, and to present to
the reader what may seem instructive in their history, while not
withholding from him the mistaken efforts which were necessarily made
before the better path was found. In this same connection, I may say
that I have no professional acquaintance with steam engineering,
as will, indeed, be apparent from the present record, but it may
be observed that none of the counsel which I obtained from those
possessing more knowledge was useful in meeting the special problems
which presented themselves to me, and which were solved, as far as
they have been solved, by constant “trial and error.”
I shall, then, as far as practicable, follow the order of dates in
presenting the work that has been done, but the reader will observe
that after the preliminary investigations and since the close of
1893, at least four or five independent investigations, attended with
constant experiment and radically distinct kinds of construction,
have been going on simultaneously. We have, for instance, the work in
the shop, which is of two essentially different kinds: first, that
on the frames and engines, which finally led to the construction
of an engine of unprecedented lightness; second, the experimental
construction of the supporting and guiding surfaces, which has
involved an entirely different set of considerations, concerned with
equilibrium and support in flight. These constructions, however
successful, are confined to the shop and are, as will be seen later,
useless without a launching apparatus. The construction of a suitable
launching apparatus itself involved difficulties which took years to
overcome. And, finally, the whole had to be tested by actual flights
in free air, which were conducted at a place some 30 miles distant
from the shop where the original construction went on. [p006]
Simultaneously with these, original experiments with the
whirling-table were being conducted along lines of research, which
though necessary have only been indicated. We have, then, at least
five subjects, so distinct that they can only be properly treated
separately, and accordingly they will be found in Chapters VII, VIII,
IX and X, and in Part Third [in preparation].
It is inevitable that in so complex a study some repetition should
present itself, especially in the narrative form chosen as the
best method of presenting the subject to the reader. Each of these
chapters, then, will contain its own historical account of its own
theme, so that each subject can be pursued continuously in the order
of its actual development, while, since they were all interdependent
and were actually going on simultaneously, the order of dates which
is followed in each chapter will be a simple and sufficient method of
reference from one to the other.
EXPERIMENTS WITH SMALL MODELS
In order to understand how the need arises for such experiments in
fixing conditions which it might appear were already determined in
the work “Experiments in Aerodynamics,”[8] it is to be constantly
borne in mind, as a consideration of the first importance, that the
latter experiments, being conducted with the whirling-table, force
the model to move in horizontal flight and at a constant angle. Now
these are ideal conditions, as they avoid such practical difficulties
as maintaining equilibrium and horizontality, and for this reason
alone give results more favorable than are to be expected in free
flight.
Besides this, the values given in “Aerodynamics” were obtained
with rigid surfaces, and these surfaces themselves were small and
therefore manageable, while larger surfaces, such as are used in
actual flight, would need to be stiffened by guys and like means,
which offer resistance to the air and still further reduce the
results obtained. It is, therefore, fairly certain, that nothing
like the lift of 200 pounds to the horse-power for a rate of 40
miles an hour,[9] obtained under these ideal conditions with the
whirling-table, will be obtained in actual flight, at least with
plane wings.
The data in “Aerodynamics” were, then, insufficient to determine the
conditions of free flight, not alone because the apparatus compels
the planes to move in horizontal flight, but because other ideally
perfect conditions are obtained by surfaces rigidly attached to the
whirling-table so as to present an angle to the wind of advance
which is invariable during the course of the experiment, whereas
the surfaces employed in actual flight may evidently change this
angle and cause [p007] the aerodrome to move upward or downward,
and thus depart from horizontal flight so widely as to bring prompt
destruction.
To secure this balance, or equilibrium, we know in theory, that
the center of gravity must be brought nearly under the center of
pressure, by which latter expression we mean the resultant of all
the forces which tend to sustain the aerodrome; but this center
of pressure, as may in fact be inferred from “Aerodynamics,”[10]
varies with the inclination of the surface. It varies also with the
nature of the surface itself, and for one and the same surface is
constantly shifted unless the whole be rigidly held, as it is on the
whirling-table, and as it cannot be in free flight.
Here, then, are conditions of the utmost importance, our knowledge of
which, as derived from ordinary aerodynamic experiments, is almost
nothing. A consideration of this led me to remark in the conclusion
of “Aerodynamics”:
“I have not asserted, without qualification, that mechanical flight
is practically possible, since this involves questions as to the
method of constructing the mechanism, of securing its safe ascent
and descent, and also of securing the indispensable condition
for the economic use of the power I have shown to be at our
disposal--the condition, I mean, of our ability to guide it in the
desired ‹horizontal› direction during transport--questions which, in
my opinion, are only to be answered by further experiment and which
belong to the inchoate art or science of ‹aerodromics› on which I do
not enter.”
It is this inchoate art of aerodromics which is begun in the
following experiments with actual flying machines.
In all discussions of flight, especially of soaring flight, the first
source to which one naturally looks for information is birds. But
here correct deductions from even the most accurate of observations
are very difficult, because the observation cannot include all of
the conditions under which the bird is doing its work. If we could
but see the wind the problem would be greatly simplified, but as
the matter stands, it may be said that much less assistance has
been derived from studious observations on bird-flight than might
have been anticipated, perhaps because it has been found thus far
impossible to reproduce in the flying machine or aerostatic model
the shape and condition of wing with its flexible and controllable
connection with the body, and especially the instinctive control of
the wing to meet the requirements of flight that are varying from
second to second, and which no automatic adjustment can adequately
meet.
At the time I commenced these experiments, almost the only
flying-machine which had really flown was a toy-like model, suggested
by A. Pénaud, a young Frenchman of singular mechanical genius, who
contributed to the world many most original and valuable papers on
Aeronautics, which may be found in the journal “L’Aeronaute.” His
aeroplane is a toy in size, with a small propeller [p008] whose
blades are usually made of two feathers, or of stiff paper, and whose
motive power is a twisted strand of rubber. This power maintains it
in the air for a few seconds and with an ordinary capacity for flight
of 50 feet or so, but it embodies a device for automatically securing
horizontal flight, which its inventor was the first to enunciate.[11]
Although Pénaud recognized that, theoretically, two screws are
necessary in an aerial propeller, as the use of a single one tends to
make the apparatus revolve on itself, he adopted the single screw on
account of the greater simplicity of construction that it permitted.
One of these little machines is shown as No. 11, Plates 1 and 2.
‹AB› is a stem about 2.5 mm. in diameter and 50 cm. long. It is bent
down at each end, with an offset which supports the rubber and the
shaft of the screw to which it is hooked. The screw ‹HH›^1 is 21
cm. in diameter, and has two blades made of stiff paper; two are
preferable, among other reasons, because they can be made so that
the machine will lie flat when it strikes in its descent. About the
middle of ‹AB› there is a “wing” surface ‹DC›, 45 cm. long and 11
cm. broad, the ends ‹C› and ‹D› being raised and a little curved.
In front of the screw is the horizontal rudder ‹GK› having a shape
like that of the first surface, with its ends also turned up, and
‹inclined at a small negative angle with this wing surface›. Along
its center is a small fin-like vertical rudder that steers the device
laterally, like the rudder of a ship.
The approximately, but not exactly, horizontal rudder serves to
hold the device in horizontal flight, and its operation can best be
understood from the side elevation. Let ‹CD› be the wing plane set
nearly in the line of the stem, which stem it is desired to maintain,
in flying, at a small positive angle, α, with the horizon, α being
so chosen that the tendency upward given by it will just counteract
the action of gravity. The weight of the aeroplane, combined with
the resistance due to the reaction of the air caused by its advance
would, under these conditions, just keep it moving onward in a
horizontal line, if there were no disturbance of the conditions.
There is, however, in the wing no power of self-restoration to the
horizontal if these conditions are disturbed. But such a power
resides in the rudder ‹GK›, which is not set parallel to the wing,
but at a negative angle (α^1) with it equal to the positive angle
of the wing with the horizon. It is obvious that, in horizontal
flight, the rudder, being set at this angle, presents its edge to
the wind of advance and consequently offers a minimum resistance as
long as the flight is horizontal. If, however, for any reason the
head drops down, the rear edge of the rudder is raised, and it is
at once subjected to the action of the air upon its upper surface,
which has a tendency to lower the rear of the machine and to restore
horizontality. Should the head rise, the lower [p009] surface of the
rudder is subjected to the impact of the air, the rear end is raised,
and horizontality again attained. In addition to this, Pénaud appears
to have contemplated giving the rudder-stem a certain elasticity,
and in this shape it is perhaps as effective a control as art could
devise with such simple means.
Of the flight of his little machine, thus directed, Pénaud says:
“If the screw be turned on itself 240 times and the whole left free
in a horizontal position, it will first drop; then, upon attaining
its speed, rise and perform a regular flight at 7 or 8 feet from the
ground for a distance of about 40 metres, requiring about 11 seconds
for its performance. Some have flown 60 metres and have remained
in the air 13 seconds.[12] The rudder controls the inclination to
ascend or descend, causing oscillations in the flight. Finally the
apparatus descends gently in an oblique line, remaining itself
horizontal.”
The motive power is a twisted hank of fine rubber strips, which
weighs 5 grammes out of a total of 16 grammes for the whole machine,
whose center of gravity should be in advance of the center of surface
‹CD›, as will be demonstrated in another place. This device attracted
little notice, and I was unfamiliar with it when I began my own first
constructions at Allegheny, in 1887.
My own earliest models employed a light wooden frame with two
propellers, which were each driven by a strand of twisted rubber.[13]
In later forms, the rubber was enclosed and the end strains taken
up by the thinnest tin-plate tubes, or better still, paper tubes
strengthened by shellac.
Little was known to me at that time as to the proper proportions
between wing surface, weight and power; and while I at first sought
to infer the relation between wing surface and weight from that of
soaring birds, where it varies from 1/2 to 1 sq. ft. of wing surface
to the pound, yet the ratio was successively increased in the earlier
models, until it became 4 sq. ft. to 1 pound. It may be well to add,
however, that the still later experiments with the steam-driven
models, in which the supporting surface was approximately 2 sq.
ft. to the pound, proved that the lack of ability of these early
rubber-driven models to properly sustain themselves even with 4 sq.
ft. of wing surface to the pound, was largely due to the fact that
the wings themselves had not been stiff enough to prevent their being
warped by the air pressure generated by their forward motion.
During the years I presently describe, these tentative constructions
were [p010] renewed at intervals without any satisfactory result,
though it became clear from repeated failures, that the motive power
at command would not suffice, even for a few seconds’ flight for
models of sufficient size to enable a real study to be made of the
conditions necessary for successful flight.
In these earliest experiments everything had to be learned about the
relative position of the center of gravity, and what I have called
the center of pressure. In regard to the latter term, it might at
first seem that since the upward pressure of the air is treated as
concentrated at one point of the supporting surface, as the weight
is at the center of gravity, this point should be always in the same
position for the same supporting surface. This relation, however,
is never constant. How paradoxical seems the statement that, if
‹ab› be such a supporting surface in the form of a plane of uniform
thickness and weight, suspended at ‹c› (‹ac› being somewhat greater
than ‹cb›) and subjected to the pressure of a wind in the direction
of the arrow, the pressure on the lesser arm ‹cb› will overpower that
on the greater arm ‹ac›! We now know, however, that this must be so,
and why, but as it was not known to the writer till determined by
experiments published later in “Experiments in Aerodynamics,” all
this was worked out by trial in the models.
[Illustration: FIG. 1. Diagram of suspended plane showing position of
C. P.]
It was also early seen that the surface of support could be
advantageously divided into two, with one behind the other, or one
over the other, and this was often, though not always, done in the
models.
At the very beginning another difficulty was met which has proved a
constant and ever-increasing one with larger models--the difficulty
of launching them in the air. It is frequently proposed by those
unfamiliar with this difficulty, to launch the aerodrome by placing
it upon a platform car or upon the deck of a steamer, and running
the car or boat at an increasing speed until the aerodrome, which is
free to rise, is lifted by the wind of advance. But this is quite
impracticable without means to prevent premature displacement, for
the large surface and slight weight renders any model of considerable
size unmanageable in the least wind, such as is always present in the
open air. It is, therefore, necessary in any launching apparatus that
the aerodrome be held rigidly until the very moment of release, and
that instant and simultaneous release from the apparatus be made at
all the sustaining points at the proper moment. [p011]
There is but a very partial analogy in this case to the launching
of a ship, which is held to her ways by her great weight. Here, the
“ship” is liable to rise from her ways or be turned over laterally at
any instant, unless it is securely fastened to them in a manner to
prevent its rising, but not to prevent its advancing.
The experiments with rubber-driven models commenced in April,
1887, at the Allegheny Observatory, were continued at intervals
(partly there, but chiefly in Washington) for three or four years,
during which time between thirty and forty independent models
were constructed, which were so greatly altered in the course of
experiment that more nearly one hundred models were in reality tried.
The result of all this extended labor was wholly inconclusive,
but as subsequent trials of other motors (such as compressed air,
carbonic-acid gas, electric batteries, and the like) proved futile,
and (before the steam engine) only the rubber gave results, however
unsatisfactory, in actual flight, from which anything could be
learned, I shall give some brief account of these experiments, which
preceded and proved the necessity of using the steam engine, or other
like energetic motor, even in experimental models.
An early attempt was made in April, 1887, with a model consisting
of a frame formed of two wooden pieces, each about 1 metre long and
4 centimetres wide, made for lightness, of star-shaped section,
braced with cross-pieces and carrying two long strips of rubber, each
about 1 mm. thick, 30 mm. wide, 2 metres long, doubled, weighing 300
grammes. Each of these strips could be wound to about 300 turns,
one end being made fast to the front of the frame, the other to the
shaft of a four-bladed propeller 30 cm. in diameter. The wings were
made of lightest pine frames, over which paper was stretched, and
were double, one being superposed upon the other. Each was 15 cm.
wide, and 120 cm. long. The distance between them was 12 cm. and the
total surface a little more than 3600 sq. cm. (4 square feet). In
flying, the rubber was so twisted that the propellers were run in
opposite directions. The weight of the whole apparatus was not quite
1 kilogramme, or about 1 pound to 2 feet of sustaining surface, which
proved to be entirely too great a weight for the power of support.
When placed upon the whirling-table, it showed a tendency to soar
at a speed of about ten miles an hour, but its own propellers were
utterly insufficient to sustain it.
In this attempt, which was useful only in showing how much was to
be learned of practical conditions, the primary difficulty lay in
making the model light enough and sufficiently strong to support its
power. This difficulty continued to be fundamental through every
later form; but besides this, the adjustment of the center of gravity
to the center of pressure of the wings, the disposition of the wings
themselves, the size of the propellers, the inclination and number
of their blades, and a great number of other details, presented
themselves for examination. [p012] Even in the first model, the
difficulty of launching the machine or giving it the necessary
preliminary impulse was disclosed—a difficulty which may perhaps not
appear serious to the reader, but which in fact required years of
experiment to remove.
By June, 1887 two other models, embodying various changes that had
suggested themselves, had been constructed. Each of these had a
single propeller (one an 18-1/2-inch propeller with eight adjustable
blades, the other a 24-inch propeller with four adjustable blades)
and was sustained by two pairs of curved wings 4 feet 7 inches long.
It is, however, unnecessary to dwell further on these details, since
these models also proved altogether too heavy in relation to their
power, and neither of them ever made an actual flight.
At this period my time became so fully occupied with the experiments
in aerodynamics (which are not here in question) that during the next
two years little additional was done in making direct investigations
in flight.
In June, 1889, however, new rubber-driven models were made in which
the wooden frames were replaced by tubes of light metal, which,
however, were still too heavy, and these subsequently by tubes of
paper covered with shellac, which proved to be the lightest and best
material in proportion to its strength that had been found. The
twisted rubber was carried within these tubes, which were made just
strong enough to withstand the end-strain it produced. The front end
of the rubber being made fast to an extremity of the tube, the other
end was attached directly to the shaft of the propeller, which in the
early models was still supplied with four blades.
A detailed description of one of these early models, No. 26, shown in
Plates 1 and 4, follows:
In each of the two tubes of paper, stiffened with shellac, which form
a part of the framing, is mounted a hank of twisted rubber, which
connects with a propeller at the rear. There are two pairs of wings,
superposed and inclined at an angle, the one above, the other below
the frame. A light stem connected with the frame bears a triangular
Pénaud tail and rudder.
Length of model 105 cm.
Spread of wings 83 "
Width of upper wings 14 "
Width of lower wings 19 "
Diameter of propeller 29 "
Area of upper wings 1134 sq. cm.
Area of lower wings 1548 "
Area of tail 144 "
Weight of wings 51 grammes
Weight of tail 7 "
Weight of frame 38 "
Weight of wheels 20 "
Weight of rubber (.09 pound) 40 "
Total weight 156 "
No. of turns of rubber 100
Time of running down 8 seconds
Horse-power from preceding data 0.001 HP
The aerodromes made at this time were too heavy, as well as too
large, to be easily launched by hand, and it was not until 1891 that
the first one was constructed light enough to actually fly. This
first flight was obtained from the north window of the dome of the
Allegheny Observatory, on March 28, 1891, and imperfect as it was,
served to show that the proper balancing of the aerodrome which would
bring the center of gravity under the center of pressure, so as to
give a horizontal flight, had yet to be obtained.
From this time on until 1893, experiments continued to be made with
rubber-driven models, of which, as has been stated, nearly 40 were
constructed, some with two propellers, some with one; some with one
propeller in front and one behind; some with plane, some with curved,
wings; some with single, some with superposed, wings; some with
two pairs of wings, one preceding and one following; some with the
Pénaud tail; and some with other forms. A few of these early forms
are indicated on the accompanying Plates 1 to 4, but it does not seem
necessary to go into the details of their construction.
No. 11 with which an early flight was made, closely resembles the
Pénaud model.
No. 13 has two propellers, one in front and one behind, with a
single wing.
No. 14 has two propellers, nearly side by side, but one slightly in
advance, with a single wing and a flat horizontal tail.
No. 15 has one leading propeller and two broad wings, placed one
behind the other.
No. 30 has the propeller shafts at an angle, and one pair of wings.
No. 31 has the propeller shafts at an angle, and two pairs of wings
superposed.
The wings in general were flat, but in some cases curved. The rubber
was usually wound to about 100 turns, and trouble continually arose
from its “kinking” and unequal unwinding, which often caused most
erratic flights.
It is sufficient to say of these that, rude as they were, much
was learned from them about the condition of the machines in free
air, which could never be learned from the whirling-table or other
constrained flight.
The advantages and also the dangers of curved wings as compared with
plane ones, were shown, and the general disposition which would
secure an even balance, was ascertained; but all this was done with
extreme difficulty, since the brief flights were full of anomalies,
arising from the imperfect conditions of observation. For instance,
the motor power was apparently exhausted more rapidly when the
propellers were allowed to turn with the model at rest, than when it
was in motion, though in theory, in the latter case more power would
seem to be expended and a greater speed of revolution obtained in
a given time. The longest flights obtainable did not exceed 6 or 8
seconds in time, nor 80 to 100 feet in distance, and were not only
so brief, but, owing to the spasmodic action of the rubber and other
causes, so irregular, that it was extremely difficult to obtain even
the imperfect results which were actually deduced from them. [p014]
ABBREVIATIONS AND SYMBOLS EMPLOYED
The following rules and symbols were adopted for determining the
relative position of points on the aerodrome, some of them during
1891, and some of them since. All are given here for convenience of
reference, though their chief application is to the larger steam
aerodromes described later. Those which immediately follow were meant
to give some of the notation of descriptive geometry in untechnical
language for the use of the workmen employed. Let ‹X›, ‹Y› and ‹Z›
be three lines at right angles to each other, and passing through
the same point in space, ‹O›, lying at any convenient distance above
the floor of the work-shop. The line ‹X› lies North and South; the
line ‹Y› lies East and West, and the line ‹Z› points to the zenith.
Now place the aerodrome on the floor so that its principal axis lies
horizontally in the plane ‹XZ›, with its head pointing North, and
in such a position that a line passing through the center of the
propellers shall coincide with the line ‹Y›.
[Illustration: FIG. 2. Diagram showing mensural coordinates.]
When measurements are made on or parallel to the line ‹X›, the point
of intersection ‹O› will be marked 1500 centimetres, and distances
toward the South will be less than, and distances toward the North
greater than 1500 centimetres.
When measurements are made on or parallel to the line ‹Z›, the point
‹O› will be considered to be marked 2500 centimetres, and distances
above will be greater than, and distances below will be less than
2500 centimetres. [p015]
Lastly, when measurements are made on or parallel to the line ‹Y›,
the point ‹O› will be marked 3500 centimetres, and distances toward
the East will be greater than, and distances toward the West will be
less than 3500 centimetres. Measurements in these latter directions
will be comparatively infrequent because the center of gravity and
center of pressure both lie in the plane ‹XZ›.
EXAMPLE
In the figure the point ‹T› in the tail, if 15 centimetres to the
South of ‹O›, would be graduated 1485 centimetres. A weight (‹W›)
25 centimetres below the axis, would be graduated 2475 centimetres.
A point 50 centimetres above the axis would be graduated 2550
centimetres, etc.
‹CG› represents the Center of Gravity of the aerodrome, or (with
subscript letters) of any specially designated part, or with
reference to some indicated condition.
‹CG›_1 ‹CG›_2 represent the Center of Gravity as referred to
the first, or horizontal, and to the second, or vertical plane,
respectively.
‹CP› represents the Center of Pressure[14] of the whole aerodrome, or
(with a subscript) of any specially designated part.
‹CF› represents the Center of Figure of the aerodrome, or of any
specially designated part.
Subscripts:
“‹fw›” refers to the front wings. “1” refers to the plane ‹XY›.
“‹rw›” refers to the rear wings. “2” refers to the plane ‹XZ›.
“‹r›” refers to a state of rest. “3” refers to the plane ‹YZ›.
“‹m›” refers to a state of motion.
“‹A›” represents the total area of the supporting surface; “‹a›”
represents the total area of the tail; ‹HP› represents the
horse-power by Prony brake measurement. “Horse-power by formula” is
given by Maxim’s formula:[15]
rev.×diam. of propeller×pitch×thrust
‹HP› = ------------------------------------.
33,000
(This formula was not in use at the time of the rubber-motor
experiments, for which the thrust was not taken. It appears to assume
the conditions where the screws from a fixed position move a mass
of still air, are the same as those of free flight. Its results,
however, are in better agreement with experiment than might be
anticipated.)
“Flying-weight” means everything borne in actual flight, including
fuel and water. [p016]
Remembering that the principal object of all these experiments is to
be able to predict that setting of the wings and tail with reference
to the center of gravity which will secure horizontal flight, we must
understand that in the following tables (see No. 30) the figures
‹CP›_m = 1516.5 cm. mean a prediction that the center of pressure
of the sustaining surfaces in motion (‹CP›_m) is to be found in a
certain position 1516.5; that is, 16.5 cm. in advance of the line
joining the propeller shafts. This prediction has been made by means
of previous calculation joined with previous experimental adjustment.
We know in a rough way where the ‹CP› will fall on the wings when
they are exposed independently if flat, and at a certain angle, and
where it will fall on the tail. From these, we can find where the
resulting ‹CP› of the whole sustaining surface will be.
It would seem that when we have obtained the center of gravity by a
simple experiment, we have only to slide the wings or tail forward
and back until the (calculated) center of pressure falls over this
observed center of gravity. But in the very act of so adjusting the
wings and tail, the center of gravity is itself altered, and the
operation has to be several times repeated in order to get the two
values (the center of pressure and center of gravity) as near each
other as they are found in the above-mentioned table, our object
being to predict the position which will make the actual flight
itself horizontal. How far this result has been obtained, experiment
in actual flight alone can show, and from a comparison of the
prediction with the results of observation, we endeavor to improve
the formula.
The difficulties of these long-continued early experiments were
enhanced by the ever-present difficulty which continued through later
ones, that it was almost impossible to build the model light enough
to enable it to fly, and at the same time strong enough to withstand
the strains which flight imposed upon it. The models were broken
up by their falls after a few flights, and had to be continually
renewed, while owing to the slightness of their construction, the
conditions of observation could not be exactly repeated; and these
flights themselves, as has already been stated, were so brief in
time (usually less than six seconds), so limited in extent (usually
less than twenty metres), and so wholly capricious and erratic,
owing to the nature of the rubber motor and other causes, that very
many experiments were insufficient to eliminate these causes of
mal-observation.
It is not necessary to take the reader through many of them, but not
to pass over altogether a labor which was so great in proportion
to the results, but whose results, such as they were, were the
foundation of all after knowledge, I will, as illustrations, take
from an almost unlimited mass of such material the observations of
November 20, 1891, which were conducted with Model No. 30 with a
single pair of wings, shown in Plate 1, and with another one, No.
31, also shown [p018] in Plate 1, with superposed wings, which was
used for the purpose of comparison. S. P. Langley was the observer,
the place of observation the larger upper hall of the Smithsonian
building, at Washington, the time being taken by a stop-watch, and
the distance by a scale laid down upon the floor. The models were in
every case held by an assistant and launched by hand, being thrown
off with a slight initial velocity. In the case of No. 30, the
preliminary calculation of the position of the center of pressure had
been made by the process already described; the center of gravity,
with reference to the horizontal plane, was determined by simply
suspending the whole by a cord.
[Illustration: PL. 1
RUBBER-MOTOR MODEL AERODROMES NOS. 11, 13, 14, 15, 26, 30, 31]
[Illustration: PL. 2
RUBBER-MOTOR MODEL AERODROMES NOS. 11, 13, 14]
[Illustration: PL. 3
RUBBER-MOTOR MODEL AERODROMES NOS. 15, 24]
[Illustration: PL. 4
RUBBER-MOTOR MODEL AERODROME NO. 26]
OBSERVATION OF NOVEMBER 20, 1891.
OBSERVER, S. P. L.
LOCALITY, UPPER HALL, SMITHSONIAN BUILDING.
------------------------+----------------------+----------------------
| No. 30. | No. 31.
| Single wings. | Superposed wings.
------------------------+----------------------+----------------------
‹CP›_m 1516.5 cm.
‹CG›_1 1515 cm. 1517 cm.
‹CF›_w 1528 cm.
Length (without fender) 120 cm. = 3.94 ft. 120 cm. = 3.94 ft.
Width over wing tips 120 cm. = 3.94 ft. 120 cm. = 3.94 ft.
Weight of rubber (72
grammes in each tube) 144 gr. = 0.32 lbs. 144 gr. = 0.32 lbs.
Total flying weight
(including tail) 432 gr. = 0.95 lbs. 506 gr. = 1.11 lbs.
Turns of rubber 30 30
Diameter of propellers 37 cm. = 1.21 ft. 37 cm. = 1.21 ft.
Width of propellers 7 cm. = 0.23 ft. 7 cm. = 0.23 ft.
Pitch of propellers 50 cm. = 1.64 ft. 50 cm. = 1.64 ft.
Each pair 1984 sq. cm.
Area of wings (each 1984 sq. cm. = 2.13 sq. ft.
992 sq. cm.) = 2.13 sq. ft. Total 3968 sq. cm.
= 4.26 sq. ft.
Area of tail 373 sq. cm. 373 sq. cm.
= 0.40 sq. ft. = 0.40 sq. ft.
------------------------+----------------------+----------------------
Area of wings and tail in No. 30, 2357 sq. cm. = 2.53 sq. ft.
2.53 sq. ft. ÷ .95 = 2.7. Therefore, there are 2.7 or nearly
3 square feet of sustaining area to the pound.
-------+----------+---------------------------------------------------
Flight.|Aerodrome.| Results.
-------+----------+---------------------------------------------------
1 No. 30 With 30 turns of the rubber, flew low through
10 metres.
2 No. 30 Flew heavily through 12 metres.
3 No. 31 Flew high and turned to left; distance not noted.
4 No. 31 The right wing having been weighted (to depress it
and correct the tendency to turn to the left),
model flew high, but the rubber ran down when it
had obtained a flight of 10 metres.
5 No. 31 The wings were moved backward until the ‹CP›
stood at 1493. The model still turned to the left;
flight lasted three and a-half seconds; distance
not noted.
6 No. 31 Vertical tail was adjusted so as to further increase
the tendency to go to the right. In spite of all
this, the model turned sharply to the left, flying
with a nearly horizontal motion; time of flight
not noted; distance not noted.
7 No. 30 Straight horizontal flight; time three and
three-fifth seconds, when rubber ran down;
distance 13 metres.
8 No. 30 Straight flight as before; time two and four-fifth
seconds; distance 13 metres.
9 No. 30 With a curved wing in the same position as the flat
wing had previously occupied, model flew up and
struck the ceiling (nearly 30 feet high), turning
to right, with a flight whose curtate length was
10 metres.
10 No. 30 Wing having been carried back 5 centimetres, model
still flew up, but not so high, and still turned
to the right.
11 No. 30 Wings carried back 5 centimetres more; model still
flew high; time two and two-fifths seconds;
distance 13 metres.
12 No. 30 Wings carried back 4 centimetres more; model still
flew high during a flight of 13 metres.
-------+----------+---------------------------------------------------
The observations now ceased, owing to the breaking up of the model.
The objects of these experiments, as of every other, were to find the
practical conditions of equilibrium and of horizontal flight, and to
compare the calculated with the observed positions of the center of
pressure. They enable us to make a comparison of the performances
given by earlier ones with a light rubber motor, with the relatively
heavy motors used to-day, as well as a comparison of single flat,
single curved, and superposed flat wings.
The average time of the running down of the rubber in flight was
something like three seconds, while the average time of its running
down when standing still was but one and a half seconds. It might
have been expected from theory that it would take longer to run
down when stationary, than in flight, and this was one of the
many anomalies observed, whose explanation was found later in the
inevitable defects of such apparatus.
The immediate inferences from the day’s work were:
1. That the calculated position of the ‹CP› at rest, as related to
the ‹CG›, is trustworthy only in the case of the plane wing.
2. The formula altogether failed with the curved wing, for which the
‹CP› had to be carried indefinitely further backward.
On comparing the previous flights of November 14, with these, it
seems that with the old rubber motor of 35 grammes and 50 turns,
the single wing, either plane or curved, is altogether inferior to
the double wing; while with the increased motor power of this day,
the single wing, whether plane or curved, seems to be as good as
the double wing. It also seems that the curved wing was rather more
efficient than the plane one.
The weight of the rubber in each tube was 72 grammes, or 0.16 pounds;
mean speed of flight in horizontal distance 4-1/2 metres (about 15
feet) per second.[16]
From experiments already referred to, there were found available
300 foot-pounds of energy in a pound of rubber as employed, and in
0.16 of a pound, 48 foot-pounds of energy were used; 48/33,000 or
0.00145 = the horse-power exerted in [p019] one minute, but as the
power was in fact expended in 1/20 of that time we have 20×0.00145 =
0.029; that is, during the brief flight, about 0.03 of a horse-power
was exerted, and this sustained a total weight of only about a pound.
In comparing this flight with the ideal conditions of horizontal
flight in “Aerodynamics,” it will be remembered that this model’s
flight was so irregular and so far from horizontal, that in one
case it flew up and struck the lofty ceiling. The angle with the
horizon is, of course, so variable as to be practically unknown, and
therefore no direct comparison can be instituted with the data given
on page 107 of “Experiments in Aerodynamics,” but we find from these
that at the lowest speed there given of about 35 feet per second,
0.03 of a horse-power exerted for three seconds would carry nearly
one pound through a distance of somewhat over 100 feet in horizontal
flight.
The number of turns of the propellers multiplied by the pitch
corresponds to a flight of about 16 metres, while the mean actual
flight was about 12. It is probable, however, that there was really
more slip than this part of the observation would indicate. It
was also observed that there seemed to be very little additional
compensatory gain in the steering of No. 30 for the weight of the
long rudder-tail it carried. It may be remarked that in subsequent
observations the superiority of the curved wing in lifting power
was confirmed, though it was found more liable to accident than
the flatter one, tending to turn the model over unless it was very
carefully adjusted.
It may also be observed that these and subsequent observations show,
as might have been anticipated, that as the motor power increased,
the necessary wing surface diminished, but that it was in general an
easier and more efficient employment of power to carry a surface of
four feet sustaining area to the pound than one of three, while one
of two feet to the pound was nearly the limit that could be used with
the rubber motor.[17]
It may be remarked that the flights this day, reckoned in horizontal
distance, were exceptionally short, but that the best flights at
other times obtained with these models (30 and 31) did not exceed
25 metres. Such observations were continued in hundreds of trials,
without any much more conclusive results. [p020]
The final results, then, of the observations with rubber-driven
models (which were commenced as early as 1887, continued actively
through the greater portion of the year 1891 and resumed, as will
be seen later, even as late as 1895), were not such as to give
information proportioned to their trouble and cost, and it was
decided to commence experiments with a steam-driven aerodrome on a
large scale.
[p021]
CHAPTER III
AVAILABLE MOTORS
In the introductory chapter to “Experiments in Aerodynamics,” it was
asserted that
“These researches have led to the result that mechanical
sustentation of heavy bodies in the air, combined with very great
speeds, is not only possible, but within the reach of mechanical
means we actually possess.”
It was, however, necessary to make a proper selection in order to
secure that source of power which is best adapted to the requirements
of mechanical flight. Pénaud had used india rubber as the cheapest
and at the same time the most available motor for the toys with
which he was experimenting, but when models were constructed that
were heavier than anything made prior to 1887, it appeared, after
the exhaustive trials with rubber referred to in the preceding
chapter, that something which could give longer and steadier flights
must be used as a motor, even for the preliminary trials, and the
construction of the large steam-driven model known as No. 0, and
elsewhere described, was begun. Even before the completion of this,
the probability of its failure grew so strong that experiments were
commenced with other motors, which it was hoped might be consistent
with a lighter construction.
These experiments which commenced in the spring of 1892 and continued
for nearly a twelvemonth, were made upon the use of compressed air,
carbonic-acid gas, electricity in primary and storage batteries, and
numerous other contrivances, with the result that the steam engine
was finally returned to, as being the only one that gave any promise
of immediate success in supporting a machine which would teach the
conditions of flight by actual trial, though it may be added that the
gas engine which was not tried at this time on account of engineering
difficulties, was regarded from the first as being the best in
theory and likely to be ultimately resorted to. All others were
fundamentally too heavy, and weight was always the greatest enemy.
It is the purpose of this chapter to pass in brief review the work
that was done and the amount of energy that was obtained with
these several types of motors, as well as the obstacles which they
presented to practical application upon working aerodromes.
INDIA RUBBER
India rubber is the source of power to which the designer of a
working model naturally turns, where it is desirable that it shall
be, above all, light and free from the necessity of using complicated
mechanism. Rubber motors were, [p022] therefore, used on all of
the earlier models, and served as the basis of calculations made
to determine the amount of power that would be required to propel
aerodromes with other sources of energy.
Some of the disadvantages inherent in the use of rubber are at
once apparent, such as the limited time during which its action is
available, the small total amount of power, and the variability in
the amount of power put forth in a unit of time between the moment of
release and the exhaustion of the power. In addition, serious, though
less obvious difficulties, present themselves in practice.
There are two ways in which rubber can be used; one by twisting a
hank of strands, and, while one end is held fast, allowing the other
to revolve; the other, by a direct longitudinal stretching of the
rubber, one end being held fast and the other attached to the moving
parts of the mechanism. The former method was adopted by Pénaud,
and was also used in all of my early constructions, but while it is
most convenient and simple in its (theoretical) application, it has,
in addition to the above drawbacks, that of knotting or kinking,
when wound too many turns, in such a way as to cause friction on
any containing tube not made impracticably large, and also that of
unwinding so irregularly as to make the result of one experiment
useless for comparison with another.
In 1895, some experiments were made in which the latter method was
used, but this was found to involve an almost impracticable weight,
because of the frame (which must be strong enough to withstand the
end pull of the rubber) and the mechanism needed to convert the pull
into a movement of rotation.
As the power put forth in a unit of time varies, so there is a
corresponding variation according to the original tension to which
the rubber is subjected. Thus in some experiments made in 1889 with
a six-bladed propeller 18.8 inches in diameter, driven by a rubber
spring 1.3 inches wide, 0.12 inch thick and 3 feet long, doubled, and
weighing 0.38 pound, the following results were obtained:
Number of twists of rubber 50 75 100
Time required to run down 7 sec. 10 sec. 12 sec.
Foot-pounds developed 37.5 63.0 124.6
Foot-pounds developed per min. 321.4 378.0 623.0
Horse-power developed 0.0097 0.0115 0.0189
Thus we see that, with twice the number of turns, more than three
times the amount of work was done and almost twice the amount of
power developed, giving as a maximum for this particular instance 328
foot-pounds per pound of rubber.
The usual method of employing the twisted rubber was to use a number
of fine strands formed into a hank looped at each end. One of these
hanks, consisting of 162 single or 81 double strands of rubber, and
weighing 73 grammes, when given 51 turns developed 55 foot-pounds
of work, which was put out in 4 seconds. This corresponds to 0.01
horse-power per minute for one pound of rubber. [p023]
The results of a large number of tests show that one pound of twisted
rubber can put forth from 450 to 500 or more foot-pounds of work,
but at the cost of an overstrain, and that a safe working factor can
hardly be taken at higher than 300 foot-pounds, if we are to avoid
the “fatigue” of the rubber, which otherwise becomes as marked as
that of a human muscle.
While twisting is an exceedingly convenient form of application
of the resilience of rubber to the turning of propelling wheels,
the direct stretch is, as has been remarked, much more efficient
in foot-pounds of energy developed by the same weight of rubber.
It was found that rubber could not, without undue “fatigue,” be
stretched to more than four and a half times its original length,
though experiments were made to determine the amount of work that a
rubber band, weighing one pound, was capable of doing, the stretching
being carried to seven times its original length. The results varied
with the rubber used and the conditions of temperature under which
the experiments were tried, ranging from 1543 foot-pounds to 2600
foot-pounds. The tests led to the conclusion that, for average
working, one pound of rubber so stretched, is capable of doing 2000
foot-pounds of work, but, owing to the weight of the supporting
frame and of the mechanism, this result can be obtained only
under conditions impracticable for a flying machine. In the more
practicable twisted form it furnishes, as has been said, less than a
fifth of that amount.
The conclusions reached from these experiments are:
1. The length of the unstretched rubber remaining the same, the
sustaining power will be directly proportional to the weight of
rubber;
2. With a given weight of rubber, the end strain is inversely
proportional to the length of the unstretched rubber;
3. With a given weight of rubber, the work done is constant, whatever
the form; hence if we let ‹w› = the work in foot-pounds, ‹g› = the
weight of the rubber in pounds, and ‹k› = a constant taken at 2000 as
given above, we have
‹w› = ‹kg› = 2000 ‹g› foot-pounds.
This is for an extension of seven units of length, so that for a unit
of extension we would have approximately
‹w› = 300 ‹g› foot-pounds
which for four units of extension corresponds very closely to the
1300 foot-pounds which Pénaud claims to have obtained.
4. The end strain varies with the cross-section for a given unit of
extension.
These results can lead to but one conclusion; that for the
development of the same amount of power when that amount shall be
1 horse-power or more, rubber weighs enormously more than a steam
engine, besides being less reliable [p024] for a sustained effort,
and, therefore, cannot be used for propelling aerodromes intended for
a flight that is to be prolonged beyond a few seconds.[18]
It may be desirable to present a tabular view of the ‹theoretical›
energy of available motors, which it will be noticed is a wholly
different thing from the results obtained in practice. Thus, we
represent the weight of rubber only, without regard to the weight
of the frame required to hold it. In the steam engine, we consider
the theoretical efficiency per pound of fuel, without regarding the
enormous waste of weight in water in such small engines as these,
or the weight of the engine itself. We treat the hot-water engine
in like manner, and in regard to carbonic acid and compressed air,
we take no note of the weight of the containing vessel, or of the
cylinders and moving parts. In the same way we have the theoretical
potency of electricity in primary and storage batteries, without
counting the weight of the necessary electromotors; and of the
inertia-engine without discussing that of the mechanism needed to
transmit its power.
Foot-pounds of energy in one pound of
Gasoline 15,625,280
Alcohol 9,721,806
Gunpowder 960,000
Hot water, under pressure of 100 atmospheres 383,712
Air, under pressure of 100 atmospheres, isothermal
expansion 120,584
Liquid carbonic acid, at temperature of 30° and pressure
of 100 atmospheres 78,800
Electric battery; short-lived, thin walled; chromic acid
and platinum 75,000
Steel ring, 8 inches in diameter, at speed of 3000 turns
per minute 19,000
Storage battery 17,560
Rubber, pulled 2,000
Rubber, twisted 300
It may be interesting to consider next, in even a roughly approximate
way, what may be expected from these various sources of energy in
practice.
STEAM ENGINE
The steam engine on a small scale, and under the actual restrictions
of the model, must necessarily be extremely wasteful of power. If we
suppose it to realize 2 per cent of the theoretical energy contained
in the fuel, we shall be assuming more than was actually obtained.
The energy of the fuel cannot be obtained at all, of course, without
boiler and engine, whose weight, for the purpose of the following
calculation, must be added to that of the fuel; and if we suppose
the weight of the boilers, engines and water, for a single minute’s
flight, to be collectively ten pounds, we shall take an optimistic
view of what may be expected under ordinary conditions. We have
in this view 1/500 of the [p025] theoretical capacity possibly
realizable under such conditions, but if we take 1/1000 we shall
probably be nearer the mark. Even in this case we have, when using
gasoline as fuel, 15,625 foot-pounds per minute, or nearly 0.50
horse-power, as against .0091 horse-power in the case of the rubber,
so that even with this waste and with the weight of the engines
necessary for a single minute’s service, the unit weight of fuel
employed in the steam engine gives 55 times the result we get with
rubber.
[Illustration: PL. 5
RUBBER-PULL MODEL AERODROME]
[Illustration: PL. 6
RUBBER-PULL MODEL AERODROME]
[Illustration: PL. 7
RUBBER-PULL MODEL AERODROME]
[Illustration: PL. 8
RUBBER-PULL MODEL AERODROME]
[Illustration: PL. 9
RUBBER-PULL MODEL AERODROME]
With alcohol we have about 2/3 the result that is furnished by
gasoline, since nearly the same boiler and engine will be used
in either case. Certain difficulties which at first appeared to
be attendant on the use of gasoline on a small scale induced me
to make the initial experiments with alcohol. This was continued
because of its convenience during a considerable time, but it was
finally displaced in favor of gasoline, not so much on account of
the superior theoretical efficiency of the latter, as for certain
practical advantages, such as its maintaining its flame while exposed
to wind, and like considerations.
GUNPOWDER
Although there are other explosives possessing a much greater energy
in proportion to their weight than gunpowder, this is the only one
which could be considered in relation to the present work, and the
conclusion was finally reached that it involved so great a weight
in the containing apparatus and so much experiment, that, although
the simplicity of its action is in its favor where crude means are
necessary, experiments with it had better be deferred until other
things had been tried.
HOT-WATER ENGINE
A great deal of attention was given to the hot-water engine, but it
was never put to practical use in the construction of an aerodrome,
partly on account of the necessary weight of a sufficiently strong
containing vessel.
COMPRESSED AIR
Compressed air, like the other possible sources of power, was
investigated, but calculations from well-authenticated data showed
that this system of propelling engines would probably be inadequate
to sustain even the models in long flights. As the chief difficulty
lies in the weight, not of the air, but of the containing vessel,
numerous experiments were made in the construction of one at once
strong and light. The best result obtained was with a steel tube
40 mm. in diameter, 428 mm. in length, closed at the ends by heads
united by wires, which safely contained 538 cubic cm. of air at an
initial pressure of 100 atmospheres for a weight of 521 grammes.
[p026]
If we suppose this to be used, by means of a proper reducing valve,
at a mean pressure of 100 pounds, for such an engine as that of
Aerodrome No. 5, which takes 60 cubic cm. of air at each stroke, we
find that (if we take no account of the loss by expansion) we have
18,329 foot-pounds of energy available, which on the engine described
will give 302 revolutions of the propellers.
There are such limits of weight, and the engines must be driven at
such high speeds, that the increased economy that might be obtained
by re-heating the air would be out of the question. The principal
object in using it would have been the avoidance of fire upon the
aerodrome, and the expansion of the unheated air would probably
have caused trouble with freezing, while the use of hot (i. e.
superheated) water was impracticable. So when, after a careful
computation, it was found that, having regard to the weight of the
containing vessel, only enough compressed air could be stored at 72
atmospheres and used at 4, to run a pair of engines with cylinders
0.9 inch in diameter by 1.6 inches stroke, at a speed of 1200
revolutions per minute for 20 seconds, all further consideration of
its adaptation to the immediate purpose was definitely abandoned.
This course, however, was not taken until after a model aerodrome
for using compressed air had been designed and partially built.
Then, after due consideration, it was decided to make the test with
carbonic-acid gas instead.
GAS
The gas engine possesses great theoretical advantages. At the time
of these experiments, the gas engine most available for the special
purposes of the models was one driven by air drawn through gasoline.
As the builders could not agree to reduce the weight of a one
horse-power engine more than one-half of the then usual model, and
as the weight of the standard engine was 470 pounds, it was obvious
that to reduce this weight to the limit of less than 3 pounds was
impracticable under the existing conditions, and all consideration
of the use of gas was abandoned provisionally, although a gasoline
engine of elementary simplicity was designed but never built. I
purposed, however, to return to this attractive form of power if I
were ever able to realize its theoretical advantages on the larger
scale which would be desirable.
ELECTRICITY
As it was not intended to build the model aerodromes for a long
flight, it was thought that the electric motor driven by a primary
or storage battery might possibly be utilized. It therefore occurred
to me that a battery might be constructed to give great power in
proportion to its weight on condition of being short-lived, and that
in this form a battery might perhaps advantageously take the place
of the dangerous compressed-air tubes that were at the time (1893)
[p027] under consideration for driving the models. I assumed that
the longest flight of the model would be less than five minutes. Any
weight of battery, then, that the model carried in consumable parts
lasting beyond this five minutes would be lost, and hence it was
proposed to build a battery, the whole active life of which would be
comprised in this time, to actuate a motor or motors driving one or
two propellers.
According to Daniell, when energy is stored in secondary batteries,
over 300,000 megergs per kilogramme of weight can be recovered and
utilized if freshly charged.
300,000 megergs = 0.696 horse-power for 1 min.
300,000 megergs = 0.139 horse-power for 5 min.
In a zinc and copper primary battery with sulphuric acid and water,
one kilogramme of zinc, oxidized, furnishes at least 1200 calories as
against 8000 for one kilogramme of carbon, but it is stated that the
zinc energy comes in so much more utilizable a form that the zinc,
weight for weight, gives practically, that is in work, 40 per cent
that of carbon. The kilogramme of carbon gives about 8000 heat units,
each equal to 107 kilogrammetres, or about 6,176,000 foot-pounds.
Of this, in light engines, from 5 to 10 per cent, or at least
308,800 foot-pounds, is utilized, and 2/5 of this, or about 124,000
foot-pounds, would seem to be what the kilogramme of zinc would give
in actual work. But to form the battery, we must have a larger weight
of fluid than of zinc, and something must be allowed for copper. If
we suppose these to bring the weight up to 1 kilogramme, we might
still hope to have 50,000 foot-pounds or 1.5 horse-power for one
minute, or 0.3 horse-power for 5 minutes.
Storage batteries were offered with a capacity of .25 horse-power for
5 minutes per kilogramme, but according to Daniell one cannot expect
to get more than 0.139 horse-power from a freshly charged battery of
that weight for the same time.
The plan of constructing a battery of a long roll of extremely thin
zinc or magnesium, winding it up with a narrower roll of copper or
platinized silver, insulating the two metals and then pouring over
enough acid to consume the major portion of the zinc in 5 minutes,
was carefully considered, but the difficulties were so discouraging,
that the work was not undertaken.
The lightest motors of 1 horse-power capacity of which any trace
could be found weighed 25 pounds, and a prominent electrician stated
that he would not attempt to construct one of that weight.
In trials with a 1/2 horse-power motor driving an 80 cm. propeller
of 1.00 pitch-ratio, I apparently obtained a development of 0.56
indicated horse-power at 1265 revolutions; but at lower speeds when
tried with the Prony brake, the brake horse-power fell to 0.10 at 546
revolutions, and even at 1650 revolutions [p028] it was but 0.262
indicated, with a brake horse-power of 0.144, or 55 per cent of that
indicated.
With these results both of theoretical calculation and practical
experiment, all thought of propelling the proposed aerodrome by
electricity was necessarily abandoned.
CARBONIC-ACID GAS
At the first inception of the idea, it seemed that carbonic-acid gas
would be the motive power best adapted for short flights. It can
be obtained in the liquid form, is compact, gives off the gas at a
uniform pressure dependent upon the temperature, and can be used in
the ordinary steam engine without any essential modifications. The
only provision that it seemed, in advance, necessary to make, was
that of some sort of a heater between the reservoir of liquid and the
engine, in order to prevent freezing, unless the liquid itself could
be heated previous to launching.
The engines in which it was first intended to use carbonic acid were
the little oscillating cylinder engines belonging to Aerodrome No.
1. The capacity of each cylinder was 21.2 cu. cm., so that 84.8 cu.
cm. of gas would be required to turn the propellers one revolution
when admitted for the full stroke, and 101,760 cubic cm. for 1200
revolutions. The density of the liquid at a temperature of 24° C. was
taken as .72, and as 1 volume of liquid gives 180 volumes of gas at a
pressure of 2-1/2 atmospheres, we have 101,760/180 = 565 cu. cm. of
liquid, or 407 grammes required for 1200 revolutions of the engines.
Thus, a theoretical calculation seemed to indicate that a kilogramme
of liquid carbonic acid would be an ample supply for a run of two
minutes. The experiments were, at first, somewhat encouraging. The
speed and apparent power of the engines were sufficient for the
purpose, but the length of time during which power could be obtained
was limited.
In 1892, 415 grammes of carbonic acid drove the engines of Aerodrome
No. 3 700 revolutions in 60 seconds, 900 in 75, and 1000 in 85
seconds, at the end of which time the gas was entirely expended. The
diameter of these cylinders was 2.4 cm., the stroke of the pistons 7
cm., and the work done, that of driving a pair of 50 cm. propellers,
when taken in comparison with the propeller tests detailed elsewhere,
amounted to an effective horse-power of about 0.10 for the output of
the engine.
The difficulties, however, that were experienced were those partially
foreseen. The expansion of the gas made such serious inroads upon the
latent heat of the liquid, that lumps of solid acid were formed in
the reservoir, and could be heard rattling against the sides when the
latter was shaken, while the expansion of the exhaust caused such a
lowering of temperature at that point, that the [p029] pipes were
soon covered with a thick layer of ice, and the free exit of the
escaping gas was prevented.
Such difficulties are to be expected with this material, but here
they were enhanced by the small scale of the construction and the
constant demand for lightness. And it was found to be very hard
to fill the small reservoirs intended to carry the supply for the
engines. When they were screwed to the large case in which the liquid
was received and the whole inverted, the small reservoir would be
filled from one-third to one-half full, and nothing that could be
done would force any more liquid to enter.
In view of these difficulties, and the objections to using a heater
of any sort for the gas, as well as the absolute lack of success
attendant upon the experiments of others who were attempting to use
liquid CO_2 as a motive power on a large scale elsewhere, experiments
were at first temporarily and afterwards permanently abandoned.
The above experiments extended over nearly a year in time, chiefly
during 1892, and involved the construction and use of the small
aerodromes Nos. 1, 2, and 3, presently described.
[p030]
CHAPTER IV
EARLY STEAM MOTORS AND OTHER MODELS
In dealing with the development of the aerodrome, subsequent to the
early rubber-driven models, the very considerable work done and
the failures incurred with other types of motors than steam, have
been briefly dealt with in the preceding chapter, but are scarcely
mentioned here, as no attempts at long flights were ever successful
with any other motor than steam, and no information was gained from
any of the experiments made with compressed air, gas, carbonic acid,
or electricity, that was of much value in the development of the
successful steam machines.
In November, 1891, after the long and unsatisfactory experiments with
rubber-driven models already referred to, and before most of the
experiments with other available motors than steam had been made,
I commenced the construction of the engines and the design of the
hull of a steam-driven aerodrome, which was intended to supplement
the experiments given in “Aerodynamics” by others made under the
conditions of actual flight.
In designing this first aerodrome, here called No. 0, there was no
precedent or example, and except for the purely theoretical conditions
ascertained by the experiments described in “Aerodynamics,” everything
was unknown. Next to nothing was known as to the size or form, as to
the requisite strength, or as to the way of attaching the sustaining
surfaces; almost nothing was known as to the weight permissible,
and nothing as to the proper scale on which to build the aerodrome,
even if the design had been obtained, while everything which related
to the actual construction of boiler and engines working under such
unprecedented conditions was yet to be determined by experiment.
The scale of the actual construction was adopted under the belief
that it must be large enough to carry certain automatic steering
apparatus which I had designed, and which possessed considerable
weight. I decided that a flying machine if not large enough to carry
a manager, should in the absence of a human directing intelligence,
have some sort of automatic substitute for it, and be large enough to
have the means of maintaining a long and steady flight, during which
the problems (which the rubber-driven models so imperfectly answered)
could be effectually solved.
When, in 1891, it was decided to attempt to build this steam
aerodrome, the only engine that had been made up to that time
with any claim to the lightness and power I was seeking, was the
Stringfellow engine, exhibited at the Crystal Palace in London, in
1868, which it was then announced developed 1 horse-power [p031] for
a total weight (boiler and engines) of 13 pounds. The original engine
came into the possession of the Institution in 1889 as an historical
curiosity, but on examination, it was at once evident that it never
had developed, and never could develop the power that had been
attributed to it, and probably not one-tenth so much.
With the results obtained on the whirling-table at Allegheny as a
basis, a theoretical computation of the weight which 1 horse-power
would cause to soar showed that, with a plane whose efficiency
should be equal to that of a 30×4.8 inch plane set at an angle of
5° and moving at a speed of 34 miles an hour, 1 horse-power would
support 120 pounds.[19] With a smaller angle even better results
could be obtained, but as the difficulties of guidance increase as
the angle diminishes, I did not venture to aim at less than this.
In this computation, no allowance was made for the fact that these
results were obtained by a mechanism which ‹forcibly maintained› the
supporting surface in the ideal condition of the best attainable
angle of attack as if in perfect equilibrium, and above all in the
equally ideal condition of perfectly horizontal flight.
Besides this, I had to consider in actual flight the air resistance
due to the guy wires and hull, but after making an allowance of as
much as three-quarters for these differences between the conditions
of experiment and those of free flight, I hoped that 1 horse-power
would serve to carry 30 pounds through the air if a supporting
surface as large as 3 feet to the pound could be provided, and this
was the basis of the construction which I will now describe.
The general form of this Aerodrome No. 0, without wings or
propellers, is shown in the accompanying photograph in Plate 10. Its
dimensions and its weights, as first designed, and as finally found
necessary, are as follows:
COMPARISON OF ESTIMATED AND ACTUAL WEIGHTS OF PARTS OF
AERODROME “0”--IN POUNDS AND OUNCES.
Estimated Actual
lbs. oz. lbs. oz.
Engines 4 0 4 1
Boilers and Burners 8 11 13 14
Pumps and Attachments 0 0 1 10
Steering Apparatus 0 6 0 0
Frame of Hull and Braces, including bowsprit
and tail tube 7 7 8 11
Oil tank covering and pipes 0 0 0 13
Shafts, ball bearings (2:1) and wooden
propellers (1:7) 1 14 3 8
Wings (5:4) and guys (0:9) 4 0 5 13
Tail 1 5 2 2
Jacket at prow 0 0 4 0
--------- ---------
Total without oil or water 27 11 44 8
(The weights attained in the actual making were, as is seen, nearly
double those first estimated, and this constant increase of weight
under the exigencies of construction was a feature which could never
be wholly eliminated.) [p032]
After studying various forms for the hull or body of the prospective
aerodrome, I was led to adopt the lines which Nature has used in the
mackerel as most advantageous so far as the resistance of the air was
concerned, but it proved to be difficult in construction to make the
lines of the bow materially different from those of the stern, and in
this first model the figure was symmetrical throughout.
As I wish that my experience may be of benefit to the reader, even
in its failures, I will add that I made the not unnatural mistake
of building on the plan on which the hull of an ordinary ship is
constructed; that is, making the hull support the projecting bowsprit
and other parts. In the aerodrome, what corresponds to the bowsprit
must project far in advance of the hull to sustain the front wings,
and a like piece must project behind it to sustain the rear wings and
the tail, or the supporting surfaces of whatever kind. The mistake
of the construction lay in disjoining these two and connecting them
indirectly by the insufficiently strong hull which supported them.
This hull was formed of longitudinal U-shaped ribs of thin steel,
which rested on rings made of an alloy of aluminum, which possessed
the lightness of the latter metal with very considerable toughness,
but which was finally unsatisfactory. I may say parenthetically that
in none of the subsequent constructions has the lightness of aluminum
been found to compensate for its very many disadvantages. The two
rods, which were each 1 metre in length, were with difficulty kept
rigorously in line, owing to the yielding of the constructionally
weak hull. It would have been better, in fact, to have carried the
rod straight through at any inconvenience to the disposition of the
boilers and the engine.
[Illustration: PL. 10
STEEL FRAMES OF AERODROMES NOS. 0, 1, 2, 3, 1891 AND 1892]
I may add that the sustaining surfaces, which were to be nearly flat
wings, composed of silk stretched from a steel tube with wooden
attachments, were to [p033] have been carried on the front rod,
but, as subsequent experience has shown, these wings would have been
inadequate to the work, both from their insufficient size and their
lack of rigidity.
The propellers, which were to be 80 cm. in diameter, 1.25
pitch-ratio, and which were expected to make from five to six hundred
revolutions a minute, were carried on the end of long tubular shafts,
not parallel, but making with each other an angle of 25 degrees, and
united by gears near the bow of the vessel in the manner shown in
Plate 10.
The first engines were of the oscillating type, with the piston-rod
connected directly to the crank; were very light, and were unprovided
with many of the usual fittings belonging to a steam engine, such as
rod or piston packing; and their construction was crude in comparison
with their successors. They were tested with the Prony brake and
found to be deficient in power, for with a steam pressure of 80
pounds to the square inch, they ran at the rate of 1170 revolutions
per minute, and developed only .363 horse-power. It soon became
evident that they were too light for the work that it was intended
that they should do, and steps were taken, even before the completion
of these tests, for the construction of a pair of more powerful
cylinders, which should also be provided with a special boiler for
the generation of the steam. Acting upon the supposition, in a saving
of steam, it was decided to work with compounded cylinders. As two
propellers were to be used, they were each fitted with a distinct
pair of cylinders working directly upon the shaft, but so connected
by gearing that they were compelled to turn at the same rate of speed.
The cylinders were of the inverted oscillating type, like the first
pair of engines, but, unlike them, they were single-acting. The
dimensions were: diameter of high-pressure cylinder 1.25 inches; low
pressure, 1.94 inches, with a common stroke of 2 inches, and with
cranks set opposite to each other so that one cylinder was always at
work. The cylinders were held at their upper ends by a strap passing
around a hollow conical trunk, which served the double purpose of
a support for the cylinders and an intermediate receiver between
them. This receiver had a mean inside diameter of 1.25 inches, with
a length of 4.75 inches, so that it had about twice the cubical
capacity of the high-pressure cylinder, while the displacement of
the low-pressure cylinder was about 2.5 times that of the high;
ratios that would have given satisfactory results, perhaps, had
the steam pressure and other conditions been favorable to the use
of the compound principle in this place. There were no valves for
the admission of the steam, for, inasmuch as the engines were
single-acting, it was possible to make ports in the cylinder-head
act as the admission and exhaust ports as the cylinder oscillated,
and thus avoid the complication and weight of eccentric and valves.
[p034]
These cylinders were set in a light frame at an angle of 25° with
each other, or 12.5° with the median line of the aerodrome, and drove
the long propeller shafts as shown in Plate 10, No. 0. At the extreme
forward end of the crank-shafts there was a pair of intermeshing
bevel gears which served to maintain the rate of revolution of the
two propellers the same.
[Illustration: FIG. 3. Boilers in use in 1891–1892.]
The boiler built for this work was a beehive-shaped arrangement
of coils of pipe. It consisted at first, as shown in Fig. 3, of
three double coils of 3/8-inch copper pipe coiled up in the shape
of a truncated cone, carrying in the central portion a pear-shaped
receiver into the upper portion of which the water circulating
through the coils discharged. Each of these receivers was connected
at the top with the bottom of a long cylindrical drum, with
hemispherical ends, which formed a steam space from which supply for
the engines was drawn. The lower ends of the coils were connected
with an injection pipe supplying the water. Each “beehive” had 23
turns of tubing, and had a base of 7.5 inches and a top diameter of
6 inches, the steam drum being 2.5 inches in diameter. I may here
say that in the selection of the general type of boiler for the work
to be done, there was never any hesitation regarding the use of
the water-tube variety. Their superiority for the quick generation
of large volumes of steam had been so pronounced that nothing else
seemed capable of competing with [p035] them in this respect,
regardless of the absolute economy of fuel that might or might not be
exhibited. Hence, to the end of my experiments nothing else was used.
Even before the “beehive” boiler was completed, I was anxious to
ascertain what could be done with a coil of pipe with a stream of
water circulating through it, as well as with various forms of
burners, for I realized that the success of the apparatus depended
not only upon getting an exceedingly effective heating surface, but
also an equally effective flame to do the heating.
For fuel I naturally turned to the liquids as being more compact
and readily regulated. Whether to use some of the more volatile
hydrocarbons or alcohol, was still an unsolved problem, but my
opinion at the time was that, on the limited scale of the model,
better results could probably be obtained with alcohol.
In the experiments made with a coil preliminary to the trial of the
“beehive” boiler, I tried a simple horizontal coil of 3/8-inch copper
pipe into which two forked burners working on the Bunsen principle
and using city illuminating gas, were thrust. The jets were about 1/2
inch apart. The arrangement primed so badly that the engines could
not get rid of the entrained water, and would only make a few turns.
I then tried the same coil with two 1.25-inch drums in the inside
and with five longitudinal water tubes at the bottom, beneath which
were the same two forked burners used in the previous experiment. The
coils were covered with a sheet of asbestos, and two round burners
were added. This boiler would hold a steam pressure of about 15
pounds and run the engine slowly; but if the pressure were allowed
to rise to 60 pounds, the engine would drive a 2-foot propeller of
18-inch pitch at the rate of about 650 turns per minute for from 80
to 90 seconds, while the steam ran down to 10 pounds, showing that
this boiler, at least, was too small. This was further shown in a
trial of the plain coil made in October, 1891; 6 pounds of water were
evaporated in 32 minutes under a pressure of 60 pounds. This was at
the rate of 11.25 pounds per hour, or, taking the U. S. Centennial
standard of 30 pounds of evaporation per horse-power, gave an
available output of less than 1/3 horse-power.
With these results before me, I decided to make a trial of the
“beehive” principle upon a smaller scale than in the boiler designed
for Aerodrome No. 0. I used a small boiler of which the inner coil
consisted of 8 turns of 3/8-inch copper tube about 28 gauge thick,
and the outer coil of 11 turns of 1/4-inch copper pipe. This gave 12
feet of 3/8-inch, and 16 feet of 1/4-inch tubing. The drum was of No.
27 gauge, hard planished copper. With this boiler consuming 6 oz. of
fuel, 80.3 oz. of water were evaporated in 28 minutes, or at the rate
of about 10.75 pounds per hour. As these coils contained but 2.22
square feet of heating surface, and as the three to be built would
contain 3.7 square feet each, it was estimated the [p036] 10 square
feet afforded by them could safely be depended upon to provide steam
for a 1 horse-power engine. As far as fuel consumption was concerned,
the rate of evaporation was about 15.6 pounds of water per pound of
gasoline, all of which was satisfactory.
The burner originally designed for use in connection with the
“beehive” boilers, consisted of a small tank in which a quantity of
gasoline was placed, the space above being filled with compressed
air. Rising from the bottom of this tank was a small pipe coiling
back and down and ending in an upturned jet from which the gas
generated in the coil would issue. The burner thus served to generate
its own gas and act as a heater for the boilers at the same time.
In the construction of Aerodrome No. 0, four of the “beehive” coils
were placed in a line fore and aft. The fuel tank was located
immediately back of the rear coil and consisted of a copper
cylinder 11 cm. in diameter and 9 cm. long. The engines were placed
immediately in front of the coils, all the apparatus being enclosed
in a light framing, as shown in the photograph (Plate 10).
Extending front and back from the hull were the tubes for supporting
the wings and tail, each one metre in length. The cross-framing for
carrying the propeller shafts was built of tubing 1.5 cm. diameter,
and the shafts themselves were of the same size. The ribs of the hull
were rings made of angle-irons measuring 1.50×1.75 cm., which were
held in place longitudinally by five 0.7 cm. channel bars.
As it had been learned in the preliminary experiments with the model
“beehive” boiler that the heated water would not of itself cause a
sufficiently rapid circulation to be maintained through the tubes to
prevent them from becoming red-hot, two circulating pumps were added
for forcing the water through the coils of the two forward and two
rear boilers respectively, the water being taken from the lower side
of the drum and delivered into the bottom of the coils, which were
united at that point for the purpose. A worm was placed upon each of
the propeller shafts, just back of the engines, meshing in with a
gear on a crank-shaft from which the pumps were driven. This shaft
rotated at the rate of 1 to 24, so that for 1200 revolutions of the
engine, it would make but 50, driving a single-acting plunger 1.2 cm.
in diameter and 2 cm. stroke.
Apparently all was going well until I began to try the apparatus.
First, there was a difficulty with the burner, which could not be
made to give forth the relative amount of heat that had been obtained
from the smaller model, and steam could not be maintained. With one
“beehive” connected with the compound engine, and a 70 cm. propeller
on the shaft, there were about 250 turns per minute for a space
of about 50 seconds, in which time the steam would fall from 90
pounds to 25 pounds, and the engine would stop. Then, as we had no
air-chamber on the pumps at the time, they would not drive the water
through the coils. Subsequent experiments, however, showed that the
boilers could be [p037] depended upon to supply the steam that the
compound engines would require; but after the whole was completed,
the weight, if nothing else, was prohibitory.
I had gone on from one thing to another, adding a little here and a
little there, strengthening this part and that, until when the hull
was finally completed with the engines and boilers in place, ready
for the application of the wings, the weight of the whole was found
(allowing 7 pounds for the weight of the wings and tail) to be almost
exactly 45 pounds, and nearly 52 pounds with fuel and water. To this
excessive weight would have to be added that of the propellers, and
as the wings would necessarily have to be made very large in order to
carry the machine, and as the difficulties of launching had still to
be met, nothing was attempted in the way of field trials, and with
great disappointment the decision was made in May, 1892 (wisely, as
it subsequently appeared) to proceed no further with this special
apparatus.
However, inasmuch as this aerodrome with its engines and boilers had
been completed at considerable expense, it was decided to use the
apparatus as far as it might be practicable, in order to learn what
must be done to secure a greater amount of success in the future. The
fundamental trouble was to get ‹heat›. In the first place there was
trouble with the burners, for it seemed to be impossible to get one
that would vaporize the gasoline in sufficient quantity to do the
work, and various forms were successively tried.
All of the early part of 1892 was passed in trying to get the boilers
to work at a steam pressure of 100 pounds per square inch. On account
of the defects in the tubes and elsewhere this required much patient
labor. The writer, even thus early, devised a plan of using a sort
of aeolipile, which should actuate its own blast, but this had to
be abandoned on account of the fact that the pear-shaped receivers
would not stand the heat. This necessitated a number of experiments
in the distillation of gas, in the course of which there was trouble
with the pumps, and a continual series of breakages and leakages,
so that the middle of April came before I had secured any further
satisfaction than to demonstrate that ‹possibly› the boilers might
have a capacity sufficient for the work laid out for them to do; but
subsequent experiments showed that even in this I was mistaken, for
it was only after additional jets had been put in between the coils
that I succeeded in getting an effective horse-power of 0.43 out of
the combination.
Finally, on the 14th of April, after having reduced the capacity
of the pumps to the dimensions given above (for the stroke was
originally 1.25 inch) I obtained the development of 1 full
horse-power by the engine for 41 seconds, with a steam pressure of
100 pounds per square inch, and a rate of revolution of 720 per
minute. But at the end of this brief period, the shafts sprung and
the worm was thrown out of gear. [p038]
I pass over numerous other experiments, for their only result was to
make it clear that the aerodrome, as it had been constructed, could
not be made to work efficiently, even if its great weight had not
served as a bar to its flight. It was, therefore, decided to proceed
with the construction of another.
After the failure of the first steam-driven model No. 0, which has
just been described, subsequent light models were constructed. These,
three in number, made with a view to the employment of carbonic acid
or compressed air, but also to the possible use of steam, are shown
in Plate 10, Nos. 1, 2, 3; on the same scale as the larger model
which had preceded them. In describing these, it will be well to
mention constructive features which were experimented on in them, as
well as to describe the engines used.
In No. 1, which was intended to be on about 2/5 the linear scale
of No. 0, the constructive fault of the latter, that of making the
support depend on a too flexible hull, was avoided, and the straight
steel tube (“midrod” it will hereafter be called) was carried
through from end to end, though at the cost of inconvenience in the
placing of the machinery, in what may be called the hull, which
now became simply a protective case built around this midrod. The
mistaken device of the long shafts meeting at an angle, was, however,
retained, and the engines first tried were a pair of very light ones
of crude construction.
These were later replaced by a pair of oscillating engines, each 3
cm. diameter by 3 cm. stroke, with a combined capacity of 42 cubic
cm. and without cut-off. The midrod was made of light steel tubing
2 cm. outside diameter. The framing for the hull was formed by a
single ring of U section, 8 cm. across and 18 cm. in depth, stayed
by five ribs of wood measuring 0.7×0.3 cm. The inclined propeller
shafts, which were connected by a pair of bevel gears as in No. 0,
were made of tubing 0.5 cm. outside diameter, and were intended
to turn propellers of from 40 to 45 cm. in diameter. The weight,
without engine or reservoir for gas, was 1161 grammes. With a weight
equivalent to that of the intended reservoir and engines plus that of
the proposed supporting surfaces, the whole weight, independent of
fuel or water, was 2.2 kilogrammes.
The engines, which were not strong enough to sustain a pressure of
over 2 atmospheres, at an actual pressure of 20 pounds drove the 45
cm. propellers through the long V shafts and lifted only about 1/7 of
the flying weight of the machine. The power developed at the Prony
brake was collectively only about .04 horse-power, giving 1200 turns
a minute to two 40 cm. propellers. This was the best result obtained.
This aerodrome was completed in June, 1892, but changes in the
engines and other attempted improvements kept it under experiment
until November of that year, when it appeared to be inexpedient to do
anything more with it.
Aerodrome No. 2 (see Plate 10), was a still smaller and still lighter
construction, in which, however, the midrod was bent (not clearly
shown in the [p039] photograph), so as to afford more room in
the hull. This introduced a constructional weakness which was not
compensated by the added convenience, but the principal improvement
was the abandonment of the inclined propeller shafts, which was done
at the suggestion of Mr. J. E. Watkins, so that the propellers were
carried on parallel shafts as in marine practice. These parallel
shafts were driven by two very small engines with cylinders 2.3 cm.
in diameter by 4 cm. stroke, with a collective capacity of 33 cu. cm.
and without cut-off, which were mounted on a cross-frame attached to
the midrod at right angles near the rear end of the hull.
These engines, driven either by steam or by carbonic-acid gas
developed 0.035 horse-power at the Prony brake, giving 750
revolutions of the 45 cm. propellers, and lifting about 1/5 of the
total weight which it was necessary to provide for in actual flight.
A higher rate of revolution and a better lift were occasionally
obtained, but there was little more hope with this than with the
preceding models of obtaining power enough to support the actual
weight in flight, although such sacrifices had been made for
lightness that every portion of the little model had been reduced to
what seemed the limit of possible frailty consistent with anything
like safety. Thus the midrod was lighter than that of No. 1, being
only 1 cm. in outside diameter. The frame was made of thin wooden
strips 5 mm.×3.5 mm., united by light steel rings. The cross framing
carrying the engines was also of wood, and was formed of four strips,
each 7 mm.×3 mm. The shafts were but 4 mm. in diameter.
As these engines did not give results that were satisfactory, when
using carbonic-acid gas, experiments were commenced to secure a
boiler that would furnish the requisite steam. As the “beehive”
boiler had proved to be too heavy, and as the steam obtained from it
had been inadequate to the requirements, something else had to be
devised. A few of the boilers used in 1892 are shown in Fig. 3. The
one marked ‹A› is one of the “beehives,” while an element of another
form tried is that marked ‹B›. It consisted of 3/8-inch copper tubes
joined to a drum of 10-oz. copper. This was made in May, 1892, and
was tested to a pressure of 50 atmospheres, when it burst without any
tearing of the metal.
In July another boiler like that shown at ‹C› in Fig. 3 was made.
This was formed of tubes 3 cm. in diameter, and weighed 348 grammes.
It carried about 300 grammes of water and stood a steam pressure of
125 pounds per square inch, but failed to maintain sufficient steam
pressure.
Accordingly, in the same month, a third boiler like that shown at
‹D› was built. It consisted of a tube 12 inches long to which were
attached fifteen 1/4-inch tubes each 7 inches long, in the manner
shown. The heating surface of this boiler, including the tubes and
the lower half of the drum, amounted to 750 square cm., and it was
thought that this would be sufficient to supply steam for a flight of
a [p040] minute and a half. But when a test was made, it also was
found to be deficient in steaming power even after changes were made
in it which occupied much time.
By the first of October, 1892, there had been built one large
aerodrome that could not possibly fly, a smaller one, No. 1, on 2/5
the linear scale of No. 0, with a pair of engines but no means of
driving them, and the still smaller No. 2 with a boiler that was yet
untried.
Aerodrome No. 3 (Plate 10) was an attempt to obtain better conditions
than had existed in the preceding model without any radical change
except that of moving the cross frame, which carried the engines
and propellers, nearer the front of the machine. Instead of the
oscillatory engines used up to this time, two stationary cylinder
engines, each 2.4 cm. in diameter and 4 cm. stroke, having a combined
capacity of 36 cu. cm. without cut-off were employed for driving
the propellers. The engines, though occasionally run in trials with
steam from a stationary boiler, were intended to be actuated either
by compressed air or carbonic-acid gas contained in a reservoir which
was not actually constructed, but whose weight was provisionally
estimated at 1 kilogramme. The weight of the aerodrome without this
reservoir was but 1050 grammes, including the estimated weight of the
sustaining surfaces, which consisted principally of two wings, each
about 1 metre in length by 30 cm. in breadth and which were in fact
so slight in their construction, that it is now certain that they
could not have retained their shape in actual flight.
The only trials made with this aerodrome, then, were in the shop,
of which it is sufficient to cite those of November 22, 1892, when
under a pressure of 30 pounds, the maximum which the engines would
bear, two 50 cm. propellers were driven at 900 revolutions per
minute, with an estimated horse-power of 0.07, about 35 per cent of
the weight of the whole machine being lifted. This was a much more
encouraging result than any which had preceded, and indicated that
it was possible to make an actual flight with the aerodrome if the
boilers could be ignored, the best result having been obtained only
with carbonic acid supplied without limit from a neighboring ample
reservoir.
This aerodrome was also tested while mounted upon a whirling-arm and
allowed to operate during its advance through the air. The conclusion
reached with it at the close of 1892, after a large part of the year
passed in experiments with carbonic-acid gas and compressed air,
was that it was necessary to revert to steam, and that whatever
difficulties lay in the way, some means must be found of getting
sufficient power without the weight which had proved prohibitory in
No. 0.
With this chapter, then, and with the end of the year 1892, I close
this very brief account of between one and two years of fruitless
experiment in the construction of models supplied with various
motors, subsequent to and on a larger scale indeed than the toy-like
ones of india rubber, but not even so efficient as those had been,
since they had never procured a single actual flight.
[p041]
CHAPTER V
ON SUSTAINING SURFACES
The following general considerations may conveniently precede the
particular description of the balancing of the aerodrome.
In “Experiments in Aerodynamics,” I have given the result of trials,
showing that the pressure (or total resistance) of a wind on a
surface 1 foot square, moving normally at the velocity of 1 foot per
second, is 0.00166 pounds, and that this pressure increases directly
as the surface of the plane, and (within our experimental condition)
as the square of the velocity,[20] results in general accordance with
those of earlier observers.
I have further shown by independent investigations that while the
shape of the plane is of secondary importance if its movement be
normal, the shape and “aspect” greatly affect the resultant pressure
when the plane is inclined at a small angle, and propelled by such
a force that its flight is horizontal, that is, under the actual
conditions of soaring flight.
I have given on page 60 of “Aerodynamics,” the primary equations,
‹P›_α = ‹P›_{90}‹F›(α) = ‹k›‹A›‹V›^2‹F›(α),
‹W› = ‹P›_α cos α = ‹k›‹A›‹V›^2‹F›(α) cos α,
‹R› = ‹P›_α sin α = ‹k›‹A››V›^2‹F›(α) sin α,
where ‹W› is the weight of the plane under examination (sometimes
called the “lift”); ‹R› the horizontal component of pressure
(sometimes called the “drift”); ‹k› is the constant already given;
‹A› the area in square feet; ‹V› the velocity in feet per second; ‹F›
a function of α (to be determined by experiment); α the angle which,
under these conditions, gives horizontal flight.
I have also given on page 66 of the same work the following table
showing the actual values obtained by experiment on a plane, 30×4.8
inches (= 1 sq. ft.), weighing 500 grammes (1.1 pounds):
Weight with planes
Angle Soaring speed Horizontal Work expended of like form that
with ‹V›. pressure per minute 1 horse-power will
horizon ‹R›. 60 ‹RV›. drive through the
α. air at velocity ‹V›.
------- ------------- ---------- ----------------- --------------------
Metres Feet Grammes. Kilogram- Foot- Kilo- Pounds.
per per metres. pounds. grammes.
sec. sec.
======= ======= ===== ========== ========= ======= ======== =======
45° 11.2 36.7 500 336 2,434 6.8 15
30 10.6 34.8 275 175 1,268 13.0 29
15 11.2 36.7 128 86 623 26.5 58
10 12.4 40.7 88 65 474 34.8 77
5 15.2 49.8 45 41 297 55.5 122
2 20.0 65.6 20 24 174 95.0 209
It cannot be too clearly kept in mind that these values refer to
‹horizontal› flight, and that for this the weight, the work, the
area, the angle and the velocity are inseparably connected by the
formulæ already given.
It is to be constantly remembered also, that they apply to results
obtained under almost perfect theoretical conditions as regards not
only the maintenance of equilibrium and horizontality, but also the
rigid maintenance of the angle α and the comparative absence of
friction, and that these conditions are especially “theoretical”
in their exclusion of the internal work of the wind observable in
experiments made in the open wind.
EXPERIMENTS IN THE OPEN WIND
I have pointed out[21] that an indefinite source of power for the
maintenance of mechanical flight, lies in what I have called the
“internal work” of the wind. It is easy to see that the actual effect
of the free wind, which is filled with almost infinitely numerous and
incessant changes of velocity and direction, must differ widely from
that of a uniform wind such as mathematicians and physicists have
almost invariably contemplated in their discussions.
Now the artificial wind produced by the whirling-table differs from
the real wind not only in being caused by the advancing object,
whose direction is not strictly linear, and in other comparatively
negligible particulars, but especially in this, that in spite of
little artificial currents the movement on the whole is regular and
uniform to a degree strikingly in contrast with that of the open wind
in nature.
In a note to the French edition of my work, I have called the
attention of the reader to the fact that the figures given in the
Smithsonian publication can show only a small part of the virtual
work of the wind, while the plane, which is used for simplicity of
exposition, is not the most advantageous form for flight; so that,
as I go on to state, the realization of the actually successful
aerodrome must take account of the more complex conditions actually
existing in nature, which were only alluded to in the memoir, whose
object was to bring to attention the little considered importance of
the then almost unobserved and unstudied minute fluctuations which
constitute the internal work of the wind. I added that I might later
publish some experimental investigations on the superior efficiency
of the real wind over that artificially created. The experiments
which were thus alluded to in 1893, were sufficient to indicate the
importance of the subject, but the data have not been preserved.
What immediately follows refers, it will be observed, more
particularly to the work of the whirling-table. [p043]
RELATION OF AREA TO WEIGHT AND POWER
In order to get a more precise idea of the character of the
alteration introduced into these theoretical conditions by the
variation of any of them, let us, still confining ourselves to the
use of the whirling-table, suppose that the plane in question while
possessing the same weight, shape, and angle of inclination, were to
have its area increased, and to fix our ideas, we will suppose that
it became 4 square feet instead of 1 as before. Then, from what has
already been said, ‹V›, the velocity, must vary inversely as the
square root of the area; that is, it must, under the given condition,
become one-half of what it had been, for if ‹V› did not alter, the
impelling force continuing the same, the plane would rise and its
flight no longer be horizontal, unless the weight, now supposed to be
constant, were itself increased so as to restore horizontality.
I have repeated Table XIII under the condition that the area be
quadrupled, while all the other conditions remain constant, except
the soaring speed, which must vary.
+--------+-----------+-----------------+----------------------+
| | Soaring | Work. | Weight. |
| | speed +-----------------+----------------------+
| α |(feet per | Work expended | Weight of like |
| | second) | per minute. | planes which |
| | ‹V′›. |‹A› = 4 sq. ft. | 1 H.P. will drive |
| | |‹W› = 500 gr. | through the air |
| | | = 1.1 lbs. | with velocity ‹V′›. |
+--------+-----------+-----------------+----------------------+
| | | ‹Foot-pounds.› | ‹Pounds.› |
| 45° | 18.4 | 1,217 | 30 |
| 30 | 17.4 | 634 | 57 |
| 15 | 18.4 | 312 | 116 |
| 10 | 20.4 | 237 | 154 |
| 5 | 24.9 | 148 | 244 |
| 2 | 32.8 | 87 | 418 |
+--------+-----------+-----------------+----------------------+
‹W› is the weight of the single plane; ‹A› is the area; ‹R› is the
horizontal “drift.” ‹Wt› is the weight of like planes which 1 H. P.
will drive at velocity ‹V›. Work is ‹RV›.
I. If Work is constant, ‹R› varies as ∛‹A›. II. If ‹R› is constant,
Work varies as 1/(∛‹A›). III. If ‹W› is constant while ‹A› varies,
the weight which 1 H. P. will support varies as √‹A›.
The reader is reminded that these are simply deductions from the
equations given in “Aerodynamics,” and that these deductions have
not been verified by direct trial, such as would show that no new
conditions have in fact been introduced in this new application.
While, however, these deductions cannot convey any confidence
beyond what is warranted by the original experiments, in their
general trustworthiness as working formulæ at this stage of the
investigations, we may, I think, feel confidence.
I may, in view of its importance, repeat my remark that the relation
of area and weight which obtain in practice, will depend upon yet
other than these theoretical considerations, for, as the flight
of the free aerodrome cannot be expected to be exactly horizontal
nor maintained at any constant small angle, the [p044] data of
“Aerodynamics” (obtained in constrained horizontal flight with the
whirling-table) are here insufficient. They are insufficient also
because these values are obtained with small rigid planes, while the
surfaces we are now to use cannot be made rigid under the necessary
requirements of weight, without the use of guy wires and other
adjuncts which introduce head resistance.
Against all these unfavorable conditions we have the favoring one
that, other things being equal, somewhat more efficiency can be
obtained with suitable curved surfaces than with planes.[22]
I have made numerous experiments with curves of various forms upon
the whirling-table, and constructed many such supporting surfaces,
some of which have been tested in actual flight. It might be expected
that fuller results from these experiments should be given than those
now presented here, but I am not yet prepared to offer any more
detailed evidence at present for the performance of curved surfaces
than will be found in Part III.[23] I do not question that curves are
in some degree more efficient, but the extreme increase of efficiency
in curves over planes understood to be asserted by Lilienthal and by
Wellner, appears to have been associated either with some imperfect
enunciation of conditions which gave little more than an apparent
advantage, or with conditions nearly impossible for us to obtain in
actual flight.
All these circumstances considered, we may anticipate that the
power required (or the proportion of supporting area to weight)
will be very much greater in actual than in theoretical (that is,
in constrained horizontal) flight, and the early experiments with
rubber-driven models were in fact successful only when there were
from three to four feet of sustaining surface to a pound of weight.
When such a relatively large area is sought in a large aerodrome,
the construction of light, yet rigid, supporting surfaces becomes
a nearly insuperable difficulty, and this must be remembered as
consequently affecting the question of the construction of boiler,
engines and hulls, whose weight cannot be increased without
increasing the wing area.
[p045]
CHAPTER VI
BALANCING THE AERODROME
By “balancing” I mean such an adjustment of the mean center of
pressure of the supporting surfaces with reference to the center
of gravity and to the line of thrust, that for a given speed the
aerodrome will be in equilibrium, and will maintain steady horizontal
flight. “Balance” and “equilibrium” as here used are nearly
convertible terms.
LATERAL STABILITY
Equilibrium may be considered with reference to lateral or
longitudinal stability. The lateral part is approximately secured
with comparative ease, by imitating Nature’s plan, and setting the
wings at a diedral angle, which I have usually made 150°. Stability
in this sense cannot be secured in what at first seems an obvious
way—by putting a considerable weight in the central plane and
far below the center of gravity of the aerodrome proper, for this
introduces rolling. Thence ensues the necessity of carrying the
center of gravity more nearly up to the center of pressure than
would otherwise be necessary, and so far introducing conditions
which tend to instability, but which seem to be imposed upon us by
the circumstances of actual flight. With these brief considerations
concerning lateral stability, I pass on to the far more difficult
subject of longitudinal stability.
LONGITUDINAL STABILITY
My most primitive observation with small gliding models was of the
fact that greater stability was obtained with two pairs of wings,
one behind the other, than with one pair (greater, that is, in the
absence of any instinctive power of adjustment).
This is connected with the fact that the upward pressure of the air
upon both pairs may be resolved into a single point which I will call
the “center of pressure,” and which, in stable flight, should (apart
from the disturbance by the propeller thrust) be over the center
of gravity. The center of pressure in an advancing inclined plane
in soaring flight is, as I have shown in “Aerodynamics,” and as is
otherwise well known, always in advance of the center of figure, and
moves forward as the angle of inclination of the sustaining surfaces
diminishes, and, to a less extent, as horizontal flight increases
in velocity. These facts furnish the elementary ideas necessary in
discussing this problem of equilibrium, whose solution is of the most
vital importance in successful flight. [p046]
The solution would be comparatively simple if the position of the
‹CP› could be accurately known beforehand, but how difficult the
solution is may be realized from a consideration of one of the facts
just stated, namely, that the position of the center of pressure in
horizontal flight shifts with the velocity of the flight itself,
much as though in marine navigation the trim of a steamboat’s hull
were to be completely altered at every change of speed. It may be
remarked here that the center of pressure, from the symmetry of the
aerodrome, necessarily lies in the vertical medial plane, but it may
be considered with reference to its position either in the plane
‹XY› (‹cp›_1) or in the plane ‹YZ› (‹cp›_2). The latter center of
pressure, as referred to in the plane ‹YZ›, is here approximately
calculated on the assumption that it lies in the intersection of this
vertical plane by a horizontal one passing through the wings half way
from root to tip.
Experiments made in Washington, later than those given in
“Aerodynamics,” show that the center of pressure, (‹cp›_1) on a plane
at slight angles of inclination, may be at least as far forward as
one-sixth the width from the front edge. From these later experiments
it appears probable also that the center of pressure moves forward
for an increased speed even when there has been no perceptible
diminution of the angle of the plane with the horizon, but these
considerations are of little value as applied to curved wings such
as are here used. Some observations of a very general nature may,
however, be made with regard to the position of the wings and tail.
In the case where there are two pairs of wings, one following the
other, the rear pair is less efficient in an indefinite degree than
the front, but the action of the wings is greatly modified by their
position with reference to the propellers, and from so many other
causes, that, as a result of a great deal of experiment, it seems
almost impossible at this time to lay down any absolute rule with
regard to the center of pressure of any pair of curved wings used in
practice.
Later experiments conducted under my direction by Mr. E. C. Huffaker,
some of which will appear in Part III, indicate that upon the curved
surfaces I employed, the center of pressure moves forward with an
increase in the (small) angle of elevation, and backward with a
decrease, so that it may lie even behind the center of the surface.
Since for some surfaces the center of pressure moves backward, and
for others forward, it would seem that there might be some other
surface for which it will be fixed. Such a surface in fact appears to
exist in the wing of the soaring bird. These experiments have been
chiefly with rigid surfaces, and though some have been made with
elastic rear surfaces, these have not been carried far enough to give
positive results.
The curved wings used on the aerodromes in late years have a rise of
one in twelve, or in some cases of one in eighteen,[24] and for these
latter the following empirical local rule has been adopted: [p047]
The center of pressure on each wing with a horizontal motion of 2000
feet per minute, is two-fifths of the distance from front to rear.
Where there are two pairs of wings of equal size, one following the
other, and placed at such a distance apart and with such a relation
to the propellers as here used, the following wing is assumed to
have two-thirds of the efficiency of the leader per unit of surface.
If it is half the size of the leader, the efficiency is assumed to
be one-half per unit of surface. If it is half as large again as
the leader, its efficiency is assumed to be eight-tenths per unit
of surface. For intermediate sizes of following wing, intermediate
values of the efficiency may be assumed.
These rules are purely empirical and only approximate. As
approximations, they are useful in giving a preliminary balance, but
the exact position of the center of pressure is rarely determinable
in either the horizontal or vertical plane, except by experiment in
actual flight. The position of the center of gravity is found with
all needed precision by suspending the aerodrome by a plumb-line in
two positions, and noting the point of intersection of the traces of
the line, and this method is so superior to that by calculation, that
it will probably continue in use even for much larger constructions
than the present.
The principal factor in the adjustment is the position of the wings
with reference to the center of gravity, but the aerodrome is moved
forward by the thrust of its propellers, and we must next recall the
fact of experiment that as it is for constructional reasons difficult
to bring the thrust line in the plane of the center of pressure of
the wings, it is in practice sufficiently below them to tend to tip
the front of the aerodrome upward, so that it may be that equilibrium
will be attained only when ‹CP›_1 is ‹not› over ‹CG›_1.
In the discussion of the equilibrium, then, we must consider also the
effect of thrust, and usually assume that this thrust-line is at some
appreciable distance below the center of pressure.
We may conveniently consider two cases:
1. That the center of pressure is not directly over the center of
gravity; that is, ‹CG›_1−‹CP›_1 = ‹a›, and estimate what the value of
a should be in order that, during horizontal flight, the aerodrome
itself shall be horizontal; or, [p048]
2. Consider that the center of pressure is directly over the center
of gravity (‹CP›_1−‹CG›_1 = 0), and in this case inquire what angle
the aerodrome itself may take during horizontal flight.
First case. The diagram (Fig. 4) represents the resultants of
the separate system of forces acting on the aerodrome, and these
resultants will lie in a vertical medial plane from the symmetry of
their disposition.
Let ‹af› represent the resultant of the vertical components of the
pressure on the wings; the horizontal component will lie in the line
‹ae›.
[Illustration: FIG. 4. Diagram showing relation under certain
conditions of thrust, C. P. and C. G.]
Let the center of gravity be in the line ‹bd›, and the resultant
thrust of the propellers be represented by ‹cd›.
Let ‹W› = weight of aerodrome.
Let ‹T› = thrust of propellers.
Then if we neglect the horizontal hull resistance, which is small
in comparison with the weight, equilibrium obtains when ‹W›×‹ab› =
‹T›×‹bd›.
Second case. The diagram (Fig. 5) represents the same system of
forces as Fig. 4, but in this case the point of support is directly
over the center of gravity ‹g›, when the axis of the aerodrome is
horizontal.
Let ‹W› = weight of aerodrome.
Let ‹T› = thrust of propellers.
Let ‹R› = distance of ‹CG›_2 below ‹CP›_2 = ‹ag›.
Let ‹S› = distance of thrust-line below ‹CP›_2 = ‹ad›.
If now the aerodrome under the action of the propellers be supposed
to turn about the ‹CP›_2 (or, ‹a›) through an angle α, so that ‹g›
takes the position ‹g′›, we [p049] obtain by the decomposition of
the force of gravity an element ‹g′k› = ‹W› sin α which acts in a
direction parallel to the thrust-line.
If we again neglect the horizontal hull resistance, equilibrium will
be obtained when
‹kg′›×‹ag′› = ‹T›×‹ad′›
or ‹WR› sin α = ‹TS›
∴ α = sin^{−1}×(TS/WR)
[Illustration: Fig. 5. Diagram showing relation under certain
conditions of thrust, C. P. and C. G.]
The practical application of these rules is greatly limited by the
uncertainty that attaches to the actual position of the center of
pressure, and this fact and also the numerical values involved may be
illustrated by examples.
CONDITION OF AERODROME NO. 6, NOVEMBER 28, 1896
The weight was 12.5 kilos. On November 28, the steam pressure was
less than 100 pounds, and the thrust may be taken at 4.5 kilos. The
distance ‹bd› was 25 cm.
Hence 12.5׋ab› = 4.5x25 cm.
‹ab› = 9 cm.
This appears to give the position of ‹CP›_1, but ‹CP›_1 is a
resultant of the pressure on both wings, and its position is
determined by the empirical rule just cited. We [p050] cannot
tell in fact, then, with exactness how to adjust the wings so that
‹CG›_1−‹CP›_1 may be 9 cm., and equilibrium was in fact obtained in
flight when (the empirically determined) ‹CG›_1−‹CP›_1 = 3 cm.
Again, let it be supposed that ‹CP›_1 was really over ‹CG›_1 . . . .
The distance of the center of gravity below the center of pressure is
43 cm. = ‹R›.
Then α = sin^{−1} {(4.5×25)/(12.5×43)} = 12° nearly.
The doubt as to the actual position of the resultant center of
pressure, then, renders the application of the rule uncertain. In
practice, we are compelled (unfortunately) after first calculating
the balance, by such rules as the above, and after it has been thus
found with approximate correctness, to try a preliminary flight.
Having witnessed the actual conditions of flight, we must then
readjust the position of the wings with reference to the center of
gravity, arbitrarily, within the range which is necessary. This
readjustment should be small.
[Illustration: FIG. 6. Diagram showing effect of Pénaud tail.]
In the preceding discussion it has been assumed that, if there is a
flat tail or horizontal rudder, it supports no portion of the weight.
This is not an indispensable condition but it is very convenient,
and we shall assume it. In this case the action of the so-called
Pénaud rudder becomes easily intelligible. This is a device, already
referred to in Chapter II, made by Alphonse Pénaud for the automatic
regulation of horizontal flight, and it is as beautiful as it is
simple.
Let ‹AB› (Fig. 6) be a schematic representation of an aerodrome whose
supporting surface is ‹Bb›, and let it be inclined to the horizon at
such an angle α that its course at a given speed may be horizontal.
So far it does not appear that, if the aerodrome be disturbed from
this horizontal course, there is any self-regulating power which
could restore it to its original course; but now let there be added
a flat tail ‹AC› set at an angle −α with the wing. This tail serves
simply for direction, and not for the support of the aerodrome,
which, as already stated, is balanced so that the ‹CG› comes under
the ‹CP› of the wing ‹Bb›.
It will be seen on a simple inspection that the tail under the given
conditions is horizontal, and that, presenting its edge to the wind
of advance, it offers no resistance to it, so that if the front rises
and the angle α increases, the wind will strike on the under side of
the tail and thereby tend to raise the rear and depress [p051] the
front again. If the angle α diminish, so that the front drops, the
wind will strike the upper surface of the tail, and equally restore
the angle α to the amount which is requisite to give horizontal
flight. If the angle α is not chosen originally with reference to the
speed so as to give horizontal flight, the device will still tend to
continue the flight in the straight line which the conditions impose,
whether that be horizontal or not.
From this description of its action, it will be seen that the Pénaud
tail has the disadvantage of giving an undulatory flight, if the tail
is made rigid. This objection, however, can be easily overcome by
giving to it a certain amount of elasticity. It does not appear that
Pénaud gave much attention to this feature, but stress is laid upon
it in the article “Flight,” in the ninth edition of the Encyclopædia
Britannica, and I have introduced a simple device for securing it.
The complete success of the device implies a strictly uniform
velocity and other conditions which cannot well he fulfilled in
practice. Nevertheless, it is as efficient a contrivance for its
object as has yet been obtained.
More elaborate devices have been proposed, and a number of them,
depending for their efficiency upon the action of a variety of
forces, have been constructed by the writer, one of which will be
described later. This has the advantage that it tends to secure
absolutely horizontal flight, but it is much inferior in simplicity
to the Pénaud tail.
Apart from considerations about the thrust, the ‹CP› is in practice
always almost directly over the ‹CG›, and this relationship is,
according to what has been suggested, obtained by moving the
supporting surfaces relatively to the ‹CG›, or ‹vice versa›,
remembering, however, that, as these surfaces have weight, any
movement of them alters the ‹CG› of the whole, so that successive
readjustments may be needed. The adjustment is further complicated
by another important consideration, namely, that those parts which
‹change› their weight during flight (like the water and the fuel)
must be kept very near the ‹CG›. As the water and fuel tanks are
fixed, it appears, then, that the center of gravity of the whole is
practically fixed also, and this consideration makes the adjustment a
much more difficult problem than it would be otherwise.[25]
[Illustration: PL. 11
STEEL FRAMES OF AERODROMES NOS. 4, 5, 6. 1893, 1895 AND 1896]
[p053]
CHAPTER VII
HISTORY OF CONSTRUCTION OF FRAME AND ENGINES OF AERODROMES
During the years 1892 and 1893, it will be recalled, four aerodromes,
known as Nos. 0, 1, 2, and 3, had been built, which were of two
general types of construction. First, that represented by No. 0, in
which a radically weak hull was made to support rods at the front and
rear, to which the wings and tail were attached. This aerodrome was
abandoned on account of the inability to provide it with sufficient
power, as well as because of its constructional weakness. Second,
that type represented by Nos. 1, 2, and 3, in which a midrod was
carried through from front to rear, around which the hull supporting
the machinery was built. These models were much lighter than No. 0,
but were all abandoned because it was found impossible to propel even
the lightest of them. While all these machines were in the strictest
sense failures, inasmuch as none of them was ever equipped with
supporting surfaces, yet the experience gained in the construction
of them was of the very greatest value in determining the points at
which strength was needed, and in indicating the mode of construction
by which strength and rigidity could be obtained.[26]
1893
Another aerodrome, known as No. 4 (shown in Plate 11), was designed
in the latter part of 1892, and by the end of March, 1893, its
construction was well under way. It was of the second type, in that
the midrod was continuous, but it differed from the preceding forms
in having the machinery (boilers, burners, and tanks) attached
directly to the midrod, the hull now taking the form of a mere
protective sheathing. As in Nos. 2 and 3, two engines were used,
which were mounted on a cross-frame of light tubing attached to the
midrod at right angles. It had, as at first constructed, no provision
for the generation of steam, but only for carrying a reservoir of
carbonic acid to supply gas for the engines.
The whole, including wings, tail, and engines, but without the
carbonic acid reservoir, weighed 1898 grammes (4.18 lbs.). A
cylindrical reservoir, weighing 521 grammes (1.14 lbs.) and capable
of holding 1506 cu. cm. (92 cu. in.) was constructed for this
purpose, and tested for 30 minutes with a pressure of 100 [p054]
atmospheres. If the weight of the cylinder, with its contents and
adjuncts, be taken as 800 grammes (1.76 lbs.), the total weight
of the aerodrome was 2698 grammes (5.95 lbs.). The wings were
plane surfaces of silk, stretched over a very light frame, with no
intermediate ribs to prevent the wing from being completely distorted
by the upward pressure of the air. Even if they had been sufficiently
strong and stiff, the total surface of both wings and tail was but
2601 sq. cm. (2.8 sq. ft.) or approximately 0.5 sq. ft. of supporting
surface to the pound, much less than was found adequate, even under
the most favorable circumstances. The weight was much more than had
been contemplated when the wings were designed, yet, if all the other
features of the aerodrome had been satisfactory, and sufficient
power had been secured, the work of providing suitable supporting
surfaces would have been attempted. But as it was found that the
engines when supplied with carbonic-acid gas were unable to develop
anything like the power necessary to propel the aerodrome, and that
the construction could be greatly improved in many other ways, this
aerodrome was entirely rebuilt. The work of the engines with carbonic
acid had been so completely unsatisfactory that the idea was entirely
abandoned, and no further attempts to develop an efficient motor
other than steam were made.
It now became realized more completely than ever before that the
primary requisite was to secure sufficient power, and that this could
be obtained only by the use of steam. This involved a number of
problems, all of which would have to be solved before any hope of a
successful machine could be entertained. In the first place, engines
of sufficient power and strength, but of the lightest possible
construction, must be built. Second, a boiler must be constructed of
the least possible weight, which would develop quickly and maintain
steadily steam at a high enough pressure to drive the engines. This
demanded some form of heating apparatus, which could work under
the adverse condition of enclosure in a narrow hull, and steadily
supply enough heat to develop the relatively large quantity of steam
required by the engines.
The first of these problems, that of procuring suitable engines, was
at least temporarily solved by the construction of two engines with
brass cylinders, which had a diameter of 2.4 cm. (0.95 in.), and a
piston stroke of 5 cm. (1.97 in.). The valve was a simple slide-valve
of the piston type, arranged to cut off steam at one-half stroke. No
packing was used for the piston or the valve, which were turned to
an accurate fit to the cylinder and the steam-chest respectively.
In the engines built up to this time, the parts had frequently been
soldered together, and a great deal of trouble and delay had arisen
from this cause. In these new engines, however, as strong and careful
a construction was made as was possible within the very narrow
limits of weight, with the result that the engines, though by no
means as efficient as those constructed later, were used in all the
experiments of 1893 and also during the first part of 1894. [p055]
As soon as these engines were completed, in February, 1893, a test
was made of one of the cylinders, steam being supplied from the
boiler of the shop-engine. The experiments were made with the Prony
brake, and showed that at a speed of 1000 revolutions per minute, the
power developed from a single cylinder was 0.208 H. P., with a mean
effective pressure in the cylinder of only about 21 pounds per square
inch of piston area, allowing a loss of 25 per cent for the internal
resistance of the engine. This pressure was so much less than should
have been obtained with the steam pressure used, that it now seems
evident that the steam passages and ports were too small to admit and
exhaust the steam with sufficient rapidity to do the work with the
same efficiency that is obtained in common practice. This, however,
was not immediately recognized. The piston speed at 1000 R. P. M.
was 328 feet per minute, at which speed the steam at a pressure of
80 pounds should have been able to follow up the piston and maintain
almost, if not quite, full boiler pressure to the point of cut-off,
but it did not do so.
The problem of generating steam was much more difficult and required
a long and tedious series of experiments, which consumed the greater
part of the year before any considerable degree of success had
been attained. In the course of these experiments many unexpected
difficulties were encountered, which necessitated the construction
of special forms of apparatus, which will be described at the
proper point. Numerous features of construction, which seemed to
be of value when first conceived, but which proved useless when
rigorously tested, will be noted here, whenever a knowledge of their
valuelessness may seem to be of advantage to the reader.
The boiler was necessarily developed simultaneously with the
development of the heating apparatus, and in the following pages, as
far as possible, they will be treated together; but often for the
sake of clearness and to avoid repetition, separate treatment will be
necessary.
At the beginning of these experiments, there was much doubt as to
whether alcohol or gasoline would be found most suitable for the
immediate purpose. An alcohol burner had been used in connection with
the earliest aerodrome, No. 0, but from the results obtained with it
at that time, there seemed to be little reason to hope for success
with it. It is to be premised that the problem, which at first seemed
insoluble, was no less than to produce steam for something like 1
H. P. by a fire-grate, which should occupy only a few cubic inches
(about the size of a clenched hand) and weigh but a few ounces.
It had to be attacked, however, and as alcohol offered the great
advantage of high calorific properties with freedom from all danger
of explosion, it was at first used.
Early in 1893, it occurred to me to modify the burner so as to make
it essentially an aeolipile, and in April of that year the first
experimental aeolipile model shown at ‹A› (Plate 12) was made. It was
very small and intended for the [p056] demonstration of a principle
rather than for actual service, but the construction of this small
aeolipile was an epoch in the history of the aerodrome. It furnished
immensely more heat than anything that had preceded it, and weighed
so little and worked so well that in May the aeolipile marked ‹B› was
made. In this design two pipes were led from the upper portion of
the cylinder, one to a large Bunsen burner which heated the boiler,
the other to a small burner placed under the tank to vaporize the
alcohol. This was followed by the one shown at ‹C›, wherein the
heating burner was smaller and the gas pipe, leading to the main
burner, larger.
Figures ‹D›, ‹E›, ‹F›, and ‹G› (Plate 12) were really continuations
and improvements of the same idea. In ‹C› there was simply a tube or
flue through the tank; in ‹F›, however, this tube discharged into a
smoke-stack fastened to the end of the cylinder, while in ‹G› the
flue turned upward within the tank itself and discharged into the
short stack on top. The object of these changes was to increase the
draft and heating power of the small flame, so that the gas would be
more rapidly generated and a greater quantity be thus made available
for use under the boiler in a unit of time. They were, however,
though improvements in a construction which was itself a great
advance, still inadequate to give out a sufficient amount of heat to
meet the excessive demands of the required quantity of steam. The
boilers in connection with which these aeolipiles were used must now
be considered.
The first boiler ‹E› (Plate 13) made during this year was a
double-coil boiler of the Serpollet type, formed of 19 feet of copper
tubing having an internal diameter of about 1/8 inch. Attached to the
boiler was a small vertical drum, from the top of which steam was
led to the engine, a pipe from the bottom leading to the pump. This
boiler was tested in April with an alcohol heater, the pump in this
trial being worked by hand. This apparatus developed a steam pressure
varying from 25 to 75 pounds, which caused the engines to drive a 60
cm. propeller of 1.25 pitch-ratio 565 revolutions per minute. The
greatest difficulty was experienced in securing a sufficient and
uniform circulation in the boiler coils. The action in the present
case was extremely irregular, as the pressure sometimes rose to 150
pounds, driving the engines at a dangerous speed and bending the
eccentric rod, while at other times it would fall so low that the
engines stopped completely.
As the pump used in this trial had proved so unsatisfactory and
unreliable, it was replaced by a reservoir of water having an
air-chamber charged to 10 atmospheres, the flow from which could
apparently be regulated with the greatest nicety by a needle valve
at the point of egress; but for some reason its performance was
unsatisfactory and remained so after weeks of experiment.
[Illustration: PL. 12
BURNERS, AEOLIPILES, AND SEPARATORS]
[Illustration: PL. 13
BOILERS OF AERODROMES]
There was used in connection with this device the double-coil boiler
shown at ‹F› (Plate 13) which was made of tubes flattened so as to
be nearly capillary. The idea of this was to obtain a larger heating
surface and a smaller volume of [p057] water, so that by proper
regulation at the needle valve, just that quantity would be delivered
which could be converted into steam in its passage through the coils,
and be ready for use in the engines as it left the boiler at the
farther extremity. The results obtained from this were an improvement
over those from the original coil, and a third set of coils (‹G›, in
Plate 13) was made. This boiler consisted of three flattened tubes
superposed one over another.
These two boilers were tried by placing them in a charcoal fire and
turning on an alcohol blast, while water from a reservoir under
constant air pressure was forced through them past a pin valve. The
result was that the two-stranded coil supplied steam at from 10 to
40 pounds pressure to run the engines at about 400 revolutions per
minute. The pressure rose steadily for about 40 seconds and then
suddenly fell away, though the coils were red-hot, and neither the
water nor the alcohol was exhausted--apparently because of the
irregularity of the supply of water, due to the time taken by it
after passing the valve to fill the considerable space intervening
between that point and the boiler.
An attempt was made to overcome this difficulty by putting a
stop-cock directly in front of the boiler so that the water, while
still under the control of the needle valve, could be turned in at
once; the alcohol blast was also arranged to be turned on or off at
pleasure, and provision was made, by taking out the end of the flue
inclosing the boiler, to provide for an increased air supply. With
this arrangement a flame eight or nine inches long was obtained,
but a test showed that not more than 25 grammes of water per minute
passed through the tubes, which was not enough.
Further tests with these boilers were so far satisfactory as to
show that with the flattened-tube Serpollet boiler, comprising from
60 to 80 feet of tubing, from 80 to 100 pounds pressure of steam
could be maintained, but not steadily. As there were difficulties
in flattening the tubes to make a boiler of this sort, a compromise
was effected in the construction of the one shown at ‹H› (Plate
13), which was made of light copper tubes 5 mm. in diameter, laid
up in three lengths of 6 metres each. The ends of these coils were
so attached to each other that the water entering at one end of
the smallest coil would pass through it and then enter the middle
coil, whence it passed through the third or outer coil. Two sets of
these coils were made and placed in the thin sheathings shown in the
photograph. Repeated experiments with these boilers demonstrated
that the pressure did not rise high enough in proportion to the heat
applied, and that even the pressures obtained were irregular and
untrustworthy. The principal difficulty still lay in maintaining
an active and uniform circulation through the coils, and for this
purpose the water reservoir under constant air pressure had proved
itself inadequate. This pointed to a return to the use of the force
pump, the construction of which had hitherto presented so many
special difficulties that it had been temporarily abandoned. [p058]
A further difficulty experienced in the use of these boilers had been
that of obtaining ‹dry› steam for the engines, as during the early
experiments the steam had been delivered directly into the engines
from the boiler coils. But in August the writer devised a chamber,
known as the “separator,” where it had an opportunity to separate
from the water and issue as dry steam, or at least approximately
dry steam. This was an arrangement familiar in principle to steam
engineers under another form, but it was one of the many things
which, in the ignorance of steam engineering the writer has already
freely admitted, he had to reinvent for himself.
At about the same time, a new pump was designed to drive the water
from the bottom of the separator, which served the double purpose of
steam drum and reservoir, into the coils. This pump had a diameter of
4.8 cm., and was run at 180 strokes per minute.
The result of the first experiments with these improvements
demonstrated that, within certain limits, the amount of water
evaporated is proportional to the circulation, and in this boiler
the circulation was still the thing that was at fault. Finally, the
results of the experiments with the two-stranded, triple-coil boiler
may be summed up in the statement that it was possible to maintain a
pressure of 80 pounds, and that with it the engines could be made to
develop from 0.3 to 0.4 H. P. at best. It weighed 650 grammes (1.43
pounds) without the asbestos jacket.
About this time the writer had the good fortune to secure the
temporary services of Dr. Carl Barus, an accomplished physicist, with
whose aid a great variety of boilers were experimented on.
The next form of boiler tested was that shown at ‹N› (Plate 13), made
on a system of coils in parallel, of which there were twenty complete
turns. In the first test it generated but 20 pounds of steam, because
the flame refused to work in the colder coils. The work of this
boiler was very unsatisfactory, and it was only with the greatest
difficulty that more than ten pounds pressure could be maintained.
There was trouble, too, with the circulation, in that when the flame
was in full play the pump seemed to meet an almost solid resistance,
so that it could not be made to do its work.
A new boiler was accordingly made, consisting of three coils of four
strands each. With this the pump worked easily, but whereas it was
expected to get 120 pounds pressure, the best that could be obtained
was 70 pounds. The outer coil was then stripped off, and a trial made
in which everything ran smoothly and the pressure mounted momentarily
to 90 pounds. After some adjustment, a mean pressure of 80 pounds was
obtained, giving 730 revolutions of the engine per minute, with an
indicated horse-power of 0.32.
It was shown in this work that, within certain limits, steam is
generated most rapidly when it is used most rapidly, so that two
engines could be used [p059] almost as well as one, the reason
apparently being that the rapid circulation increased the steam
generating power of the boiler, and that the engines worked best at
about 80 pounds. It was also found that a larger tubing was better
than the small, weight for weight, this fact being due to the greater
ease with which circulation could be maintained, since fewer coils
were necessary in order to obtain the same external heating surface.
The pressure in the coils and the separator was also much more nearly
equalized. The result was that the boiler temporarily approved was
one made of tubing 6.35 mm. (0.25 inch) in diameter, bent into a
two-coil, two-stranded boiler, having sixteen complete turns for each
strand in each coil. The total weight was 560 grammes (1.23 pounds)
with a total heating surface of 1300 sq. cm. (1.4 sq. ft.).
The separator used in the experiments made during August and
September was of a form in which the water was forced below a series
of partitions that prevented it from following the steam over into
the cylinders of the engines. It weighed 410 grammes (0.9 pound) and
was most conveniently worked with 700 grammes (1.54 pounds) of water.
The boiler and separator together weighed 970 grammes (2.1 pounds).
A new separator was, however, designed, which was horizontal instead
of vertical, as it was intended that it should be placed just below
the midrod. Another form, devised for constructional reasons,
consisted of a cylinder in which a pump was imbedded. Heretofore the
pump used had been single-acting, but it was now proposed to make a
double-acting pump. Upon testing this apparatus, it was found that
when using an aeolipile, it took 150 grammes of alcohol to evaporate
600 grammes of water. It was evident that the latter was used very
wastefully, so that the thermal efficiency of the engine was not
over one per cent; but it was also evident that, under the necessity
of sacrificing everything to lightness, this waste was largely
inevitable.
About the middle of October, another boiler (O, Plate 13) was
made, which consisted of two coils wound in right and left hand
screw-threads, one fitting loosely over the other, so as to make a
cylindrical lattice-work 32 cm. (12.6 in.) long. Each coil contained
two strands of copper tube 0.3 mm. thick, and weighing 54 grammes to
the metre (0.036 pound to the foot). The inner coil had a diameter
of 5.63 cm. (2.22 in.), with nine turns of tube to the strand, the
two strands making a length of 319 cm. (10.5 feet) for the coil. The
outer coil had a mean diameter of 6.88 cm. (2.71 in.) and a length of
388 cm. (12.7 feet) for the two strands. The total length of the two
coils was, therefore, 707 cm. (23.2 feet), with a heating surface of
about 1415 sq. cm. (1.52 sq. ft.) and a total weight of 382 grammes
(0.84 pound).
The results obtained with this boiler were so far satisfactory as to
show that, under the most favorable conditions, when air was supplied
in unlimited quantities and there were no disturbing currents to
put out or interfere with the work [p060] of the burners, steam
could be supplied at a sufficient pressure to run the engines. It
was realized, however, that the conditions in flight would be very
different, and that in order to protect the apparatus from the wind,
some sort of protecting covering would have to be devised, which
would of itself introduce new difficulties in providing the burners
with a proper and uniform draft.
[Illustration: FIG. 10. Diagram of pendulum.]
The hull, as at first constructed, consisted of a cylindrical
sheathing open in front, through the rear end of which the boiler
and aeolipile projected inward, so that the air taken in at the
front would be drawn through the boiler and hearth to the exclusion
of lateral currents. In the first tests, however, after the hull
had been applied, it was impossible to secure a proper rate of
combustion, nearly the whole hull being filled with a bluish flame,
while only a very small portion of the gases of combustion passed
into the coils of the boiler. The remedy for this lay in obtaining
an increased draft, and a small stack was, therefore, arranged to
carry off the products of combustion. This proved inadequate, and
it was only after several weeks of experiment with various types of
smoke-stack, and constant alteration of the aeolipile, that it was
possible to make the apparatus work [p061] efficiently when it was
inside the hull. Finally such a degree of success was attained that
the burners could be kept lighted even when the aerodrome was placed
in a considerable artificial breeze, created by a blower in the shop.
In connection with these tests of the engines and boilers, some
method was desired, in addition to the Prony brake tests, by which
the thrust of the propellers when driven by the engines at various
speeds could be measured accurately and in terms which would be
readily available in judging whether the aerodromes were ready to be
given an actual trial in free flight. Such a method was found in the
use of an apparatus known as the “pendulum,” which was introduced
near the end of 1892, but was not generally used until the end of
1893. After this time, however, this test was made a condition
prerequisite to taking any of the aerodromes into the field, and
proved of the greatest assistance in estimating the probable outcome
of the trials.
The apparatus used, which is diagrammatically shown in Fig. 10, was
extremely simple both in theory and operation. It consisted primarily
of a horizontal arm ‹AC› carrying the knife-edge ‹B› by which it
is pivoted on each side on supporting beams not shown. Depending
from ‹AC› is the light vertical arm ‹DE›, rigidly joined to it and
carrying the lower horizontal arm ‹FG›, all of which are braced
together so as to maintain the arm ‹DE› constantly perpendicular
to ‹AC›. To this arm ‹FG› the model was rigidly attached with its
center of gravity in line with the vertical arm ‹DE› and its weight
increased by the addition of properly disposed flat weights, in
order to make the angle of lift for a given thrust of the propellers
smaller and less likely to interfere with the working of the boiler
and separator.
Before the actual test of the “lift” could be made, it was necessary
to know the exact distance of the vertical center of gravity of
the model and the extra weights from the knife-edge ‹B›. This was
determined by the following method: A known weight was suspended
from the arm ‹AB› at some arbitrarily selected distance from the
point ‹B›. This weight caused the perpendicular arms ‹AB› and ‹DE›
to rotate through an angle, θ, which was measured on the scale ‹KL›.
Knowing, then, the weight on the arm ‹AB›, its point of application,
the weight of the aerodrome suspended on the arm ‹DE›, and the angle
of rotation, it is easy, by a simple application of trigonometric
functions, to determine the distance of the center of gravity of the
model from the point ‹B›.
In a test of Aerodrome No. 6 made on September 23, 1898, the weight
suspended from ‹AB› was 10,000 grammes, its point of application
50 cm., the model was weighted to 20,450 grammes, and the angle of
rotation, θ, was 7° 2′. Letting y equal the distance of the ‹CG› from
‹B›, we may equate the balanced forces thus:
10,000×50 cos 7° 2′ = 20,450׋y› sin 7° 2′
10,000×50 cot 7° 2′ = 20,450‹y›
‹y› = 198.2 cm.
Having determined this distance, the weight on ‹AB› was removed and
the aerodrome was allowed to regain its former position. The distance
of the center of thrust from ‹B› was then measured. The engine was
next started and the number of revolutions of the propellers counted
by a tachometer. The thrust of the propellers, acting perpendicularly
to the arm ‹BD›, produced rotation around the point ‹B›, the angle of
which was measured as above.
In the power test of No. 6, the following data were obtained:
‹W› = weight of aerodrome = 20,450 grammes.
θ = angle of lift = 19° 30′.
Distance of ‹CG› from center of rotation = 198.2 cm.
Distance of center of thrust from center of rotation = 186.3 cm.
As the propeller thrust and the weight of the model are forces acting
in opposite directions at known distances from a center of rotation,
letting ‹L› equal the “dead lift,” we may express the equation thus:
‹W› sin θ×198.2 = ‹L›×186.3,
‹L› = (198.2/186.3)×sin 19° 30′×20,450,
‹L› = 7,263 grammes “dead lift.”
The flying weight of Aerodrome No. 6 was 12,064 grammes, and the per
cent of this weight lifted was, therefore,
7,263/12,064 = 60.3.
This was much more than was necessary for flight, but in order to
insure successful flights and avoid delay, the rule was made in
1895 that no aerodrome was to be launched until it had previously
demonstrated its ability to generate enough power to maintain for at
least two minutes a lift of 50 per cent of the total flying weight.
At the same time other important data were obtained, such as the
steam-pressure, the time required to raise sufficient steam, the
total time of the run, and the general working of the boilers and
engines.
As will easily be seen, these tests afforded a most satisfactory
basis of judging what the aerodromes might be expected to do in
actual flight if the balancing were correct.
At this time, October, 1893, the aerodrome (Old No. 4) was
practically complete, and the most anxious thought was given to
lightening it in every way consistent with the ever-present demand
for more power, which necessitated an increase in the weight of both
burners and boilers to supply the requisite steam.
On November 14, when the aerodrome was prepared to be shipped
to Quantico for trial, its condition was about as follows. The
steam-generating apparatus--the parts of which were of substantially
the forms last described, although some slight improvements had
been introduced—had been developed to [p063] such a point that a
pressure of from 70 to 80 pounds of steam could be maintained for 70
seconds, when it was tested in the shop. What it would do under the
unfavorable conditions imposed by flight was to be learned only by
trial.
At this pressure, the engines, the efficiency of which had been
increased by an improvement in packing, would develop approximately
0.4 indicated H. P., while at 105 pounds pressure they at times
developed as much as 0.8 H. P. When the aerodrome was tested on the
pendulum, these engines, when making less than 700 revolutions per
minute, lifted over 40 per cent of the total flying weight.
The propellers used at this time were accurate helices, having a
diameter of 60 cm., a width of blade of approximately 36 degrees, and
a pitch-ratio of 1.25. They were formed of wood, and were bushed with
brass where they were attached to the shafts.
AERODROME OLD NO. 4 AS PREPARED FOR FLIGHT BEFORE BEING SHIPPED FOR
TRIAL ON NOVEMBER 14, 1893
------------------------+-------+------+------+-----+
Part. |Copper.|Steel.|Brass.|Iron.
| | | |
| | | |
------------------------+-------+------+------+-----+
| gms. | gms. | gms. | gms.
Aeolipile | 200 | .. | 92 | ..
Boiler | 350 | .. | 37 | ..
Separator and pumps | 300 | 30 | 100 | 20
Engine and frame | .. | 350 | 570 | ..
Midrod (200 cm. long) | .. | 220 | .. | ..
Two smoke-stacks | 70 | .. | .. | ..
Asbestos jacketing | .. | .. | .. | ..
Air chamber | .. | .. | .. | 82
Spider between boiler | | | |
and burner | 32 | .. | .. | ..
Intake valve | .. | .. | 15 | ..
|-------|------|------|-----+
Total | 952 | 600 | 814 | 102
|-------|------|------|-----+
Hull | 50 | .. | 50 | ..
Pins for starter | .. | 15 | .. | ..
Two large wings and tail| .. | .. | .. | ..
Buffer and steerer | .. | .. | .. | ..
Propellers | .. | .. | .. | ..
|-------|------|------|-----+
Total | 50 | 15 | 50 | ..
|-------|------|------|-----+
Grand total | 1002 | 615 | 864 | 102
|=======|======|======|=====+
Density | 8.9 | 7.8 | 8.5 | 7.5
Volume (cu. cms.) | 113 | 79 | 102 | 136
Alcohol | .. | .. | .. | ..
Water | .. | .. | .. | ..
|-------|------|------|-----+
Total | .. | .. | .. | ..
Density | .. | .. | .. | ..
| | | |
Volume (cu. cm.) | .. | .. | .. | ..
------------------------+-------+------+------+-----+
------------------------+------+---------+------+---------------------
Part. | Wood |Mica and |Fluid.| Total and mean
| and |asbestos.| | weights.
| silk.| | |
------------------------+------+---------+------+---------------------
| gms.| gms. | gms. | gms.
Aeolipile | .. | .. | .. | 292
Boiler | .. | .. | .. | 387
Separator and pumps | .. | .. | .. | 450
Engine and frame | .. | .. | .. | 920
Midrod (200 cm. long) | .. | .. | .. | 220
Two smoke-stacks | .. | .. | .. | 70
Asbestos jacketing | .. | 50 | .. | 50
Air chamber | .. | .. | .. | 82
Spider between boiler | | | |
and burner | .. | .. | .. | 32
Intake valve | .. | .. | .. | 15
|------|---------|------|-----
Total | .. | 50 | .. | 2518 = 5.54 lbs.
|------|---------|------|-----
Hull | .. | 25 | .. | 125
Pins for starter | .. | .. | .. | 15
Two large wings and tail| 571 | .. | .. | 571
Buffer and steerer | 53 | .. | .. | 53
Propellers | 250 | .. | .. | 250
|------|---------|------|-----
Total | 874 | 25 | .. | 1014 = 2.33 lbs.
|------|---------|------|----- ----
Grand total | 874 | 75 | .. | 3532 = ┐ 7.77 lbs.
|======|=========|======|===== │
Density | 0.8 | 3.0 | .. | 2.48 │ I.
Volume (cu. cms.) | 1092 | 25 | .. | 1425 = ┘ 87 cu. ins.
Alcohol | .. | .. | 100 | 100
Water | .. | .. | 500 | 500
|------|---------|------|-----
Total | .. | .. | .. | 4132 = ┐ 9.1 lbs.
Density | .. | .. |┌125┐ | │
| | |└500┘ | 2.01 │ II.
Volume (cu. cm.) | .. | .. | .. | 2050 ┘
------------------------+------+---------+------+---------------------
Permanent air spaces:
in midrod, vol. = 355 cc.┐
in engine frame, vol. = 100 cc.│ ┐
volume as per II. 2050 cc.│ Density = 4132/2505 = 1.65│ III.
--------│ ┘
2505 cc.┘
The total flying weight of Old No. 4, including fuel and water,
was 4132 grammes (9.1 lbs.), a much larger weight than had been
contemplated when the original designs were made. A detailed
statement of the weights of the various parts of the aerodrome,
together with some data as to its density, is given on the preceding
page. There were provided in the wings and tail approximately 2 sq.
ft. of supporting surface to the pound of weight, which would have
been barely sufficient to sustain the aerodrome, even if it had been
successfully launched and the wings had been built much stronger than
the flimsy construction in use at this time.
An air chamber, which served the double purpose of floating the
aerodrome and of providing a moveable weight by which the center of
gravity could be shifted to the proper position relatively to the
center of pressure, was constructed of the thinnest sheet-iron and
attached to the midrod.
This aerodrome, the fifth in actual construction, and the first,
after years of experiment, to be carried into the field, was
transported to Quantico, where the first trial with it was made on
November 20, under the conditions described in Chapter IX.
1894
The aerodrome, No. 4, which has just been described, had not been
put to the test of an actual flight, for reasons connected with the
difficulties of launching, which are more fully described elsewhere;
but, when the completed machine was more fully studied in connection
with the unfavorable conditions which it was seen would be imposed
on it in trials in the open air, many possibilities for improvement
presented themselves. It was seen, for instance, that a better
design might be made, in which the engines, boiler and aeolipile
might be placed so that the center of gravity of each would lie in
the same vertical plane as the central line of the aerodrome. In
order to do this the construction of a single midrod, which was the
distinguishing feature of Old No. 4, had to be essentially departed
from, the midrod of this new one, No. 5, being opened out into two
rods, so to speak, which were bent out so that the open space between
them furnished a sufficiently large hull space to hold the entire
power generating apparatus. In arranging the machinery within this
hull, it was provided that, as the water and fuel were expended, the
center of gravity of the aerodrome would shift little, and, if at
all, backward relatively to the center of pressure.
Instead of the two small engines, which it will be remembered were
mounted on the cross-frame in No. 4, a single engine with a larger
cylinder, having a diameter of 3.3 cm. (1.3 in.) and a stroke of 7
cm. (2.76 in.), capable of developing about 1 H. P. was used. This
engine was mounted within the hull near the forward end and drove the
propellers by suitable gearing. [p065]
In addition to these radical changes many important improvements were
made in the different parts. Internal compartments were built in the
separator, so that even if the water was displaced by the pitching
of the aerodrome, it could still perform its functions properly.
The pump was provided with a ratchet, so that it could be worked by
hand after the burners were lighted, and before enough steam had
been raised to enable the engine to run it. An active circulation
was thus maintained in the coils of the boiler as soon as the burner
was lighted and before the engine was started, which prevented the
tubing from being burnt out, as had frequently happened previously.
The wing construction was also improved and many other changes were
introduced, which will be treated separately.
In the meantime, No. 4, which had been damaged in the attempted
launching in November, 1893, was strengthened and prepared for
another trial which took place in January, 1894.
By the end of the first week in February, the engine of No. 5 was
ready for trial, and with a boiler pressure of about 80 pounds per
square inch, apparently developed 0.56 H. P. on the Prony brake, when
making 800 revolutions per minute. To accomplish this called for such
good distribution of steam in the cylinder, that it is doubtful if
the power could be exceeded at that speed and pressure.
It was, however, apparent that it was desirable to have a boiler
capable of supplying steam for at least one horse-power, and that in
order to do this, there must be an improvement in the aeolipiles.
The problem consisted in arranging to evaporate more than 500 cu.
cm., and in fact as nearly as possible 1000 cu. cm. (61 cu. in.)
of water per minute, and, since from 200 to 300 cu. cm. per minute
had already been evaporated, this was not regarded as impossible of
accomplishment. The theoretical advantages of gasoline had for a
long time been recognized, as well as the very practical advantage
possessed by it of keeping lighted in a breeze, and several attempts
had been made during the latter part of the previous year to
construct a suitable burner for use with it. These had not been
very successful; but in view of the increasing demand for a flame
of greater efficiency than that of the alcohol aeolipiles, it was
decided to resume the experiments with it.
Accordingly, a gasoline evaporator was tried, consisting in the first
experiment of a gasoline tank with nine flues, through which steam
was passed. A flow of steam gave a rapid evaporation of gasoline
when the pressure did not exceed 5 pounds. The chief difficulty with
the burner employed was that the supply of gasoline gas would rise
and fall as the steam rose and fell, conditions just the opposite
of what was really desired. On the other hand, it was thought that
this gasoline tank would form a real condenser for the steam, so
that a [p066] portion of the exhaust steam would be condensed and
be available for use in the boiler again. The gasoline vapor had
many advantages over the alcohol; but it was at first possible to
evaporate only 120 cu. cm. of gasoline in a minute.
In the experiments that were made at this time (March 9) with
gasoline, the main object in view was to obtain a smooth blue flame
at 10 pounds pressure. There had been failures to accomplish this,
owing to the high boiling point of the liquid, and while the work
was in progress it was still evident that the problem of the boiler
and the flame which was to heat it had not been solved. A Prony
brake test gave, at 130 pounds pressure, 1.1 H. P. with about 1000
revolutions of the propellers; but this was with steam supplied from
the boiler of the stationary shop engine.
On April 1, 1894, the following record was made of the condition of
Aerodrome No. 5:
“The wings, the tail, and the two 80 cm. propellers, as well as the
two smaller propellers, are ready. The cylinders, gear, pump, and
every essential of the running gear, are in place. The boilers,
separators, and adjuncts are still under experiment, but may be
hoped to be ready in a few days. At present, the boilers give from
450 to 600 grammes of mixed steam and water per minute. With 130
pounds of steam, the engine has actually developed at the brake,
without cut-off, considerably more than 1 H. P., so that it may be
confidently considered that at 150 pounds, with cut-off, it will
give at least 0.8 H. P., if it works proportionately well.”
The delays incident to the accomplishment of the work in hand were
always greater than anticipated, as is instanced by the fact that
it was the latter part of September before the work was actually
completed. The greater part of this delay was due to the necessity
for a constant series of experiments during the spring and summer to
determine the power that it was possible to obtain with the various
styles of boilers, aeolipiles, and gasoline burners.
While No. 5 was thus under construction, new and somewhat larger
engines had been built for No. 4, the work on them having been
begun in January. The cylinders of these engines, which are more
fully described in connection with Aerodrome No. 6, were 2.8 cm. in
diameter, with a 5 cm. stroke, each cylinder thus having a capacity
of 30.8 cu. cm., which was an increase of 36 per cent over that of
the old brass cylinder engines, which had previously been used on
No. 4. On April 28, under a pressure of 70 pounds, these engines
drove the two 60 cm. propellers at a rate of 900 R. P. M., and
lifted on the pendulum nearly 40 per cent of the total flying weight
of Aerodrome No. 4, which was now approximately 5 kilos. A trial
was made at Quantico in the latter part of May, which is described
in Chapter IX. It is only necessary to mention in this connection
that there was a great deal of trouble experienced with the alcohol
aeolipile, the flame being extinguished in the moderate wind to which
the [p067] aerodrome was subjected while preparations for the launch
were being made. Moreover the flame was so nearly invisible in the
sunlight that it was uncertain whether it was burning in the critical
instants just before the launch, when doubt might be fatal. These
conditions resulted in a final decision in favor of gasoline, on
account of its greater inflammability, and in the provision of such
hull covering that the fires could be lighted and maintained in a
breeze.
In June, I tried a modification of the burner, in which the gasoline
was delivered under the pressure of air to the evaporating coil. In
the first trial steam was raised to a final pressure of about 70
pounds, and a run of 45 seconds was secured under a pressure of 40
pounds in the gasoline tank, which was thought to be altogether too
high; for, at the end of the run, the whole apparatus was enveloped
in flames, because of the gasoline that was projected through the
burner-tips.
Continual experiments with different forms of burner, illustrated
in Plate 12, occupied the time, with delays and imperfect results,
which were trying to the investigator, but are omitted as of little
interest to the reader. They had, however, the incidental result
of proving the practical superiority of gasoline over alcohol, and
culminated in the evolution of the burner that was finally used
successfully. It consisted of a tank for the gasoline, from which
compressed air delivered the liquid to a small coil surrounded by
asbestos, in which it was vaporized. At the rear end of this coil
three pipes were led off, one of which was a small “bleeder,” which
fed the burner for heating the gasoline, the other two leading to the
main burners. After the generation of gas in the small coil had been
started, the heat from the small burner was expected to continue the
vaporization, so that nothing but gas would be able to reach the main
burners. A device was also introduced, which had greatly increased
the amount and uniformity of the draft and consequently made the
burners and boilers more efficient than before. This consisted simply
in passing the exhaust steam from the engines into the smoke-stack,
and it is remarkable that it was not thought of earlier.
By the middle of September, 1894, both aerodromes were completed and
ready for another test. On September 27 the condition of Aerodrome
No. 4 was as follows: The general type of construction, namely, that
of a single midrod, to which all the steam generating apparatus was
attached, and which supported also the cross-frame and the wings, was
the same as in the construction of 1893. On account of the increased
weight of the model, and the substitution of an inferior piece of
tubing in place of the former midrod, it was found necessary to
stiffen it by the use of temporary trusses. Permanent bearing points
for holding the aerodrome securely to the newly devised launching
apparatus were also attached to this midrod. [p068]
The engines in use at this time were the small steel cylinders
described above, which were mounted on the cross-frame, and drove the
propellers directly. These engines were capable of delivering to the
propellers, as had been proved by repeated tests, at least 0.66 brake
horse-power.
The boiler consisted of two inner coils and an enveloping outer coil,
loosely wound and connected in series. The inner coils, each of which
had about 17 turns of 8 mm. diameter, 0.2 mm. thick tubing, developed
about 80 per cent of the steam; the outer coil of 8 turns, while not
exactly useless as a steam generator, afforded an efficient means of
fastening the smoke-stack and cover of the boiler, and for attaching
the latter to the midrod. This boiler was externally 30 cm. long, 16
cm. wide, and 10 cm. deep, weighing with its cover approximately 650
grammes. The stack for the burnt gases, into which exhaust steam was
led from a central jet, was about 1 foot long. At best this boiler
was capable of developing slightly over 100 pounds of steam.
The separator was of the form last described, except that the steam
dome had been moved toward the front, to prevent the jerk of the
launching car in starting from causing water to be pitched over
into the engines. It was constructed of sheet aluminum-bronze, and
weighed, together with its pump, 580 grammes. The pump, which was
double-acting and fitted with ball valves, was capable of discharging
4.5 grammes of cold water per stroke, its efficiency being only about
one-half as great with hot water.
The gasoline burner, which had been finally adopted in place of the
alcohol aeolipiles, had now been perfected to the form in which
it was finally used. Two Bunsen burners of special construction
were provided with gasoline gas by the heat of an intermediate
accessory burner, which played upon a coil to which all three
burners were connected. Gasoline was furnished from a tank made
of aluminum-bronze, under an air pressure of about 20 pounds, the
fluid being under the control of a screw stop-cock. This tank, which
was capable of holding 100 to 150 cu. cm. of gasoline, weighed 180
grammes, and the burners with an outer sheathing weighed 302 grammes.
It was calculated that about 3300 cu. cm. (201 cu. in.) of air
space would be required to float the aerodrome in water, and this
was supplied by an air chamber, having a capacity of 2700 cu. cm.
(165 cu. in.), which could be shifted to adjust the longitudinal
equilibrium of the aerodrome, and about 900 cu. cm. (55 cu. in.) of
space in the gasoline tank and the midrod. The reel and float, which
served to indicate the location of the aerodrome, if for any reason
it should be submerged, were in one piece, and so moored that there
was no danger of fouling the propellers.
The total weight of the aerodrome was about 6 kilogrammes (13.2
lbs.), or, with a maximum quantity of fuel (850 cu. cm. of water,
150 cu. cm. of gasoline), [p069] less than 7 kilogrammes. From 60
to 90 pounds of steam could be maintained by the boilers for about
2 minutes, at which pressure the engines developed about 0.66 brake
horse-power, driving the 70 cm., 1.25 pitch-ratio propellers at 700
R. P. M., and giving a lift of from 2.6 to 3.0 kilos (5.7 to 6.6
pounds), or about 40 per cent of the flying weight.
The wings and tail had a total surface of 2.62 sq. m. (28.2 sq. ft.),
giving a ratio of 2.7 kilos to 1 sq. m. of wing surface (1.8 sq. ft.
per pound). If the hull resistance be neglected, the soaring speed of
this aerodrome was about 5.9 metres (19 feet) per second, or 13 miles
per hour.
Turning now to the completed No. 5, its frame was of the “double
midrod” type described above, the two tubes which formed the
frame being prolonged at the front and rear to afford points of
attachment for the wings and tail. The range through which the wings
could be shifted to adjust the position of the center of pressure
was, however, very small. The hull, which, it will be remembered,
contained all the power generating apparatus, was much stronger
and heavier than that of No. 4, and resembled somewhat the hull of
a ship. It had a frame-work of steel tubing brazed to the midrod,
to which an outer sheathing of sheet aluminum 0.3 mm. thick was
attached. It was, however, excessively heavy, weighing nearly 800
grammes.
The engine, which was mounted near the front of the hull, was the
single cylinder, one horse-power engine, described above, which drove
the two propellers by suitable gearing. The remaining parts of the
power plant were identical with those already described in connection
with No. 4, but the more advantageous location of them in No. 5
rendered them somewhat more efficient.
It had been planned to use 80 cm. propellers of 1.25 pitch-ratio on
No. 5, but it was found in the shop tests of the aerodrome that the
cross-frame was not strong enough to withstand the strains, and that
the engine could be made to work much more steadily with a smaller
propeller. Accordingly, propellers of 70 cm. diameter and 1.25
pitch-ratio, similar to those used on No. 4, were finally substituted.
For floating the aerodrome, when it descended into the water, an
air-chamber similar to that of No. 4, but of a larger capacity was
provided. With this in place on the aerodrome, it was calculated
that, if all the parts except this float and the gasoline tank
were filled with water, there would still be a buoyancy of over 2
kilogrammes.
The total weight of No. 5 was 8200 grammes, or with its full supply
of fuel and water 9200 grammes. In this aerodrome the same boilers
used in No. 4 were capable of maintaining for at least a minute 115
pounds of steam, so that the engine now gave the maximum of one
brake horse-power for which it was designed, and, driving the 70
cm. propellers, lifted repeatedly nearly 45 per cent of the flying
weight. [p070]
The wings and tail constructed for No. 5 were identical with those
of No. 4, being slightly curved and containing 2.62 sq. m. (28.2 sq.
ft.), equivalent to 1.4 sq. ft. to the pound, which with the flimsy
construction of the wings gave an entirely inadequate support to the
aerodrome.
During the summer a launching apparatus of a new and improved type,
which is described in Chapter X, had been perfected, and with it
repeated tests were made of both aerodromes in October, November,
and December, with the unsatisfactory results recorded in Chapter
IX. In the course of these experiments, many slight modifications
of the burners and boilers were made, but no important changes were
introduced except that the cross-frame of No. 5 was enlarged and
strengthened so as to admit of its carrying one metre propellers
safely. The results, however, which were obtained, did not compensate
for the increased weight of the larger frame.
Viewing the work of this year from the standpoint of results obtained
in the numerous attempts at flight, it would seem that very little
progress had been made, and that there was small reason to expect
to achieve final success. However, if the work be examined more
particularly, it will be seen that two of the most difficult problems
had been solved, one completely as far as the models were concerned,
and the other to a very satisfactory degree. First, a launching
apparatus, with which it was possible to give the aerodrome any
desired initial velocity, had been devised, and so far perfected
that no trouble was ever experienced with it in testing the models.
Second, as a result of the extended and systematic series of
experiments, which had been conducted under the direction of Dr.
Barus, a steam pressure of 115 pounds could be maintained steadily
in the boilers for at least a minute, and the burners could be kept
lighted even in a considerable breeze.
A summary of these experiments, together with some account of the
difficulties encountered and the results finally obtained with the
apparatus in use at the end of the year, is given in the following
report, which was prepared by Dr. Barus in December, 1894.
“If water be sprayed upon a surface kept in a permanent state of
ignition, any quantity of steam might be generated per time unit.
Similarly advantageous conditions would be given if threads of
water could be passed through a flame. In practice this method
would encounter two serious difficulties, the importance of which
is accentuated when the boiler apparatus is to be kept within the
degree of lightness essential in aerodromics. These difficulties are
(1) the danger of chilling the flame below the point of ignition
or of combustion of the gases, and (2) the practical impossibility
of maintaining threads of water in the flame. For it is clear
that the threads must be joined in multiple arc, so as to allow a
large bulk of water to circulate through the boiler, whereas even
when there are but two independent passages for the water through
the furnace, it is hard to keep both supplied with liquid without
unduly straining the pump. If the water be even slightly deficient,
circumstances will arise in which one of [p071] the passages
is better than the other. This conduit will then generate more
steam and drive the water under force through the other passage,
increasing the temperature discrepancy between them. Eventually
the hot passage reaches ignition and either bursts or melts. This
is what sooner or later takes place in boilers adapted for flying
machines and consisting of tubes joined in multiple arc, when a
single moderately strong circulating pump supplies the system.
“To avoid these annoyances, ‹i. e.›, to increase the length of life
of the boiler, the boiler tubes are joined in series to the effect
that a single current of water may flow successively through all of
them. It is needful therefore to select wide tubes, such as will
admit of an easy circulation in consideration of the length of
tubing employed without straining the pump and at the same time to
allow sufficient room for the efflux of steam. Other considerations
enter here, the bearing of which will be seen presently: if the
tube be too wide the difficulty of coiling it on a mandrel of
small diameter is increased, while at the same time the tube loses
strength (‹cæt. par.›) in virtue of the increased width.
[Illustration: FIG. 11.
Diagram 1. Diagram 2.]
“It is from considerations such as these that, in the course of many
experiments, copper tubing about 8 mm. in diameter has been adopted.
Copper is selected because of its freedom from internal corrosion,
easy coiling, and because of its availability in the market. The
thinnest tube to be had (walls only 0.1 mm. thick) will withstand
more pressure than can be entrusted to the larger steam receivers
in circuit with the boiler. The boiler weight is thus a negligible
factor, and it is quite feasible to reduce the thickness of boiler
tubing, by the superficial application of moderately strong nitric
acid, to 200–400 grammes per horse-power of steam supplied. External
corrosion due to flames occurs only in case of deficient water, and
if the boiler be made of tubing with the walls 0.2 mm. thick, it is
in view of the possibility of such accidents. Boilers may then be
tested to 25 atm. without endangering the metal.
“Boilers are wound or coiled with regard to the two points above
suggested, viz.: to avoid chilling the flame the successive turns
are spaced on all sides, and to bring the water as nearly into the
flame as possible, the diameter of the coils is chosen as small
as expedient. Further reasons for this will presently be adduced.
The type of boiler eventually adopted is shown in the accompanying
diagrams, 1 and 2, Fig. 11.
“Diagram 1, is a perspective diagram showing the plan of winding
and Diagram 2, an end view. The circulation is indicated. There are
two inner coils [p072] each containing about 17 turns, wound on a
mandrel 5 cm. in diameter. The turns are spaced so as to allow about
1 cm. clear between successive turns. The outer coil envelopes both,
and in this there are about 3 cm. between successive turns, and 8
turns in all. Length, say, 30 cm., breadth 16 cm., thickness 10 cm.,
give the external dimensions of the boiler. The shell space between
outer and inner layers of tubing must nowhere be less than 1 cm.
When so wound, the inner coils (here as in other boiler forms) raise
about 80 per cent or more of the steam; the outer or enveloping
coil, while not quite useless, make the most effective frame work
for the boiler jacket which has been devised. The coils are brazed
together by blind tubes, as shown in Diagram 2, to keep the whole in
shape. Weight with couplings and cover when complete 535 grammes.
“The cover is preferably of mica, through which the flame within
the boiler may be seen, and in which lightness, nonconduction, and
resistance to the disintegrating effects of high temperature are
met with in a pronounced degree. This jacket is held down by copper
bands and the end band is continuous with the long smoke-stack, as
will be presently shown.
“The wide form of boiler with two coils within the envelope is not
absolutely essential. The same amount of steam can be generated from
one coil in an envelope in other respects equal to Diagram 1 if a
sufficiently hot flame be passed axially through the coils. Such a
flame, however, is unstable, and for this reason two milder flames
with a good air access are to be preferred on practical grounds even
if the weight is thereby increased.
“To further understand the boiler construction it is advisable
to consider the action of the flame. Inasmuch as wide tubes must
be used, the problem of evaporating water as fast as possible
is equivalent to getting heat into the current (water and steam
circulating through the coils) as fast as possible from without.
If, therefore, ‹t› is the mean temperature of the fluids within the
coils, and ‹T› the effective temperature surrounding the tube, then
the rate at which heat will flow into the tubes is proportional to
‹T›−‹t›. Now ‹t› the temperature of the steam is nearly constant
(100°–150°) whereas ‹T› the effective flame temperature may vary
from 800° to, say, 1600°. It is for this reason that the heat
sponged up by the boiler depends almost directly on the flame
temperature.
“What conditions, therefore, will make the flame effectively hot?
“(1) The coils must obviously be brought as nearly into the flame as
feasible: for this purpose the cylindrical helix is better than any
other form. But
“(2) The turns and coils must not be so crowded together as to chill
the flame into imperfect combustion in various parts of its extent.
Hence the loose form of winding. Again
“(3) There must be oxygen enough to allow complete combustion, and
“(4) The flame itself must be hot and the radiation checked by good
jacketing.
“To take up the last points: the effective heat of the flame depends
not only on the combustion heat of the fuel used; it depends also,
among other things, on the speed with which this combustion takes
place. A flame burning from a low pressure of alcohol gas will be at
low temperature as compared with a flame burning from high pressures
of the gas. If the flame be burnt from a Bunsen burner in the usual
way it is an interesting question to know how flame temperature
will vary with gas pressure. At present we know it merely in steam
pressures incidently produced in a given engine (No. 4) as for
instance:
Flame pressure, 10 lbs., 20 lbs., 30 lbs. ┐
Steam pressure, 40 lbs., 80 lbs., 120 lbs. ┘ in the running engine.
“Unfortunately there is a limit set to this process of increasing
the steam supply, quite aside from conditions inherent in the
method. This is due to the fact that a certain speed of efflux
cannot be exceeded without putting the flame out. Suppose, for
instance, in Fig. 12, that a gas generated from a liquid is ignited
at the end of the Bunsen burner ‹F›; then if the velocity of efflux
of mixed gas and air in the direction ‹AB› from the mouth of ‹F›
exceeds the velocity of combustion in the direction ‹BA›, the flame
will obviously be carried away from the mouth of the tube and
dissipated. This state of things is actually realized at pressures
exceeding about 15 lbs., depending on the degree of mixture of
the combustible gases used, and therefore on apparently haphazard
conditions connected with the jet, the air holes, the air supply,
etc.
[Illustration: FIG. 12.]
[Illustration: FIG. 13.]
“If, however, the velocity of the jet at the point of efflux be
checked by an obstruction like a cylinder ‹C›, Fig. 13, placed
co-axially with the burner tube ‹F›, the speed of combustion will no
longer be exceeded (supposing ‹C› properly chosen) and flames will
then burn from high-pressure gas. In this way flames were maintained
generated from alcohol gas at even 40 lbs. and above.
[Illustration: FIG. 14.]
“The gas escaping from the Bunsen burner is never sufficiently
aërated to burn completely. Otherwise there would (in general) be
explosions in the tube ‹F›. A part of this air is supplied at the
mouth of the boiler ‹B›, Fig. 14, and the amount available here will
depend on the velocity of the jet ‹F›. Hence it does not follow
that a high-pressure burner like that in Fig. 11 will supply a
proportionate amount of heat, since its jet suction is not intense
and the combustion within the boiler is incomplete. This difficulty
may be remedied by placing [p074] air holes in the jacket of the
boiler, provided the boiler be wrapped loosely enough not to chill
the flame below ignition. It is with reference to this effect that
the boilers, Fig. 11, were wound. A number of rifts ‹aaa›, Fig. 15,
are then left in the jacket through which air may enter in virtue of
the burner flame acting as a jet at the mouth of the boiler.
“When so constructed the flame at first enters the inner coil only;
but after a little while it suddenly spreads out throughout the
whole interior space and envelops the coils. This sudden expansion
is due, probably, to the assumption of the spheroidal state by the
water within the coils, the current now flaring on an enveloping
cushion of steam. The pump must work well, for deficient water means
a hot tube and deficient steam, or eventually a rupture of the tube.
“Thus far the dependence for draft has been on the burner jet and
the suction of the smoke-stack in virtue of the inertia of the
moving gases. But even with this ventilated boiler, this method is
limited to certain dimensions of the boiler. Thus a boiler 80 cm.
long yielded about the same quantity of steam as a boiler half as
long and otherwise similar. Only the initial parts of the boiler
are, therefore, relatively efficient, and the reason of this seems
to be that, apart from shape, etc., the flame as a heat-producing
agent is practically defunct, when a certain amount of heat has
been taken out of it: in other words, even with fair ventilation
the flame is eventually chilled off by the voluminous products
of combustion continually accumulating in the boiler. The same
choking action accompanies the presence of unburnt gases. If, for
instance, the flame be burnt in the air, it is slender and much
smaller in volume than in the boiler. The flame is also of small
volume and burns completely in a wide boiler, but the steam is
always deficient, because of the distance between flame and coils
(see above). With the above apparatus about 1/2 lb. of dry steam per
minute per square foot of heating surface was attained.
[Illustration: FIG. 15.]
“This introduces the final condition for rapid steam generation.
There must be artificial suction at the smoke-stack. By passing the
exhaust steam in the form of a central jet through the smoke-stack
the yield of steam was increased 20 to 30 per cent. In fact as the
supply of gas from the burner is given, the artificial suction in
question means more air in the boiler for the same amount of gas
and it means also a more rapid removal of the exhaust gases. The
experiments with steam suction are yet to be completed, and with
them the boiler question is to be finally laid at rest. The chief
points at issue are these:
“1. Seeing that the jet suction increases with the length of the
smoke-stack, up to a certain length at least, how long and how wide
must the efficient smoke-stack be made? Thus a smoke-stack 10 cm.
long is all but useless. Good results are obtained when the stack
measures 30 cm. in length beyond the end of the steam jet. [p075]
“2. What is the relative efficiency of the initial and final halves
of the length of the boiler? This will show in how far it is
useful to increase the length of the boiler for a given burner and
steam jet. It will also show what advantage is to be gained from
triplicate boilers with three burners, as compared with duplicate
boilers with two burners, or single boilers with one burner, ‹when
the same weight› of tubing is used throughout.
“3. What is the effect of pressure on the aeolipile tank, or in
how far does the steam generated depend on what may be called the
pressure of the flame? This is also an important point which remains
for quantitative solution. It can be approached in two ways: either
by finding the steam evaporated in terms of the tank pressure, or by
finding the temperature of the flame pyrometrically.
“4. What speed of water circulation best conduces to steam
generation? A good pump is now installed by which the circulation
can be varied. If water can be put into the boiler just fast enough
to come out dry steam at the other end, the efficiency ought to be
a maximum, but it does not follow that it will be so, for one can
imagine a wet circulation sponging up more heat than one which is
just dry at the end.”
1895
During January and February, 1895, the experiments with boilers and
burners were continued and even better and more uniform results than
those given above were obtained. The boilers of Aerodrome No. 5 were
finally brought to such a state of efficiency, that under favorable
conditions a lift of nearly sixty per cent of the flying weight was
secured. This was much more than was required for flight, but it was
decided to postpone the trials until No. 4 could also be made ready
for a test and the frame of No. 5 could itself be strengthened in
many weak places.
Upon examining No. 4, which had been put aside since the trials in
December, it was found to have rusted so badly throughout and to be
so unfit in every way for trial, that a complete reconstruction of
the whole would be necessary. So many advantages had been gained in
No. 5 by the double midrod type of construction that it was decided
to rebuild No. 4 on a modification of the same plan, as shown in
Plate 11, retaining, however, the same engines which had been used
before.
In this a very guarded return was made to the type which had proved
so unsatisfactory in No. 0, that is, making the hull support rods at
the front and rear for attaching the wings and tail. In this case,
however, the hull was constructed very rigidly, and the tubes at the
front and rear were firmly attached and braced so that they could
withstand a considerable strain without undue distortion. The work on
this frame was completed in March, but the other parts were not in
entirely efficient condition even in May, when the aerodromes were
taken to Quantico for trial. Moreover, it was found that the weight
of this aerodrome had increased far beyond the original estimates.
[p076]
In view of the disasters from trials in the field, due to inability
to obtain automatic equilibrium in flight and to the flexure of the
large wings rather than to defects of the engines, the conditions at
this time, after three years of failure, seemed so nearly hopeless,
that without abandoning the work on these steam aerodromes, I again
had recourse to the early plan of constructing smaller models driven
by India rubber, in which the small wings employed could be made
of the requisite stiffness. Instead of employing twisted rubber,
however, the defects of which had been amply proved in previous
trials, these new constructions were meant to employ rubber directly
stretched and pulling. In this condition the rubber exercises nearly
six times the power in proportion to weight that it does when
twisted, but on the other hand it requires a very strong frame and
subordinate parts.
I spent an inordinate amount of time and labor during this year in
attempting to employ this latter form of construction and finally got
a few useful results from it, but none in proportion to the labor
expended.
During March, Aerodrome No. 5, the frame of which had proved on
test to be radically weak, was completely refinished except for
the wings. The propellers had hitherto been made of wood, but in
May, I commenced a new construction of steel, wood and cloth, on a
plan giving a figure which, though not rigorously helicoidal, was
practically near enough to the theoretical form and was also both
lighter and more elastic than the wooden construction.
On May 8 and June 7 Aerodrome No. 5 was again tried at Quantico, and
although the tests were unsuccessful, in that the aerodrome failed
to fly, partly because of the fact that so much time was spent in
raising steam that practically the entire supply of fuel and water
was exhausted before the aerodrome was actually launched, yet it had
come so much nearer flying than any machine had previously done,
that it was felt that if either the power could be increased or the
weight decreased even a slight amount, the aerodrome would probably
fly. In view of the great care that had been exercised in keeping
down the weight, it seemed almost hopeless to attempt to reduce it,
and it also seemed equally hopeless to attempt to get more power
without increasing the weight. However, something had to be done to
increase the ratio of power to weight, and as it was seen that this
would involve extensive changes in No. 5, it was decided to entirely
rebuild No. 4 with this idea in view, though it was evident that it
involved a plan of construction even lighter than the dangerously
light plan on which No. 4 had already been constructed.
During Mr. Langley’s absence in Europe in the summer, Aerodrome
No. 4 was entirely reconstructed and made to embody many new
characteristics, the changes introduced being so radical that
this model was henceforth designated as “New No. 4.” The new
characteristics of this model were its unprecedentedly [p077] light
frame and the elevation of the transverse frame 12 centimeters
above the midrod, whereby the position of the line of thrust was
raised so that it was 20 centimetres from the center of pressure,
which from theory seemed to be very nearly its correct position.
The total flying weight was but 6400 grammes (14 pounds), with a
total supporting surface of fifty-four square feet, equivalent to
very nearly four square feet per pound. It was hoped that with
this extremely light construction the “dead lift” would amount to
a large percentage of the flying weight, and as much as sixty per
cent was actually lifted on the pendulum. As, however, the aerodrome
approached completion it became more and more evident that the
construction was hopelessly fragile, the frame being scarcely able
to support itself in the shop. By November this conclusion became
certain, and this aerodrome (New No. 4) was never put to an actual
test in the field. The very expensive set of wings covered with gold
beater’s skin, which were also constructed at this time for this
model, proved so weak under test that they were entirely abandoned.
When Mr. Langley returned to Washington in the fall, many important
points, which had been under special consideration during the past
year, particularly those relating to the disposition of sustaining
surfaces, and the provision of automatic equilibrium, were still
not definitely determined. It was not yet decided whether two sets
of wings of equal area should be used for the aerodrome, or what
the efficiency per unit of area of the following surfaces was in
comparison with the leading surfaces. To aid in determining these and
other important points concerning the relative position of the center
of gravity and the center of pressure in the horizontal planes, he
had several small gliding models made, which could be used with
either one or two pairs of wings, and afforded an opportunity for
testing and comparing several types of curved surfaces.
These models were built so that the center of gravity could be
adjusted to any desired point, and had in addition, as a means of
assisting in preserving equilibrium, a small tail-rudder, shaped
somewhat like a child’s dart, which was intended to support no part
of the weight.
The tests with these models were very satisfactory and aided greatly
in the final development of what is known as the “Langley type.”
Indeed, in the single month of November all the points, which had
hitherto been more or less indefinite, were finally decided upon, and
the tests of the following spring proved these decisions correct.
Two sets of wings of equal area were hereafter provided for every
aerodrome, which not only greatly increased the stability, but also
overcame the difficulty hitherto experienced in bringing the ‹CP›
over the ‹CG›. The tail-rudder, formed of planes intersecting at
right angles, was adopted as the means of control. In use on the
aerodromes it was set at a negative angle, and given a certain [p078]
degree of elasticity, which was at first provided in the frame of
the rudder, but was later given by a flat wooden spring, by which it
was attached to the aerodrome. The tail in this form now became the
sole means of controlling the equilibrium, and the results obtained
with it were so very satisfactory that no further attention was given
either to the gyroscopic control built during the previous summer, or
to any of the electrical forms of control constructed prior to that
time, all of which involved more or less delicate apparatus.
The definite form into which these ideas crystallized is perhaps best
exemplified in the letter of instructions issued by Mr. Langley on
November 30, 1895 to the men employed on the work. The text of this
letter is given in the Appendix, and the forms referred to in it for
recording the weights and adjustments of the aerodromes are those
used in the data sheets after this time.
In October work was resumed on Aerodrome No. 5, on which nothing had
been done since its test on June 7. The reconstruction of “Old No.
4” into “New No. 4” which had occupied the entire summer, and the
final result of which was the production of a machine so radically
weak as to be useless, had been so discouraging that it seemed vain
to attempt in any way to decrease the weight of No. 5. The addition
of the rear wings in place of the tail had, however, so greatly
increased the supporting surface that it seemed possible that No. 5
might now be able to fly with no greater engine power than it had on
June 7. Some weak places in its frame were, therefore, strengthened
and the midrod at the front was raised five centimetres in order to
raise the center of pressure farther above the center of gravity
and give the front wings a greater range of adjustment. Some slight
changes were also made in the gearing which drove the pump, so as
to make it work faster, and new burners, boilers and a gasoline
tank were constructed during November. Later the midrod, which had
formerly consisted of two separate pieces attached at the front
and rear respectively of the main frame, was made continuous, and
in order to avoid passing it through the smoke-stack, the stack
was made to fork at this point. These changes are clearly shown in
Plates 14 and 15, which are photographs taken on December 3. This
plan was, however, soon changed so that the midrod passed through the
smoke-stack and was rigidly attached to the frame at several points,
and a new pump and new boilers were substituted for those which had
been worn out. Aside from these changes, which although small, added
very materially to the general strength of the frame, no important
changes were made in No. 5 prior to its remarkable flight of May 6,
1896.
[Illustration: PL. 14
AERODROME NO. 5, DECEMBER 3, 1895. PLAN VIEW. RUDDER REMOVED]
[Illustration: PL. 15
AERODROME NO. 5, DECEMBER 3, 1895. SIDE VIEW]
While these changes were being made in No. 5, similar ones were
also being carried out in New No. 4, and the addition of the rear
wings to No. 4, together with other slight changes, made it such a
distinctively different machine from what it had been, that it was
now designated as No. 6. After making extensive [p079] repairs
to the extremely light frame of No. 6 (formerly New No. 4) it was
thought to be in suitable condition for flight and was accordingly
boxed preparatory to sending it to Quantico.
The year, therefore, closed with No. 6 apparently in condition for
test, but it was decided not to take it to Quantico until No. 5,
which was still undergoing repairs, could also be got ready.
1896
A few days after the beginning of the new year, while the repairs on
No. 5 were being completed, it was decided that the frame of No. 6
which had been boxed ready to be carried into the field for trial,
was so weak that before putting it to an actual test in flight it
would be best to make some tests on the strength of its frame. While
testing the frame for torsional strength, it broke under the moderate
test of a weight of 500 grammes placed at the tips of the wings, the
angle of deflection just prior to its breaking being 35°, while the
frame of Old No. 4 in March, 1895, had shown a deflection of only
10.5° under a similar test. This breaking of the frame showed very
plainly that the worst fears in regard to it had been realized and
that by some means or other the frame must be strengthened. This was
finally accomplished by making the midrod continuous through the
smoke-stack as had already been done in No. 5, and at the same time
an additional improvement was made in the means of attaching the
Pénaud tail, whereby it was lowered in order to give it a greater
clearance in passing under the launching car in actual test. Later
the boilers proved defective and new ones were substituted, but
except for some minute details no further changes were made in
Aerodrome No. 6 prior to its test in May.
On May 6, No. 6 was unsuccessfully tried at Quantico just prior to
the very successful test of No. 5. In this test no serious damage
was done to the frame, but before going to Europe in the summer, Mr.
Langley ordered that both aerodromes be completely overhauled and put
in condition for further experiments in the fall. In this remodelling
practically no changes were introduced in the frame of either No. 5
or No. 6, but the engines of No. 6 were refitted and a new boiler was
substituted, which, with slight improvements in the burner, resulted
in a somewhat increased power in the engines.
A complete description, giving all essential details of both
Aerodromes Nos. 5 and 6, will be found in Chapter X.
[p080]
CHAPTER VIII
HISTORY OF CONSTRUCTION OF SUSTAINING AND GUIDING SURFACES
OF AERODROMES 4, 5 AND 6
INTRODUCTION
In some early experiments in 1887 with the small models without motor
power, which have not been particularly described, two pairs of
wings, in the same plane, were employed for reasons connected with
stability. Afterward, in many of the rubber-driven motor models,
which have been described in Chapter II, two large front wings
were employed and the following pair were diminished into what may
properly be called a tail. This plan was a retrogression in design,
and it was pursued by the writer with a pertinacity which was not
justified by the results obtained, being used even on the early
rubber-driven models.
In this construction, it will be observed that the flat tail was
in fact not only a guiding but a sustaining surface, since it bore
its own share of the weight. It was not until a much later date
(November, 1895) that the writer returned to his earlier construction
of two pairs of wings in the same plane bearing the whole weight of
the aerodrome, to which was now added a flat tail, whose function
was not to support, but wholly to guide. This was developed into the
final construction by the addition of a vertical rudder or rudders.
The present chapter is not concerned with the history of the earlier
attempts with small models, or of those numerous constructions of
sustaining surfaces which were never put to actual trial; nor does it
give any description of the experiments which were made in placing
one set of surfaces over the other, according to a method suggested
in “Experiments in Aerodynamics.”[27]
The experiments in “Aerodynamics,” and the theoretical considerations
given in Chapter V on sustaining surfaces, would never alone have led
to the construction which was finally reached, which was largely due
to the hard lessons taught by incessant accident and failure in the
field. The present chapter, therefore, should be read in connection
not only with the pages of “Aerodynamics,” but with Chapters V and IX
of this book.
[Illustration: PL. 16
EARLY TYPES OF WINGS AND SYSTEMS OF GUYING]
It is to be remembered that, while the center of gravity of the
aerodrome could be determined readily and exactly, the center of
pressure could be determined only approximately in advance of trial
in actual flight. The positions [p081] of the supporting surfaces
given in this chapter are, then, approximations made from rules for
“balancing,” ‹i. e.›, for obtaining equilibrium in actual flight,
rules which are in fact tentative, since they are founded on ‹a
priori› considerations with partial correction from the empirical
knowledge gained by previous field trials. For these rules see
Chapter VI.
1893
With reference to the supporting and guiding surfaces of Aerodromes
Nos. 4, 5, and 6, Aerodrome No. 4, in its earliest condition
mentioned in the preceding chapter, was taken into the field, but
never brought to trial in the air. It is sufficient to say that in
the largest of the three sets of wings constructed, each wing was
17×51 inches, and therefore contained about six square feet, so that
with the tail (which was at this time a supporting surface), whose
area was one-half that of the two wings, the total supporting surface
was 18 square feet, or since the flying weight was 9.1 pounds, the
proportion of surface to weight was somewhat less than 2 square feet
to the pound. The wings were at this time ribless, it being expected
that the silk cover which was purposely left loose would take its
curve from the air filling it, which subsequent experience has
shown would have led to certain disaster if the aerodrome had been
launched. It may be added that there was a vertical rudder of what is
now seen to have been a wholly inadequate size. These remarks may be
applied with little modification to the attempted flight with No. 4
on May 25, except that the vertical rudder had been made larger, but
was still much too small.
1894
From the account of the field trials to be given in Chapter IX, it
will be seen that in numerous attempts at flight prior to October
6, 1894, the cause of failure can in every instance be traced to
imperfections more fundamental than those of the sustaining surfaces,
either the launching device or some other part failing to work
satisfactorily. I therefore commence a description of the sustaining
surfaces with those of Nos. 4 and 5 as used on that day.
The construction of the wings of No. 4 and No. 5, which were nearly
identical, is shown in Fig. ‹A› Plate 16. A rod of hickory, tapering
from 1/2 inch in diameter at the larger end to 1/4 inch at the
smaller, was steamed and bent, as shown in the drawing, to form the
main front rib of the wing. This was firmly clamped to the midrod,
and to the rib in turn were attached a number of cross-ribs of
hickory, slightly curved, the inner one of which was fastened to the
hull at its inner extremity, while the whole was covered with silk.
The length of each wing was 162 cm. (63.75 inches), and the width
54 cm. (21.25 inches). The tail was plane and equal in area to one
of the wings, so that the joint area of the wings and tail was 2.62
square metres (28.2 sq. ft.). [p082]
Each wing was attached to the midrod by a single clamp, different
forms of which are shown at ‹F›, ‹G›, ‹H›, ‹I› (Fig. 16). The clamp
consisted of two short split tubes, into which the main front ribs
were securely clamped by means of screws. They were set at an angle
and united to a grooved frame, by which the wings could be readily
attached to a second piece clamped about the midrod. The tail clamp,
like the wing clamp, was composed of two pieces, sliding one upon
the other, but as the tail formed a single surface, one part was
permanently attached to it. Clamps ‹F›, ‹G› were fitted to aerodrome
No. 4, and ‹H›, ‹I› to No. 5. The wings were set at a diedral angle
of about 150°, but as they were not guyed in any way, this angle in
flight and under the upward pressure of the air probably became much
less. The tail was plane but ribbed like the wings.
[Illustration: FIG. 16. Wing clamps, 1892–1896.]
In preparing the machine for flight, the wings and tail of No. 4 were
set at a very small root angle with the midrod, perhaps not exceeding
3°, but while this angle might be maintained at the firmly held root
of the wing, it was later seen to be probable that the extremity
of the wing was flexed by the upward pressure of the air after
launching, though the full extent and evil effect of this flexure
was not recognized at the time. In the approximative calculations
for “balance,” made at this time, the tail was treated as bearing
1/3 of the weight of the aerodrome, as it was 1/3 of the supporting
area, for though it was recognized that its position in the “lee” of
the wings rendered it less efficient, the degree of this diminution
of efficiency was not realized. A vertical rudder 20 cm.×70 cm. (8
in.×28 in.), with an area of 0.14 metres (1.5 sq. ft.) was used.
[p083]
The particulars of the launch will be found in Chapter IX. In the
present connection, it is sufficient to say that though launched with
the requisite velocity and without accident, it fell into the water
at a distance of about 15 metres (49 feet) with the midrod nearly
horizontal, the combined effect of engines and initial impulse having
in fact kept it in the air for less than two seconds. The true cause
of this failure not then being recognized, it was attributed to the
angle of the wings with the midrod having been too small.
The launch of No. 5 followed almost immediately, but taking warning
by the supposed cause of failure of No. 4, its wings were set at
a root angle of 20°, and a hurried adjustment was made to secure
greater rigidity, the tip being partly secured against twisting by
a light cross-piece, and guyed so that the wing as a whole was not
only at a greater angle, but stiffer than in the case of No. 4.
These changes it was hoped would cause the aerodrome to advance at a
considerable initial angle with the horizontal, and it did so, for
instantly after the launch, as the aerodrome escaped from its bonds
into free air, the inclination of the midrod increased until it stood
at about 60°, when the machine, after struggling a moment to maintain
itself, slid ‹backward› into the water (with its engines working at
full speed) after advancing about 12 metres (39 feet), and remaining
in the air about 3 seconds.
On the whole, the result of the first actual trial of an aerodrome
in the field was disconcerting, for unless the result was due to the
wings being placed in a position wholly unfavorable to support, there
seemed to be no doubt that either the engine power or the supporting
surface was insufficient. Now this engine power was by computation
between three and four times what was necessary to support the
aerodrome in horizontal flight at an angle of 20°, and after making
every allowance for slip, there should have been still an excess of
power for the first flight of No. 4, whereas actual trial indicated
that it was insufficient. But on the other hand, the experiment with
No. 5, which momentarily held its position in the air at an angle of
60°, seemed to indicate that the engine power was abundant, and that
the failure must be traced to some other cause.
As a result of these experiments it was concluded, “that it is
an all-important thing that the angle of the front wing shall be
correct, and that this cannot be calculated unless it is known how
much the tip will turn up under pressure of the weight.” I felt,
then, that I had learned something from the failures as to the
need of greater rigidity of the wings, though how to obtain this
without adding to their weight was a trying problem. It was thus
at an early stage suspected that the evil to be guarded against in
wing construction was the distortion of the form of the wing under
pressure, chiefly by torsion, which is specially hard to provide
against without a construction which is [p084] necessarily heavy.
This suspicion was a correct one, though the full extent of the evil
was not yet surmised.
In the light of subsequent experiment it may now be confidently
stated that the trouble was with the wings, which at the moment
after launching were flexed wholly out of the shape which they were
designed to have, and which they retained up to that critical moment.
After returning to Washington, one of the wings was inverted, and a
quantity of sand, equal in weight to the pressure upon the wing in
flight, was added, under which the yielding at the tip amounted to
65°, or from +20° to −45°, showing that the wings were entirely too
weak to sustain the aerodrome.
In speaking of the efforts to strengthen the wings, it must be
constantly remembered that this could hardly be done in any way which
did not involve increased weight; that is, it could hardly be done at
all, since increased weight was forbidden.
The first attempt at systematic guying was made on October 27. As
shown in Fig. ‹B›, Plate 16, two guy-posts extending beneath the
midrod were connected by guy-wires with the outer extremities of
the wing, by means of which it was sought to hold the wing in place
and prevent its extremity from twisting upward, while a third wire
connecting with the bowsprit prevented its moving backward. In
addition, two aluminum wires, stretched across above from wing to
wing, kept the lower guys tight.
On October 27, Aerodrome No. 5, equipped with large new wings and
tail, having a combined area of 3.7 square metres (40 sq. ft.), the
wings being each 64 cm.×192 cm. (25.25 in.×75.75 in.), turned sharply
and completely round, apparently through some internal current of
the main wind against which it was advancing. Owing to this almost
instantaneous turn, it lost headway and came down. This led to the
subsequent construction and use of a much larger vertical rudder,
intended to prevent in future any such sudden pivoting and consequent
loss of momentum. The wings showed a tendency to “pocket”[28] and
bag, which indicated some serious fault in their construction.
As a result of these experiments, it was decided on October 29 to
attempt to make the wings stiffer (though their weight was almost
prohibitory), by inserting more cross-pieces, cross-pinning and
guying them so as to make them more rigid as a whole, and less liable
to pocket.
At this time an automatic device in the form of a sliding tail was
designed, which it was thought would cause the center of pressure to
move backward when the aerodrome reared, and forward when it plunged
downward, but the device, though afterward constructed, was never
brought to trial in the field.
Aerodrome No. 5, equipped with a new set of wings similar to those
used [p085] on October 27, and guyed as in the previous experiment,
was again launched on November 21, with the results recorded in
Chapter IX. The failure was attributed to the twisting of the wings
under pressure to such an extent that not only was their effective
area greatly reduced, but the outer portions were upturned so as to
catch the air upon the upper surfaces, the result being in part a
downward pressure.
On the following day a pair of the wings was inverted and a weight
of sand equal to the air pressure to which they were subjected in
flight, was distributed over their surfaces. Under the action of
this, the twisting of the wing was seen to increase from the root,
which was held with comparative rigidity, up to the tip, where in
spite of the cross-ribs it amounted to 45°. The resistance to torsion
lay chiefly in the front rib, which, in addition, could be bent
easily, allowing the surface to become distorted with great loss of
lifting power.
The experiments of 1894 had demonstrated the urgent necessity
for greater rigidity in the sustaining surfaces, which might, as
it seemed, be obtained either by increasing the strength of the
framing (which meant additional weight) or by resorting to some new
and untried construction, or by a proper system of guying. Guying
seemingly offered the most feasible solution of the problem; but
although the system of wire guying was thoroughly tried, the result
was very unsatisfactory, as the wings continued to twist and bag in a
way that was extremely discouraging.
1895
I accordingly had recourse in 1895 to the system of wooden guy-sticks
shown in Fig. ‹D›, Plate 16, which necessarily added greatly to
the weight of the sustaining surfaces. Each wing was separately
strengthened by means of a light rod of spruce, in cross-section
about the size of the main front rib, extending across the upper
surface of the wing, at a distance of about one-third the width of
the wing behind the front rib. It was tied to each of the cross-ribs
and to the outer bent portion of the front rib, and at its root was
fastened to the frame of the aerodrome.
This effectually prevented the bending of the front rib and the
consequent bagging of the cover, and to that extent marked a decided
advance in wing construction. But it was faulty, in that, not being
supplemented by wire guying, it offered little resistance to the
twisting of the wing about the main front rib, the rear tip of the
wing being free to turn up under pressure, as it had done on former
occasions. A similar guy-stick was stretched across the tail. To
guard against torsion, rods extending diagonally across the wings and
tail were used, which, with the aid of the guy-sticks just described,
prevented the surfaces from twisting greatly. In addition, a rod
joining the front ribs and stretching across from wing to wing tended
to maintain a fixed diedral angle. [p086]
The wings as thus guyed were rigid enough, and in the field-trials of
No. 5 on May 8 and June 6, did not yield noticeably under pressure,
and there seemed to be no serious default in their lifting power, but
the guy-sticks were heavy and the system was not again employed. The
wings used in these trials, shown in Fig. ‹C›, Plate 16, had a frame
of hickory, consisting of a front rib and nine cross-ribs, over which
the silk was tightly stretched. The curvature of the wings, which is
shown in the cross-sectional drawing, had a rise of about one-twelfth
the width, the highest point of curvature occurring about one-fourth
the distance from front to rear. Each wing was 64 cm.×192 cm. (25.25
in.×75.75 in.), the two with the tail, in surface equal to a single
wing, having an area of 3.7 square metres (40 sq. ft.). The combined
weight of the wings was 1150 grammes (2.53 pounds), and of the tail,
583 grammes (1.28 pounds).
The evolution of a vertical rudder had meanwhile been going steadily
forward. Those first used had been small, rectangular, stiff, and
heavy, but in the experiments of May 8 a much lighter and larger
construction, consisting of a frame 92 cm.×76 cm. (36 in.×30 in.)
covered with paper, was used, and on June 7 this was replaced by a
long, diamond-shaped rudder, having a spruce frame covered with silk,
very light and seemingly more effective than any hitherto used.
I had in the meantime designed a “tail-rudder,” consisting of a
horizontal tail and vertical rudder combined, each having an area of
about 0.6 square metres (6.5 sq. ft.) which, however, was not used
until 1896.
In August was begun the construction of a deeply curved and arched
pair of wings for No. 4, which consisted of a light framing of
spruce elaborately guyed and covered with gold-beater’s skin drawn
tight as a drum-head with pyroxelene varnish. In their construction
a new feature, foreshadowed in the method of guying the separate
wings used in the field-trials of May and June, was introduced,
which was adopted in all subsequent constructions--the guy-stick,
previously described as stretching lengthwise across the wing being
now made a part of the wing itself, which was thus provided with
two longitudinal ribs instead of one. The additional rib occupied a
central position, and like the front rib was attached to the midrod
by means of a strong wing clamp. Its outer end was united to the
front rib, which was here bent into a quadrant of a circle. This pair
of wings had an expanse of 435 cm. (14.3 feet), an area of 2.5 square
metres (26.8 sq. ft.), a weight of 660 grammes (1.45 pounds), and a
depth of curvature equal to one-tenth their width.
This construction offered a two-fold advantage in its resistance
to both torsion and bagging, for as the pressures upon the wing
were nearly balanced about the middle rib, the tendency to twist
was reduced to a minimum, while the bagging, which results from the
bending of the framework, as distinct from [p087] its twisting, was
greatly reduced by the manner in which the frame was put together,
the whole construction permitting a return to the system of wire
guying at first adopted, which had been found inapplicable to a wing
having but a single longitudinal rib forming its front margin. When
completed, the wings were strongly guyed with piano wire, both above
and below, to guy-posts attached to the midrod, and each cross-rib
was separately guyed with wire chords. Although these wings had cost
much in time and labor, and contained many points of improvement,
they were eventually found to be too weak to support the aerodrome,
and were therefore abandoned without a trial in the field.
For the plane horizontal tail hitherto used a pair of curved wings
was substituted, similar in all respects to those just described, but
having only half their area, and these were later replaced by a pair
equal in size and in every way the counterpart of the front wings.
The tail as hitherto used accordingly disappeared, and gave place
to another having a wholly different function to perform; for while
the old tail, like the rear pair of wings which superseded it, was
intended to bear a definite part of the weight of the aerodrome, the
new tail which was now added behind the rear pair of wings was not
supposed to bear any part whatever of the weight, but to act solely
as a guide, and this new feature, first introduced in October, 1895,
was continued to the end.
This arrangement of the surfaces is quite different from that adopted
by Pénaud in 1872, in which the tail became automatic in its action
through its small angle of elevation as compared with that of the
wings, while still acting as a supporting surface, whereas in the
present arrangement the function of the tail was solely one of
guidance. This, I believe, was one of the important changes which
perhaps as much as any other led to final success.
During the fall of 1895 a large number of experiments were made both
in free flight with gliding models, and in constrained flight with
the whirling-table, to determine the relative lifting power of the
front and rear wings per unit of area, and from these the following
new rules were deduced for finding the center of pressure:
If a following wing is the size of the leader, assume that its
efficiency is 66 per cent per unit of surface.
If it is half the size of the leader, assume that its efficiency is
50 per cent per unit of surface.
If it is half as large again as the leader, assume that its
efficiency is 80 per cent per unit of surface.
For intermediate sizes of surface, proportionate values per unit of
surface may be assumed.
If we consider the area of the front wing to be unity, and that of
the rear wing to be ‹n›, and if ‹m› be the efficiency of the rear
wing per unit of surface, [p088] the above is expressed in the
following formulæ, which it will be remembered take account only of
wings following each other in the same or nearly the same plane,
and are not applicable where one wing is either above or below the
plane of the other. In the formulæ, ‹CP› is the resultant center
of pressure upon both wings expressed in the notation described in
Chapter II, ‹CP›_{fw} is the center of pressure of the front wings,
and ‹CP›_{rw} the center of pressure of the rear wings.
If the value of ‹n› lie between one-half and unity,
‹n› + 1
‹m› = -------;
3
while if the value of ‹n› lie between unity and 1-1/2,
6 + 4‹n›
‹m› = --------- .
15.
In either case
‹CP›_{fw} + ‹mn›‹CP›_{rw}
‹CP› = ---------------------- ;
1 + ‹mn›
where the leading and following wings are equal
3‹CP›_{fw} + 2‹CP›_{rw}
‹n› = 1, ‹m› = 2/3 and ‹CP› = ----------------------- .
5
The steady flight of one of the gliding models referred to led to
the construction of a new set of wings for No. 5, patterned after
those used on the gliding model. These wings, shown in Plate 17, were
rectangular in outline, 200 cm.×80 cm. (6.56 ft.×2.62 ft.), each
wing having an area of 1.6 square metres (17.1 sq. ft.) They were
constructed with spruce framing covered with China silk, and were
strongly guyed with piano wire in much the same manner as the light,
skin-covered wings already described, which had preceded them. The
combined weight of the two pair was 1950 grammes (4.3 pounds).
The long central rib was now much the larger of the two which, as
in the preceding wing, formed the foundation of the structure. It
occupied a position two-fifths the distance from front to rear, and
presumably coincided at all points with the center of pressure of
fore and aft sections of the wings, so that the pressure in front of
the rib was at all points balanced by the pressure in the rear, and
there was consequently little tendency in the wing to twist under
pressure of the wind. The two main ribs were rigidly connected by
cross-ribs of spruce, 20 cm. (8 inches) apart, steamed and bent to
the desired form. The curvature of these ribs was the same for all,
and in depth was one-twelfth the width of the wing, while the highest
point of curvature was one-sixth of the distance from front to rear,
these ratios having been chosen as approximating those found in the
wing of the soaring bird. These wings were subsequently used in the
first successful flights of the following year.
[Illustration: PL. 17
AERODROME NO. 5. PLAN OF WINGS AND SYSTEM OF GUYING]
[p089]
During the year 1895 but two field-trials were made with the steam
aerodromes, and neither of these was successful; but a great step
forward had been taken in the construction, guying and arrangement
of the sustaining surfaces. The wings had been made stronger with
no increase in weight per unit of area. On the contrary, the ratio
of weight of sustaining surfaces to area had been actually reduced
from 43 to 28 grammes per square foot, so that the surfaces were both
lighter and stronger.
Two longitudinal ribs had taken the place of the single one before
used, a second wing clamp had been added to correspond to the midrib,
the difficult problem of torsion had been effectually solved, the
system of guying greatly improved, and it appeared that in the next
trial the wings might be expected to bear the weight of the aerodrome
without serious distortion.
1896
In January, 1896, two new pairs of wings were designed for No. 6,
and in order to give a greater efficiency to the rear wings, they
were made larger than the front ones, the area of the latter being 22
square feet, and of the former 27 square feet, and whereas the width
of each wing had formerly been one-third of its length, it was now
increased to two-fifths to correspond to those of No. 5.
The progress made in construction and guying is shown by the fact
that when on January 28 one pair of the wings of No. 5 was inverted
and sanded, the yielding at the tip was less than 5° greater than at
the root, whereas at one time it had been 65°. A similar test applied
to a pair of wings of No. 6 on March 4 gave even better results, as
the yield at the root was but 1° 45′, and at the tip 2° 30′.
The successive stages of the development of the wing clamps are shown
in Fig. 16. In its final form the front wing clamp, or that which
held the main front rib, shown at ‹AB› (1896), had adjustable sliding
pieces, by means of which the wings could be set at any desired angle
of elevation, the wing as a whole revolving about the rear wing
clamp, shown at ‹CD› (1896).
The general system of guying the wings, as shown in Plate 17, had
been greatly improved. In the present form a bowsprit and guy-posts
firmly attached to the midrod furnished points of attachment for
the piano wires with which the wings were guyed and held rigidly in
place, other wires being stretched across from wing to wing so as to
maintain them at a constant diedral angle of about 150°. The clamps
by which the guy-posts were attached to the midrod, are shown at ‹EF›
(Fig. 16).
In the successful flights of No. 5 on May 6, the completed wings
already described weighed together 1950 grammes (4.29 pounds), and
had a total sustaining area of 6.4 square metres (68.8 square feet),
the flying weight of the [p090] aerodrome was 11,775 grammes (26
pounds), and the sustaining surfaces therefore amounted to 2.6 square
feet to the pound, which, as the event proved, was amply sufficient.
The “tail-rudder,” shown in Plate 17, comprised a vertical and
horizontal surface of silk intersecting in a central rod or axis,
having a length of 115 cm. (3.8 feet). The framing was of spruce and
consisted of two sets of four arms, each radiating from the central
rod, the hexagonal outline of the surfaces being formed of piano
wire, over which the silk was drawn and sewed. The area of each
surface was about 0.6 square metres (6.45 square feet), and the total
weight was 371 grammes (0.8 pounds).
A flat steel spring inserted in the forward end between the rudder
and the midrod gave it a certain desirable degree of elasticity in
a vertical direction. The rudder was held in place by a pin passing
through the midrod, and was so set as to coincide with the line of
direct flight, its purpose, as already explained, being to guide the
aerodrome, but to take no part in its sustention.
In balancing Aerodrome No. 5 on May 6, the wings were so adjusted
that in accordance with the notation given above, p. 15:
‹CP›_{fw} = 1575
‹CP›_{rw} = 1415.5;
and as the wings were of equal size, from what has preceded in the
present
3‹CP›_{fw} + 2‹CP›_{rw}
‹CP›_1 = ------------------------------ = 1501.2.
5
The center of gravity was located at 1497, so that there should have
been a very slight tendency on the part of the aerodrome to rise, as
was actually the case. The formula was perhaps not quite so accurate
as the prolonged flight of the aerodrome would seem to indicate, as
it takes no account of the thrust of the propellers, which in action
tended to elevate the aerodrome in front while their resistance
would tend to depress it when they had ceased to revolve, which
consideration accounts for the action of the aerodrome on May 6, as
described in Chapter IX. The formula may, however, be regarded as
approximately correct.
In the final successful trial with No. 6 on November 28, 1896, the
wings used were similar in general construction and manner of guying
to those of No. 5 on May 6, but, as shown in the photograph (Plate
29A, Chapter X), the front rib at its outer extremity was bent to a
quadrant to connect with the midrib, this construction being somewhat
stronger than that adopted in the wings of No. 5. The curvature was
but one-eighteenth of the width of the wing instead of one-twelfth as
in No. 5. The front and rear pairs were similar and equal and had a
combined area of 5 square metres (54 sq. ft.), and a weight of 2154
grammes [p091] (4.74 lbs.). The flying weight of the aerodrome was
12,120 grammes (26.7 lbs.), the sustaining surface thus amounting to
slightly more than 2 square feet to the pound.
The position of the wings, in accordance with the notation adopted,
was
‹CP›_{fw} = 1563.2,
‹CP›_{rw} = 1374.
Since the wings were equal in size,
3‹CP›_{fw} + 2‹CP›_{rw}
‹CP›_1 = ----------------------------- = 1487.5.
5
The center of gravity was located at 1484, which was 3.5 cm. in
the rear of the center of pressure. The flight was approximately
horizontal, and the setting seems to have been as accurate as could
be desired. The angle of elevation of the wings at the root was 10°
30′, and so well were they guyed that there was no visible yielding
at any point during the flight. As the midrod during flight was
approximately horizontal the angle of elevation of the wings may be
taken as 10° 30′; the efficiency of the rear wings was two-thirds
that of the front wings, and the effective area was therefore
27+27×2/3 = 45 square feet.
The wings being very nearly plane we have therefore the data for
determining the soaring speed from the formula of “Aerodynamics”
(Chapter VI, p. 60).
‹W› = ‹P›_α cos α = ‹k›‹A›‹V›^2‹F›(α) cos α,
in which ‹W› = 26.7 pounds; A = 45 sq. ft.; ‹k› = 0.00166; α = 10°
30′; ‹F› (α) cos α = 0.353. By substituting these values in the
formula we obtain ‹V› = 32 feet per second.
The speed actually attained, however, was about 30 miles an hour,
or 44 feet per second, which seems to indicate that the angle of
elevation under pressure was reduced to much less than 10° 30′. For a
velocity of 44 feet per second, the theoretical value of α would be
but 6°. In this calculation, however, the hull resistance and that of
the system of guy-wires, which must have been comparatively large,
has been omitted. It would appear, therefore, that the actual results
obtainable in flight are much more favorable than calculations based
on experimental data would presuppose.
[p092]
CHAPTER IX
HISTORY OF LAUNCHING APPARATUS AND FIELD-TRIALS OF
AERODROMES 4, 5 AND 6
LAUNCHING APPARATUS
I have elsewhere mentioned that the difficulties of launching even
a very small model aerodrome are considerable. Early experiments
were tried with an apparatus something like a gigantic cross-bow,
and in later years with various forms of pendulum, all of which
latter brought out the inherent theoretical defect of the movement of
rotation of the aerodrome, and were otherwise practically inefficient.
A device, consisting of two pendulums, one behind the other,
connected by a rigid rod, from which the aerodrome could be
suspended and cast off without rotation, was at one time considered,
but abandoned. Experiments were also made with several forms of
railroad, upon which the aerodrome was to run up to the moment of
release, before the form of launching apparatus, which finally proved
successful, was adopted.
All these had failed chiefly for two reasons; first, it was difficult
to cause the aerodrome to be released just at the moment it attained
sufficient speed to soar; second, the extensive surface presented to
the wind by the wings of the aerodrome, made it necessary to provide
means for holding the machine securely at several points up to the
moment of release without danger of interfering in any way with
the aerodrome when it was cast into the air. This proved a serious
problem, which can be appreciated only by one who has seen such a
machine in the open air, where its wings are subject to movement
and distortion by the slightest breeze. The steps by which these
difficulties were removed and the final type of launching apparatus
perfected are recorded in the following pages in connection with the
field-trials of the model aerodromes.
1892
As the end of the year 1892 approached and with it the completion of
an aerodrome of large size which had to be started upon its flight in
some way, the method and place of launching it pressed for decision.
One thing at least seemed clear. In the present stage of experiment,
it was desirable that the aerodrome should-—if it must fall-—fall
into water where it would suffer little injury and be readily
recovered, rather than anywhere on land, where it would almost
certainly be badly damaged. [p093]
The shores of the Potomac on both banks were scrutinized for this
purpose, from a point about two miles above Washington to below
Chopawamsic Island, some thirty miles below the city. Several lofty
and secluded positions were found, but in all these there was the
danger that the aerodrome might be wrecked before reaching the
water, or, turning in its course, fly inland; but more than this, it
could be launched only on the rare occasions when the exact wind was
blowing which the local conditions demanded.
Finally, the idea, which seems obvious enough when stated, presented
itself of building a kind of house-boat, not to get up initial motion
by the boat’s own velocity, but to furnish an elevated platform,
which could be placed in the midst of a considerable expanse of
water, if desired, under conditions which admitted of turning in the
direction of the wind, as it need hardly be repeated that it was
indispensable to the machine, as it is to the bird, to rise in the
face of a wind, if there be any wind at all.
The house-boat in question was nothing more than a scow about 30 feet
long by 12 feet wide, upon which a small house was erected, to be
used for the occasional storing of the aerodromes. On account of the
accidents which were certain to occur in the first attempts, it was
fitted up with the means of making small repairs. On the roof of the
house there was a platform upon which the operator stood when making
a launch, and upon which were mounted the launching devices hereafter
described.
This boat, shown in Plate 18, was completed in November, 1892.
1893
By the kindness of the Superintendent of the Coast Survey, the
house-boat was towed in May, 1893, down to Chopawamsic Island, a
small island near the western bank of the Potomac River, not far from
the Quantico station of the Washington and Richmond Railroad Company.
A map of the island and the adjacent land and water is shown in Plate
19.
The house-boat was at all times moored somewhere on the west side of
the island, in the stretch of quiet water between that and the west
shore of the river. The waters shown here are, with the exception of
a narrow channel, very shallow, and, indeed, partly dry at low tide,
so that there was no danger of an aerodrome being lost, unless its
flight carried it a long distance away and over the land.
FIELD TRIALS[29]
Aerodrome No. 4, as shown in Plate 11, had a single midrod, a flying
weight of 9 pounds,[30] and supporting surface, consisting of wings
and tail, of 18 square feet. [p094] Its engines, with about 100
pounds pressure, developed an aggregate of 0.4 H. P., and lifted 50
per cent of the flying weight. The propellers were 60 cm. (2 feet) in
diameter and 1-1/4 pitch ratio.
The aerodrome was intended to be launched by a contrivance called the
“starter,” which was an inclined rod, hinged at the bottom, on the
top of which the aerodrome was supported on a rod which was thrown
down at the instant of flight, giving the aerodrome a slight forward
impulse, with the expectation that it would get up sufficient initial
speed to soar from the action of its propellers.
On November 18 the writer (L), with Dr. Barus (B) and the two
mechanics (R and M), went to Quantico by an early train, and
superintended with interested expectation the arrangements for this
first trial in the open air of the mechanism which had now been over
two years in preparation.
We met with an unexpected difficulty—that of launching the aerodrome
at all, for though the wind was only a very gentle breeze, it was
only by holding it down with the hands that it was possible to keep
the aerodrome in position for the launch, during the few minutes
which passed from the time it was placed upon the apparatus to the
time of releasing it. Whether the launching device itself might be
effective or not could not be ascertained, since it was found that
nothing which could even be called an attempt to launch could be made
except in an absolute calm; a condition of things very difficult for
any one to understand who has not passed through the experience. The
writer returned to Washington at the close of the day without having
done anything, but having learned a great deal.
November 20. L, with B and M, came down again, and waited until 4.20,
when, the breeze having fallen to almost a calm, the aerodrome was
maintained in place on the launching apparatus with great difficulty,
while it was repeatedly set on fire by the scattering liquid fuel.
Finally it was let go, and fell close to the house-boat, the tail
striking the edge of the platform. The immediate cause of failure was
the defective launching apparatus, for the design of which the writer
felt himself responsible.
November 24. L, with B, M, and R came down again to Quantico, but the
very moderate wind proved completely prohibitory to any attempt at
launching, and all returned again to Washington.
November 27. L, with B and M, came down to try a new launching
apparatus, not different in principle from the preceding one, but
of better construction. The morning was exceptionally calm, but the
engines were found to be out of order, and precious time was spent
in slight repairs which should have been made in the shop. At 3.30
p. m., when the engines were at last ready, the exceptional calm
gave place to a very gentle and almost imperceptible breeze, [p095]
which, nevertheless, again proved prohibitory to the launching, and
with extreme disappointment the party returned to Washington, it
being at last fully recognized that unless some ways were found of
holding down all the extended supporting surfaces upon the launching
piece, and at the same time of firmly clamping the body of the
aerodrome until it could be dropped, as well as of releasing all
this simultaneously at the critical instant, no attempt at launching
was likely to succeed except in such an entire and perfect calm as
rarely occurs. Independent of this launching difficulty, some way of
protecting the fires from the wind had to be found, which was by no
means easy, since an efficient protection meant an enclosure of them
and a diminished influx of air, of which it was essential that there
should be an unlimited supply.
December 1. L, with B, R, and M proceeded to Quantico. The same
conditions presented themselves and the party returned, without
effecting anything.
December 7. L, B, R, and M present; day overcast but perfectly calm.
Taught by experience, we had everything ready, and a little after one
o’clock the launch was made. The aerodrome fell directly into the
boat, the rod of the starter having broken. It was little damaged,
but in view of the injury and the rising wind, all other attempts
were abandoned for the day.
December 11. Present, L, with B, R, and M. A new “starter” had been
devised and brought down, but was not yet quite ready for use, and
an attempt was made to employ the old one with the improvements
suggested by experience, but, after two attempts to launch, the work
was abandoned for the day, owing this time not to the launching
apparatus, but to troubles in the engines and pumps, due probably to
injuries received in the fall of the 7th, which were not detected
until the time of the actual trial.
December 20. L, with B, M, and G, present; engine and aerodrome
in order and everything apparently favorable. What seemed to be
an almost entire calm came toward evening, yet once more the all
but imperceptible breeze which prevailed was found to defeat all
arrangements for holding the aerodrome to the launching ways before
it was let go.
Trips to Quantico were also made on November 24, and December 1
and 21, of which no account is given as the very moderate wind
which prevailed in each case precluded any attempt at launching the
aerodrome.
It will be seen that eight trips were made to Quantico, and that, far
from any flight having been made, not once even was the aerodrome
launched at all. The principal cause for this lay in the unrecognized
amount of difficulty introduced by the very smallest wind,
irrespective of the unfitness of the launching apparatus to give the
desired initial speed and direction.
In all these trials, the aerodrome rested on the launching apparatus,
by which it was projected forward by means of a spring in such a way
as not to interfere with the propellers. [p096]
Previous tests with the rubber-driven models had demonstrated the
futility of all simple pendulum types of “cast off,” and likewise all
the trials hitherto of a railroad form of launching apparatus, in
which the aerodrome was mounted on a car, which had itself to get out
of the way, were equally failures, so that when the device referred
to above proved to be worthless, it seemed that almost every plan had
been exhausted. There were, moreover, other difficulties, some of
which have been indicated above, such as that of making the burners
work properly in even a moderate wind during the very short time
required for attaching the wings and so adjusting the aerodrome on
the launching apparatus.
These difficulties, which, now that they have been overcome, seem
difficulties no longer, but which then seemed insuperable, were all
connected with the ever-present problem of weight. It would have
been easy to make rigid sustaining surfaces which would not bend in
the wind; to make fires which would not go out; and easy to overcome
all the impediments which seem so trivial in description and were
so formidable in practice, were it not that the mandate of absolute
necessity forbade this being done by any contrivance which would add
to the weight of an already phenomenally light construction. The
difficulties of the flight as they were seen in the workshop were
multiplied, then, beyond measure by the actual experiments in the
field, and the year closed with a most discouraging outlook.
1894
The new year began without any essential improvement in the means
already described, though a new launching apparatus had been devised
by the writer, which was scarcely so much an apparatus for launching,
in the ordinary sense of the word, as one of holding the aerodrome
out over the water, and simply letting it drop from a height of
about 25 feet, during which fall it was hoped (exact data being
unobtainable in advance of experiment) that there would be time for
the propellers to give the aerodrome the necessary soaring speed
before reaching the water. This device consisted of an inverted
tripod, which held the aerodrome comparatively steady by three
bearing points, while a cross-bar of wood was added to prevent the
wings from swaying before the launch. Previously, the supporting
surfaces, wings and tail, had been put on only at the last minute.
Now it became possible to keep them on in a gentle breeze for an
indefinite time before launching.
January 9. The previous day having been spent in practicing the
steps preliminary to launching, so as to avoid delay in assembling
and mounting the aerodrome, the writer, with Dr. Graham Bell, went
to Quantico. The day was calm, and every condition seemed favorable.
The aerodrome was dropped fairly, under full steam, and it fell in
a nearly horizontal position, but touched the water at a distance
of only 50 or 60 feet, evidently before the necessary initial speed
[p097] could be impressed on it by its engines. The conclusion
should have been that by this method nothing but a practically
unsuitable height would suffice to start the aerodrome in a calm,
though it might perhaps be done in the face of a considerable breeze.
May 25. After a considerable interval of delay, due to the river
being closed by ice and other causes, Aerodrome No. 4 was again
dropped from the starter under nearly the same conditions as in
the trial of January 9, and with a quite similar result, the final
conclusion being that this method must be abandoned. It may be added
that a vertical rudder was tried on this day.
June 12. No. 4, with an improved blast, was tried at Quantico, Mr.
Goode being present. The day ended in failure from another cause, the
improved blast, which worked well in the shelter of the shop, but
proved useless in the field, being extinguished by the feeblest wind.
At this time (in June and July) I designed a horizontal railroad
with launching springs and track, underneath which ran a car which
held the aerodrome firmly until the moment of automatic release.
This apparatus finally proved to be the successful solution of the
launching problem. The description given later, with the drawing in
Plate 18, shows the after-improvements, but no specific change from
that in use from the first.
About this time I also arranged for certain changes in the boilers
and burners, having decided that I would not go into the field
without some ground for confidence not only that the aerodrome could
be launched successfully, but that a steady flame could be maintained
under the boilers.
October 6. No. 4, as remodelled, having a flying weight of about 14.5
pounds, a supporting surface of about 28 square feet, with a total
engine power of about 0.5 H. P., and having lifted 40 per cent of
its weight on the pendulum, was taken down the river for trial with
the new railroad launching apparatus, and several days were spent in
erecting the launching apparatus on the house-boat, and in launching
“dummy” aerodromes from it for practice.
Aerodrome No. 4 then being fitted under conditions which apparently
insured a good start (the center of pressure being nearly over the
center of gravity, the root angle of the wing being zero, the midrod
nearly horizontal, the engine working well, and with apparently ample
sustaining surface) was finally successfully launched, but the hopes
which were reasonably entertained proved to be unfounded. The result
of this first actual trial of a “flying machine” in free air was most
disconcerting, for the aerodrome, which had ‹in theory› many times
the power required for horizontal flight, plunged into the water with
its engines working at full speed, after a course hardly longer than
that performed by the dummy. This result was at first inexplicable.
No. 4, then, did not fly at all, from some at first inscrutable
cause, and it was decided to make a trial of No. 5, though it was
hard to put the result of so much [p098] time, painstaking and cost
to the hazard of destruction. With the experience just acquired from
the trial of No. 4, the wing of No. 5 was set at an angle of about
20° with the midrod, and the tip was secured by a light cross-piece,
so guyed that the wing as a whole, while set at this considerably
greater angle with the rod, was stiffer than before. In addition to
this, the air chamber was moved back so that the center of gravity
was from 6 to 10 cm. behind the (calculated) center of pressure.
These changes were made in order to insure that the front should at
any rate keep up, and it did.
The aerodrome was launched successfully with the engines working
under a pressure of 110 pounds of steam. The head rose continually
until the mid-rod stood up at an angle of about 60°, checking all
further advance. It remained in the air in a stationary position for
nearly a second, and then slid ‹backward› into the water, striking on
the end of the rudder and bending it. The distance flown was about 12
metres, and the time of flight 3 seconds. One of the propellers was
broken short off, and the shaft was bent.
It thus became clearly evident that some cause prevented the proper
balancing of the machine, which was necessary to secure even
approximately the theoretically simple condition of horizontal
flight. It was all-important that the angle of the front wing should
be correct, but its position could not be accurately known in advance
of experiment, and this experiment could only be made with the
machine itself, and involved the risk of wrecking it.
These trials gave a very vivid object lesson of what had already
been anticipated,[31] that the difficulties of actual flight would
probably lie even more in obtaining exact balance than in the first
and more obvious difficulty of obtaining the mere engine power to
sustain a machine in the air. The immediate problem was to account
for the totally different behavior of the two aerodromes in the two
flights, under not very different conditions.
Observations of the movement of the two aerodromes through the air,
as seen by the writer from the shore, seemed to show, however, that
the wings did not remain in their original form, but that at the
moment of launching there was a sudden flexure and distortion due to
the upward pressure of the air. The time of flight was too short,
and the speed too great, to be sure of just what did occur, but it
seemed probable that the wings flexed under the initial pressure
of the weight which came upon them at the moment of launching, and
that they were in fact, while in the air, a wholly different thing
from what they were an instant before, so that a very slight initial
difference in the angle at which they first met the air might cause
the air to strike in the one case on the top of the wings and throw
the head down, and in the other case so as to throw the head up.
To ascertain the extent and character of this flexure, caused, it
will be observed, by [p099] the ‹weight› of the aerodrome suddenly
thrown on the wings, I inverted the aerodrome and distributed a
weight of dry sand equal to that of the whole machine evenly over the
supporting surfaces. It was found that under the weight of the sand
the extremity of the wings bent to an angle of 45° downwards (and
consequently must have bent to an angle of 45° upwards in the air),
a condition of affairs worse than anything that had been suspected,
and seeming to demand the entire reconstruction of the wings with
a strength and consequent weight for which there was no means of
providing.
There had been some injuries to the machines in the trials of the 5th
and 6th, and these were repaired. A new float had been made for No.
4, and a new set of larger wings for No. 5. Each of these wings had
a length of 76 inches and a breadth of 25 inches, making the total
surface of the two 26.4 sq. ft., while that of the tail was 13.2 sq.
ft., or about 40 sq. ft. in all.
October 22. When No. 5 was finally prepared for another trial, its
condition was as follows:
Flying weight 22 pounds
Area of supporting surfaces (wings and tail) 40 sq. ft.
Sq. ft. of surface per pound of weight 1.8[32]
Engine power with 115 lbs. steam pressure 1.0 H. P.[33]
Power necessary to soar 0.35 H. P.
Theoretical soaring speed (plane wings at 20°) 24 ft. per sec.
Previous lift on pendulum 40 per cent of
flying weight
October 25. The aerodromes having been taken to Quantico on October
23, and satisfactory experiments made with dummies in order to
test the launching apparatus, the house-boat was carried out into
midstream and moored.
Aerodrome No. 4 was launched in the face of a wind of about 1100
feet per minute. The midrod was at a very small inclination with
the horizontal, about 3°. The angle (α) of the chord of the curved
wing measured at the rod, where it was rigidly held, was 15°. The
adjustment was such as to bring the ‹CG› immediately under the ‹CP›,
without any allowance for the fact that the line of propeller thrust
was below the ‹CP›.[34] The aerodrome under these conditions was
launched with the head high. It made a real, though brief, flight of
about 130 feet in 4-1/2 seconds, when it swung abruptly round through
90°, and, losing headway, sank continuously, finally falling backward
into the water.
October 27. Aerodrome No. 4, having been repaired and guyed with
wires from the wings to vertical guy-posts beneath, was launched
again, but one of the [p100] guy-wires caught on the launching car,
and threw the aerodrome immediately into the water with but little
damage.
On the same day No. 5 was launched. The theoretical ‹CP›−‹CG› was
nominally 0, but, for the reasons stated in the footnote on p. 99,
was really something positive, that is to say, the ‹CP› was really
somewhat in advance of the ‹CG›; inclination of midrod less than α
(=20°). The aerodrome under these circumstances, while keeping its
head up, at first fell rapidly, yet seemed about to rise just as it
struck the water, conveying the idea that if the launching had been
made with a greater initial velocity it would have risen and cleared
the water. The wings visibly pocketed, however, and it was clear that
some better disposition must still be made for them. The flight was
3-1/2 seconds.
No. 5 was tried again on the same day with larger wings, whose area
was 40 square feet. These wings, though stiffer, pocketed a little,
α=20° as before. It flew rapidly, and at first horizontally, to a
distance of 100 feet or more against a five-mile breeze. It then
turned abruptly round through 180°, at first falling (from loss of
headway), then distinctly rising, and at the same time throwing its
head up until it reached an angle of nearly 60° with the vertical,
when it fell backward after a flight of between 6 and 7 seconds. The
wings were evidently not yet strong enough to resist flexure.
November 21. No. 5, in nearly the same condition as before. Two extra
springs had been placed on the launching car, in order to give the
aerodrome a greater initial velocity than before. Everything appeared
favorable, but as it left the launching track a piece flew out of the
port propeller, in spite of which the aerodrome, after dropping 5
feet, rose bodily at an angle of 45° and fell backward into the water
(time, 5 seconds).
Another trial was made the same day with the same aerodrome, under
similar conditions, except that the angle of inclination (α) was
reduced to 7°. It now, with all the other circumstances of launching
like those immediately before, behaved entirely differently, plunging
head downward into the water at a distance of 30 feet. Once more it
was shown beyond dispute that the wings must somehow be made even
stiffer.
December 8. Another trial was undertaken with No. 5, the ‹CG› being
10 cm. in front of the ‹CP› at rest. The root angle of the wings was
18°, tip angle 27°, elevation of midrod 1 in 24. The other changes
made since the previous trial consisted chiefly in the increased
weight due to the longer and stronger frames and shafts that were
made to carry 100 cm. propellers. The flight obtained was so short
that it was as unsatisfactory as before.
The aerodrome rose in the air after leaving the launching apparatus,
and then slid back into the water in the plane of its own wings. On
the first trial, it struck the boat, and was slightly injured; on
the second, with root angle of [p101] wings 10°, tip angle 20°, the
flight partook of the same character, but the machine struck the
water clear of the boat.
The fact that with the ‹CG› 10 cm. in advance of the calculated ‹CP›
the aerodrome steadily rose in front, seems to indicate that the rule
used at that time for calculating the ‹CP› (see Chapter II) was not
very accurate. This rule was based upon the assumption that the tail,
having an area equal to one-third the entire sustaining surface,
supported one-third the total weight (expressed by the formula ‹CP› =
(2‹CP›_{wm} + ‹CP›_{tm})/3, where ‹CP›_{wm} and ‹CP›_{tm} represent
respectively the ‹CP› of the wings and tail in motion), and that the
‹CP› of each surface was one-fifth its width in front of the center
of figure.
December 12. Four days later, the tail had been moved back 21 cm.,
thus carrying the ‹CG› back 7 cm., but the vertical rudder (weighing
105 grammes), for which there was now no room, was taken off, which
in a measure counteracted this change.
A trial was then made with the wings set at an initial angle of 8°
at the root and 20° at the tip. The aerodrome was released with the
engines working under a steam pressure of 90 pounds, and soared off
horizontally for some distance, when suddenly it swerved to the right
as though something on that side had given out, and turning quite
through 180° headed toward the boat, striking the water about 76 feet
away. The time of the flight was 4 seconds.
It was found upon the recovery of the machine that one of the
propellers had been twisted through 90°, so that the two were no
longer symmetrical. The turning may have been due to this twist or to
unequal influence of the wind upon the two wings; for when I applied
the sand test to the wings after returning them to Washington, it
was found that they deflected so much that the grains would not lie
upon them, which, to a great extent, explains the failure to secure a
better flight.
Thus the end of another year had been reached, and what might be
called a real flight had not yet been secured. The only progress
that seemed to have been made was that the aerodromes were not quite
so unmanageable and erratic in their flights as at the beginning
of the year, and that it had been demonstrated, at least to the
writer’s satisfaction, that the power was sufficient for the work to
be done. The launching device had been so perfected that it worked
satisfactorily, but the problem of balancing seemed as far from
solution as before.
1895
While, for convenience in narrating the progress of the work with the
aerodromes, each year has been treated as a unit, it is, of course,
understood that the work itself shows no especial difference between
the closing of one year and the beginning of another. Changes which
had important effects were introduced [p102] at various times,
but were, of course, made as they suggested themselves without any
reference to time or season. But while it was customary to make,
from time to time, a résumé of the progress of the work, yet at
the closing of the calendar year it was the custom to make a more
complete digest of just what had been accomplished during the year.
Upon thus reviewing the progress of the work during 1894, it was
felt that the results which had been accomplished for such a large
expenditure of time seemed small, since no real flight had been
made by any of the aerodromes, and no definite assurance that a
successful flight would be obtained within the immediate future
seemed warranted by what had already been accomplished. But now that
the principal difficulties connected with the launching apparatus
had been overcome, thus permitting the aerodromes themselves to be
given a fair trial, the belief was encouraged that the continuance of
the actual tests of the machines, with slight changes which previous
tests had shown advisable, would finally result in a successful
flight.
The early weeks of 1895 were spent in a series of pendulum tests on
No. 5, and in making such slight changes as these tests indicated
would be advisable. As a result of small improvements introduced in
the boilers, No. 5 had by the middle of March shown a repeated lift
of considerably over 50 per cent, and in some tests as much as 62
per cent of its flying weight. Certain radical changes previously
described in Chapter VII were also made in Aerodrome No. 4, and in
the pendulum tests of it a lift of 44 per cent of its flying weight
had been obtained.
Encouraged by the better results which the aerodromes had shown in
the above tests, it was decided to test them again in free flight,
and they were accordingly sent down to Quantico in charge of the two
mechanics, R and M, Mr. Langley, accompanied by Dr. Graham Bell,
whom he had invited to witness the tests, following on May 8. On the
evening of May 8 No. 5 was mounted on the launching apparatus in
order to drill the mechanics so that when favorable weather presented
itself the aerodrome could be got ready for launching with the
minimum delay.
On May 9 Mr. Langley and Dr. Bell reached the house-boat at 5 a. m.,
but even with the drill of the previous evening the mechanics were
not able to have No. 5 ready for trial until 6.15 a. m. The principal
conditions of No. 5 at this time were:
Total weight 11,200 grammes (24.6 pounds), including 800 grammes of
fuel and water. Previous lift on the pendulum 54 per cent, with a
steam pressure of 150 pounds. With this steam pressure the engine
made about 600 R. P. M. when driving the 95 cm. propellers, which
through their reduction gearing made about 500 R. P. M. [p103]
When the aerodrome was balanced for flight so as to bring the
theoretical “center of pressure in motion” over the center of
gravity, it was found that it was not possible to carry the center of
gravity in front of this point, although it was known by experience
to be necessary. Accordingly in the first trial the outer ends of the
tail were pressed down by the guys so that the wind of advance tended
to lift the tail and throw the head down more than if the tail had
been flat. Furthermore, the float, weighing 200 grammes, instead of
being placed in its normal position near the base of the bowsprit,
was carried out to its extremity, this change in the position of the
float alone being sufficient to carry the center of gravity forward
three or four centimetres. The curved wings were set at an angle of
nine degrees at the root and eleven degrees at the tip. They were
well guyed, and in flight appeared to be not materially twisted or
altered.
It was anticipated that the pressing down of the outer ends of the
tail and the shifting of the center of gravity would cause the
aerodrome to point downward in flight, and this anticipation was
verified in the test. At 6.15 a. m. the aerodrome was launched at a
steam pressure of 120 pounds. A perfect calm prevailed at the time
and the machine started straight ahead. There was no perceptible drop
at the moment it was released from the launching car, but a smooth
and steady descent until it struck the water, nose down, at a point
approximately 200 feet from the boat. Dr. Bell noted that the length
of time the aerodrome was in the air was 2.8 seconds. One of the
propellers was broken and the other one was found to have twisted its
shaft one-fourth of a turn.
At 9.45 a. m., the wings having been dried, No. 5 was again tried.
The float was moved back to its normal position at the base of the
bowsprit, and the guys, by which the outer ends of the tail had been
depressed in the previous trial, were so adjusted that the tail was
flat. The machine was, therefore, in the condition of theoretical
equilibrium for rapid motion with a plane wing. All the other
conditions were precisely as in the previous trial, except that
the round-end 100-centimetre propellers were substituted for the
95-centimetre ones which had been broken, and a new paper-covered
tail was used. The mechanic in charge was directed to let the steam
reach its highest pressure consistent with a flight of one-half a
minute, before launching the machine, but he seemed to have lost all
sense of the length of time the fuel and water would last, as he let
the engines run until almost the whole charge was exhausted before
launching it. The aerodrome went off almost horizontally, then turned
up into the wind and rose to an angle of about twenty degrees; then
(while moving forward) slowly sank as though the engine power had
given out, as in fact it doubtless had. The actual distance travelled
was 123 feet and the length of time 7.2 seconds. While the exhaustion
of the fuel and water prior to launching the machine had prevented
what apparently would otherwise have been an [p104] exceedingly good
flight, yet the fact that the aerodrome rose immediately after being
launched, and continued to do so until the power gave out, was in
itself very encouraging.
At 1.40 p. m. No. 5 was again ready for trial (the third one for
the day), and this time Mr. Langley and Dr. Bell witnessed it from
a greater distance in hopes of being able more clearly to study its
behavior when actually in the air.
The previous trial having missed success through the fuel and water
having been consumed before the machine was launched, special
instructions were given to avoid the recurrence of this mistake. But
the machine was held for probably two minutes after the burners were
lighted, with very much the same result as before. The conditions of
the aerodrome were the same as in the previous trial, except that the
tail was a little flatter, so as to tend to make the head slightly
lower in flight. It was launched at an angle of about thirty degrees
with the very gentle wind that was blowing, and, apparently under
the direction of the rudder, turned into the wind, the midrod rising
to an angle of about twenty degrees and (as noted in Mr. Langley’s
record book) “The whole machine absolutely rising during five or
six seconds--a fine spectacle! Then the power visibly gave out, the
propellers revolving slower. It settled forward and lost nearly all
of its forward motion at the end of about seven seconds, but did not
finally touch the water until ten and a quarter seconds.”
While the length of time that the aerodrome had been sustained in
the air was so short that no actual flight had really been achieved,
yet the results encouraged the belief that with the aerodrome more
accurately balanced, it could reasonably be hoped that a somewhat
longer flight would be obtained. It was, however, very evident that,
although the correct balancing which would insure equilibrium for
a few minutes might soon be attained, the machine, lacking a human
intelligence to control it, must be provided with some mechanism
which would tend to restore the equilibrium, the conditions of which
must necessarily change in a machine depending on the air for its
support. In order to see what could be done in this direction, it
was, therefore, decided to return immediately to Washington with
the machines and make some minor changes in them before attempting
further flights.
By the end of May, Nos. 4 and 5 were again in readiness for a trial,
and the mechanics were accordingly sent to Quantico to complete
preparations for the tests. During May Mr. A. M. Herring, who had
been experimenting with model machines for several years, was engaged
for a few months as an assistant, and he was immediately put in
charge of the field trials of Nos. 4 and 5, which were now about to
be made. On June 6 Mr. Langley, accompanied by Mr. Herring, went to
Quantico, and on June 7, at 5 a. m., Aerodrome No. 5 was ready for
trial, but the wind was so high that nothing could be done. The wind
later diminished in intensity, but the house-boat had become stuck
on the beach [p105] and it was impossible to make the launching
apparatus point directly into the wind, which was blowing from the
rear of the boat. An attempt was made to launch the aerodrome even
with the wind blowing at its rear, but it was found impossible to
make the fires burn and the test was accordingly postponed. Later
in the afternoon the house-boat was floated and the preparations
for a test were immediately completed. At 5.42 p. m. the fires were
lighted, but the burners did not work properly and the proper steam
pressure could not be obtained. At 6.20 p. m. the fires were again
lighted, and at 6.22 the aerodrome was launched, its midrod having
an upward angle of 25 degrees, or more, with the launching track.
The aerodrome moved off nearly horizontally, but seemed to be very
sluggish in its movement and fell in the water about seventy feet
from the boat, after having been in the air only 4.8 seconds. The
damage consisted of a broken propeller and a slight strain in the
main frame, the extent of which, however, was not immediately seen.
The steam pressure at the time of launching was 110 pounds, which was
obviously insufficient. The aerodrome had lifted fifty per cent of
its weight on the pendulum, and its sluggishness of movement seemed,
therefore, unaccountable even for this pressure. It seemed probable,
however, that the pressure ran down immediately after the machine was
launched, on account either of the use of the light-weight burners in
place of the larger and heavier ones, or of the diminution of the air
pressure in the gas tank.
At 7.55 the aerodrome was again launched, and this time made a still
shorter flight than before, being in the air only three seconds. A
serious leak in the engine cylinder was, however, discovered just as
the machine was launched, and this probably accounted for the lack of
power.
Not only had the tests which have just been described indicated that
there was a lack of power during flight, although previous pendulum
tests had repeatedly shown lifts greater than fifty per cent, but,
furthermore, the wings themselves, while appearing perfectly capable
of supporting the aerodrome when viewed with the machine stationary,
were seen to flex to such an extent in flight that it seemed probable
that much of the power was consumed in merely overcoming the head
resistance of a large portion of the wings which had lost all lifting
effect.
During the fall and winter, as recorded in Chapters VII and VIII,
Aerodrome “New No. 4,” which had been reconstructed during the
summer, and which upon test was found radically weak, was almost
entirely rebuilt and afterwards known as No. 6. Important changes
were also made in No. 5, which greatly increased its strength and
power. The improvements, however, which contributed more than
anything else to the marked success achieved in the next trial of
the aerodromes, were those which had to do with the nature and
disposition of the sustaining surfaces and the means for securing
equilibrium. [p106]
It will be recalled that in the more recent trials the apparent
causes of failure had been the inability to provide sufficiently
rigid wings, the great difficulty of properly adjusting the relative
positions of the centers of pressure and gravity, and the lack of any
means of regaining equilibrium when the balance of the aerodrome had
in any way been disturbed. In the fall of 1895, accordingly, it was
finally decided to employ a second pair of wings equal in size to the
first or leading pair. This not only added greatly to the stability
of the aerodrome, but also made it possible, without any alteration
in the plan of the frame, to bring the center of pressure into the
proper position relative to the center of gravity. In addition the
plan of constructing the wings was modified by the introduction of
a second main rib, which, placed at approximately the center of
pressure of the wings, made them much stiffer, both against bending
and torsion. The two pairs of wings now became the sole means of
support, and the tail which had hitherto been made to bear part of
the weight of the aerodrome, as well as assist in preserving the
longitudinal equilibrium, was now intended to perform only the latter
function. It was placed in the rear of the wings and was combined
with the vertical rudder. Further, in adjusting it on the aerodrome,
it was set at a small negative angle and given a certain degree of
elasticity, as described above. This device proved to be a most
efficient means of maintaining and restoring the equilibrium, when it
was disturbed, and its value was apparent in all future tests of the
models.
1896
The important changes in the steam-driven models which had been
begun in the previous fall, and which in the case of No. 4 had been
so extensive as to convert it into a new aerodrome, No. 6, were
continued during the early spring, and it was not until the last of
April that the models Nos. 5 and 6 were ready for actual test in free
flight.
[Illustration: PL. 18
HOUSE-BOAT WITH OVERHEAD LAUNCHING APPARATUS, 1896]
The condition of No. 5, which made the first successful flight, is
given in the data sheet for May 6, 1896, and its general form at this
time may be seen in the photograph of May 11, Plate 27A. Although the
changes described above, as well as the modifications in the boilers
and burners of both aerodromes had undoubtedly effected a great
improvement in every detail of the machines, the disappointments
experienced in the preceding years prevented any great feeling
of confidence that the trials which were now to be made would be
entirely successful. On May 4, however, the two mechanics, Mr. Reed
and Mr. Maltby, were sent down to Quantico with Aerodromes Nos. 5
and 6, and Mr. Langley, accompanied by Dr. Graham Bell, who had been
invited to witness the tests, followed on the afternoon of the 5th.
On May 6 the wind was so very high all the morning that a test was
found impracticable. During the forenoon, however, the wind gradually
died down, and by 1 p. m. was blowing from six to ten miles an
hour [p107] from the northeast. At 1.10 p. m. Aerodrome No. 6 was
launched, but the guy-wire uniting the wings having apparently caught
on one of the fixed wooden strips which held the wings down, the left
wing was broken before the aerodrome was really launched, and the
result was that the machine slowly settled down in the water by the
boat, breaking the propellers and slightly injuring the Pénaud tail.
After removing No. 6 from the water, No. 5 was placed on the
launching car and immediately prepared for a test. At 3.05 p. m.
it was launched at a steam pressure of 150 pounds and started
directly ahead into the gentle breeze which was then blowing. The
height of the launching track above the water was about twenty feet.
Immediately after leaving the launching track, the aerodrome slowly
descended three or four feet, but immediately began to rise, its
midrod pointing upward at an increasing angle until it made about ten
degrees with the horizon and then remained remarkably constant at
this angle through the flight. Shortly after leaving the launching
track the aerodrome began to circle to the right and moved around
with great steadiness, traversing a spiral path, as shown in the
diagram (Plate 19). From an inspection of the diagram it will be
noticed that the aerodrome made two complete turns and started on the
third one. During the first two turns the machine was constantly and
steadily ascending, and at the end of the second turn it had reached
a height variously estimated by the different observers at from 70
to 100 feet. When at this height, and after the lapse of one minute
and twenty seconds, the propellers were seen to be moving perceptibly
slower and the machine began to descend slowly, at the same time
moving forward and changing the angle of inclination of the midrod
until the bow pointed slightly downward. It finally touched the water
to the south of the house-boat at the position shown, the time the
machine was in the air having been one minute and thirty seconds
from the moment of launching. The distance actually traversed, as
estimated by plotting its curved path on the coast-survey chart and
then measuring this path, was approximately 3300 feet, which is the
mean of three independent estimates. This estimate of the distance
was checked by noting the number of revolutions of the propellers
as recorded by the revolution counter, which was set in motion at
the moment the machine was launched. On the assumption that the
slip of the propellers was not greater than fifty per cent, the
1166 revolutions as shown by the counter would indicate a distance
travelled of 2430 feet. As it was felt very certain that the slip
of the propellers could not have amounted to as much as fifty per
cent, it seemed a conservative estimate to place the length of flight
at 3000 feet, which would mean a rate of travel of between 20 and
25 miles an hour. The circular path traversed by the aerodrome was
accounted for by the fact that the guy-wires on one of the wings had
not been tightened up properly, thus causing a difference in the
lifting effect of the two sides. [p108]
The aerodrome was immediately recovered from the water and
preparations made for a second test, the machine being launched again
at 5.10 p. m. at a steam pressure of 160 pounds. The conditions were
the same as at the first trial, except that the wind had changed
from north to south and was perhaps of less velocity than before.
The path traversed by the aerodrome in this second trial was almost
a duplicate of the previous one, except that on account of the
change in the direction of the wind the machine was launched in the
opposite direction. In tightening up the guy-wires, which had not
been properly adjusted in the previous test, they were probably
tightened somewhat too much, since in this second test the aerodrome
circled towards the left, whereas in the first flight it had circled
towards the right. The aerodrome made three complete turns, rising to
a height of approximately sixty feet with its midrod inclined to the
horizon at a slightly greater angle than before. The propellers again
ceased turning while the machine was high in the air and it glided
forward and downward and finally settled on the water after having
been in the air one minute and thirty-one seconds. The distance
travelled was estimated as before, by plotting the path on the
coast-survey chart, and was found to be 2300 feet.
During these flights several photographs were secured of the machine
while it was actually in the air, some of the pictures being taken
by Dr. Bell and others by Mr. F. E. Fowle. The clearest of these are
shown in Plates 20, 21, and 22.
Just what these flights meant to Mr. Langley can be readily
understood. They meant success! For the first time in the history of
the world a device produced by man had actually flown through the
air, and had preserved its equilibrium without the aid of a guiding
human intelligence. Not only had this device flown, but it had been
given a second trial and had again flown and had demonstrated that
the result obtained in the first test was no mere accident.
Shortly after returning to Washington, Mr. Langley left for Europe,
but before doing so he gave instructions to the workmen to remedy the
small weaknesses and defects which had been found in Aerodrome No. 6,
and to have both aerodromes ready for trial before his return in the
fall.
[Illustration: PL. 19
PATH OF AERODROME FLIGHTS, MAY 6 AND NOVEMBER 28, 1896, NEAR
QUANTICO, VA., ON THE POTOMAC RIVER]
[Illustration: PL. 20
INSTANTANEOUS PHOTOGRAPH OF THE AERODROME AT THE MOMENT AFTER
LAUNCHING IN ITS FLIGHT AT QUANTICO ON THE POTOMAC RIVER, MAY 6,
1896. ENLARGED TEN TIMES]
[Illustration: PL. 21
INSTANTANEOUS PHOTOGRAPH OF THE AERODROME AT A DISTANCE IN THE AIR
DURING ITS FLIGHT AT QUANTICO ON THE POTOMAC RIVER, MAY 6, 1896.
ENLARGED TEN TIMES]
[Illustration: PL. 22
INSTANTANEOUS PHOTOGRAPH OF THE AERODROME AT A DISTANCE IN THE AIR
DURING ITS FLIGHT AT QUANTICO ON THE POTOMAC RIVER, MAY 6, 1896.
ENLARGED TEN TIMES]
[Illustration: PL. 23
OVERHEAD LAUNCHING APPARATUS]
[Illustration: PL. 24
OVERHEAD LAUNCHING APPARATUS]
After returning in the fall, Mr. Langley again had Aerodromes Nos.
5 and 6 taken down to Quantico for trial, and this time had as his
invited guest Mr. Frank G. Carpenter. On November 27 a test was
made of Aerodrome No. 6, the general disposition of which at this
time may be learned from the description in Chapter X, and the
photographs in Plates 29A, 29B. The model was launched at 4.25 p.
m. with a steam pressure of 125 pounds. The aerodrome went nearly
horizontally against the wind, and descended into the water in six
and a quarter seconds at a distance of perhaps 100 yards. After
the machine had been recovered from the water, it was found that
a pin had broken in the synchronizing rod which connects the two
propeller shafts together, and that the counter, which showed 495
revolutions of the propellers, had been caused to register [p109]
inaccurately on this account. The balancing of Aerodrome
No. 6 had been made the same as that of No. 5, but in No. 6 the line
of thrust was twelve centimetres higher, and this fact, which had not
been taken into account in determining the proper balancing for No.
6, seemed to be sufficient cause for the aerodrome coming down into
the water so soon after being launched. Darkness had descended before
the aerodrome could be recovered and prepared for a second trial. On
the next day, November 28, a high wind prevailed in the morning, but
in the afternoon it became comparatively calm, and No. 6 was launched
at 4.20 p. m. under the same conditions as on the preceding day,
except that the float, which weighed 275 grammes, was moved back from
the bowsprit eighty centimetres in order to make the machine lighter
in front. The aerodrome was launched at a steam pressure of not much
over 100 pounds, the air draft for the burners being temporarily
bad. The midrod made an angle of approximately three degrees with
the horizontal. On account of a slight rain, which had occurred just
before the machine was launched, the wings were wet and the weight
of the entire aerodrome was doubtless as much as twelve kilos.
Immediately on being launched the aerodrome started directly ahead in
a gentle south wind, moving horizontally and slowly turning to the
right and appearing to approach dangerously near to some thick woods
on the west shore. However, it fortunately continued turning until
it pointed directly up the beach with the wind in the rear. It then
moved more rapidly forward, dipped and rose but once, and this very
slightly, and continued its remarkable horizontal flight, varying not
more than two yards out of a horizontal course, and this only for a
moment, until it finally descended into the bay at a point nearly in
a line between the house-boat and the railroad station at Quantico.
Upon being recovered, it was found to be absolutely uninjured, and
another flight would have been made with it immediately but darkness
had descended. The time of flight, as determined independently by
two stop-watches, was one minute and forty-five seconds. The number
of revolutions of the propellers was 2801, or at the rate of 1600
R. P. M., which, with an allowance of fifty per cent slip, should
have carried the aerodrome a distance of 4600 feet in one and
three-quarter minutes. While the distance from the house-boat in a
straight line to the point at which the aerodrome descended was only
about 1600 feet, yet it was estimated by those present that this
straight-line distance was certainly not greater than one-third the
total length of the path traversed, which would mean a distance of
something like 4800 feet. The length of the course, as plotted on
the coast-survey map and afterwards measured, was 4200 feet, and it,
therefore, seemed safe to say that the total distance travelled was
about three-quarters of a mile, and the speed was, therefore, about
thirty miles an hour.
[p110]
CHAPTER X
DESCRIPTION OF THE LAUNCHING APPARATUS AND OF
AERODROMES Nos. 5 AND 6
Reference has already been made to the development of the “cast-off”
apparatus that was used at Quantico for launching the aerodrome. An
initial velocity is indispensable, and after long experiment with
other forms which proved failures, an apparatus was designed by me,
which gave a sufficient linear velocity in any direction. It had,
moreover, been found that, when the aerodrome was attached to any
apparatus upon the roof of the house-boat, such slight changes in
the direction and intensity of the wind as would ordinarily pass
unperceived, would tend to distort or loosen it from its support,
so that only the most rigid of fastenings at three independent
bearing points were of any use in holding it, while the wings must
be separately fastened down, lest they should be torn from their
sockets. It was, then, necessary to be able to fasten the aerodrome
very firmly to the cast-off apparatus, to start it upon its journey
in any direction with an initial linear velocity that should equal
its soaring speed, and to release it simultaneously at all points at
the very same instant, while at the same time the points of contact
of the launching device, to which it had just been fastened, were
themselves drawn up out of the way of the passing propellers and guys.
All these requirements and others were met by the apparatus finally
adopted, which is shown in Plates 23 and 24. It consists of a strong
timber frame-work, carrying a track, consisting of two flat iron
rails set on edge, upon which runs the launching car, suspended from
two small wheels on each side. At the front end of the frame there
are two cylindrical air buffers to receive the buffing pistons and
thus stop the car after the aerodrome has been released. The car is
drawn to the rear end of the track and held by the bell-crank lever
‹A› (Plate 23). The contact points ‹BB› and ‹C› are turned down and
the clutch-hook ‹D› set over the clutch-post ‹K›. The aerodrome
is thus held firmly up against the three points ‹BB› and ‹C› by
the clutch ‹D›, and a distortion from its proper position rendered
impossible. All these points are thrown up out of the way of the
projecting portions of the aerodrome at the instant of release. This
result is accomplished as follows: when the car has reached the
proper point in its forward course, the cam ‹E›, which is hinged at
1, is depressed by a roller fixed to the framework of the device. In
this motion it pushes down the adjustable connections ‹FF›, which
are attached at their lower ends to the bell-crank arms ‹GG›, which
turn about a central pivot at 2. Thus the downward movement of the
connections ‹FF› opens the jaws of the [p111] clutch ‹D›. While the
clutch ‹D› is rigidly attached to ‹G› to prevent transverse movement,
it is hinged to the latter at 3 so that it can fold in a longitudinal
direction. Screwed to the clutch ‹D› is a narrow plate 4, which,
when the clutch is closed, is behind the lug 5, thus preventing any
turning about the hinge 3.
But when the arms of ‹G› and the jaws of the clamp are thrown out by
the depression of ‹F›, the plate 4 is moved out from behind the lug
5 and the clamp is free to fold to the front. The strut, hinged at
6, is under a constant tension from the spring 7 to fold up, and is
prevented from doing so only by the connections 8, by which it is
held down until the release of the plate 4 from behind the lug 5,
when the spring snaps them instantly up and out of the way.
As the struts ‹BB› have no fixed connection with the aerodrome,
they are released by the relaxation in the rigidity of the other
connections and are thrown up by their spring 9 and held in that
position by the clip 10 catching beneath the upper cross-piece.
The power for the propulsion of the car is obtained by means of from
one to nine helical springs working under tension, and multiplying
their own motion four times by means of a movable two-sheave pulley,
as shown in the drawing.
DESCRIPTION OF AERODROME NO. 5
When the details of the aerodrome, whose description is to follow,
are considered from the standpoint of the engineer accustomed to
make every provision against breakage and accident and to allow an
ample factor of safety in every part, they will be found far too
weak to stand the stresses that were put upon them. But it must be
remembered that in designing this machine, all precedent had to be
laid aside and new rules, adapted to the new conditions, applied.
It was absolutely necessary, in order to insure success, that the
weight should be cut down to the lowest possible point, and when
this was reached it was found that the factor of safety had been
almost entirely done away with, and that the stresses applied and the
strength of material were almost equal.
The same observation holds true of the boilers, aeolipile, and
engines, when regarded from the point of view of the economical
generation and use of steam. It was fully recognized that the waste
of heat in the coil boilers was excessive, but as it was necessary
that there should be an exceedingly rapid generation of steam with a
small heating surface, this was regarded as inevitable.
In the engine the three points aimed at in the design were lightness,
strength and power, but lightness above all, and necessarily in a
degree which long seemed incompatible with strength. No attempt was
made to secure the requirements of modern steam-engine construction,
either in the distribution of the steam or the protection of the
cylinder against the radiation of heat by a suitable jacketing.
The very narrow limits of weight permissible required that the
[p112] barrel of the cylinder should be as thin as possible, that
no protective jacketing should be used, and that the valve motion
should be of the simplest description. To obtaining the greatest
lightness consistent with indispensable power, everything else was
subordinated; and hence, all expectation of ordinary economical
efficiency had to be abandoned at the outset.
It was only after long trials in other directions that Mr. Langley
introduced the aeolipile device, which for the first time provided
sufficient heat. Even in the aeolipile, however, is was apparent that
nothing short of the most complete combustion accompanied by the
highest possible temperature of the flame would be sufficient for
the extreme demand. To secure this result under all conditions of
wind and weather, with the aerodrome at rest and in motion, required
the long series of experiments that are given in another chapter. In
respect to the generation of heat, then, it is probable that it would
be difficult to exceed the performance of the final type of burner in
practical work, but in the utilization of this heat in the boiler, as
well as in the utilization of the steam there generated, the waste
was so great as to be prohibitive under ordinary conditions. But this
was not ordinary work, and the simplest protection against radiation
from boiler, separator, and engine could not well be used.
The framework of the aerodrome is made of thin steel tubes, the main
or midrod extending the whole length of the machine and carrying the
attachments to which the wings are fastened. Suspended from this
midrod by rigid connections is a skeleton hull of steel tubing,
shaped somewhat like the framework of a boat, from which, directly
abeam of the engines, arms are run out like the outriggers of a
rowboat for carrying the propellers. Within this central hull are
placed the aeolipile, the boiler, and the engine, which with their
auxiliary parts, the pump and the separator, constitute the entire
power-generating apparatus.
The aeolipile consists of four essential parts: the spherical air
chamber containing the supply of compressed air by which the gasoline
in the reservoir tank is forced into the burner; the reservoir
tank containing the gasoline that is to be used as a fuel; the gas
generator wherein the liquid gasoline is heated and converted into
gas; and the burners where it is finally utilized to heat the boilers.
The air chamber ‹D›, Plate 25, is a spherical vessel 120 mm. in
diameter, located at the extreme front end of the hull. It is made
of copper 0.25 mm. thick and has two openings. The front opening
has a copper pipe 1 cm. outside diameter, to which the air pump for
charging the chamber is connected. From the back a copper pipe 5 mm.
outside diameter extends to the top of the gasoline reservoir.
[Illustration: PL. 25
SIDEVIEW OF STEEL FRAME OF AERODROME NO. 5 SUSPENDED FROM
LAUNCHING-CAR, OCTOBER 24, 1896]
This reservoir, shown at ‹I›, Plate 25, is also a light, hollow
sphere 120 mm. in diameter; both this and the air chamber being
made by soldering hemispheres [p113] of copper together at their
circumferences. There are three openings in the reservoir tank; two
at the top and one at the bottom. One of those at the top serves for
the admission of the 5-mm. pipe bringing compressed air from the
air chamber; the other is connected with a pipe 1 cm. in diameter,
through which gasoline is supplied to the tank, and which is closed
by a simple plug at the top. The hole in the bottom serves as the
outlet for the gasoline to the burners. Close to the bottom of the
tank there is placed a small needle valve, which serves to regulate
the flow of oil, for, were the pipe left open, the compressed air
would force the oil out with such rapidity that the burners would be
flooded and the intensity of the flame impaired. The construction
of this valve is clearly shown in Plate 26A. It consists of a brass
shell having one end (‹a›) soldered to the bottom of the tank. The
needle enters through a stuffing box whose gland is held by two small
screws. The stem of the needle is threaded and engages in a thread
cut in the body of the casting and is operated by a fine wire on the
outside. It will readily be seen that this device affords a means of
making a very accurate adjustment of the flow of the liquid to the
burners.
After leaving the needle valve the gasoline flows along the pipe
‹S›, Plate 25, until it reaches the evaporating coil, ‹N›. In order
to subject the oil to as large a heating surface as possible, in
comparison with the sectional area through which it is flowing, the
pipe, which left the needle valve with a diameter of 6 mm. soon
contracted to 5 mm., is here flattened to a width of 7 mm. and a
thickness of 2 mm. There are seven complete turns of this flattened
tubing coiled to an outside diameter of 30 mm. At the end of the
seventh coil the pipe is enlarged to a diameter of 1 cm. and two
coils of this size are added, the inside diameter being the same as
that of the flattened coil. This enlarged portion serves as a sort
of expansion chamber for the complete gasification of the gasoline,
which is then led back through a turn of the enlarged pipe, beneath
the coils and to the front. At the front end of the coil a small
branch is led off, forming a “bleeder,” which takes sufficient gas to
supply the burner by which the coil is heated, the products of whose
combustion pass into and between the coils of the boiler like those
of the regular heating burners. The gas pipe rises in front of the
coil and by a ‹T› connection branches to the two burners that are
placed in front of the coils of the boiler. These burner pipes are 5
mm. in diameter and enter sheet-iron hoods forming regular burners of
the Bunsen type, which are fully shown in all their details in the
accompanying engraving, Plate 26. The pipe is plugged at the end, and
a hole 0.9 mm. in diameter drilled for the nipple of the burner in
front of the coil where the water first enters from the separator,
and 0.85 mm. for the one in front of the return coil. The face of the
burner shell stands exactly central with and 41 mm. in front of the
coils.[35] [p114]
This constitutes the heat-generating portion of the machine, and
with it it is probable that a flame of as high a temperature is
produced as can be reached, with the fuel used, by any practical
device.
The boiler or steam-generating apparatus may be said to consist of
three parts: the separator, the circulating pumps, and the generating
coils.
The separator (‹M› in Plate 25) is a device which has attained its
present form after a long course of development. As at present
constructed, it is formed of a hollow sphere 190 mm. in diameter and
is located as nearly as possible over the center of gravity of the
whole apparatus. It serves the double purpose of water reservoir and
steam drum, and is called a “separator” on account of the function
which it performs of separating the water from the steam as it enters
from the coils. There is a straight vertical pipe 10 mm. in diameter
rising from the top of the sphere and fastened to the right-hand side
of the midrod. This is used for filling the separator with water.
Upon the other side of the midrod there is a small steam dome 42 mm.
in diameter with a semi-spherical top rising to a height of 70 mm.
above the top of the sphere. From this dome two steam pipes are led
off, one to the engine and the other to the steam gauge.
As already stated elsewhere, it was found in the experiments with
the coil boiler that an artificial forcing of the circulation of
the water was a necessity, as the natural circulation was too slow
to be of any service. Accordingly, but only after numerous devices
involving less weight had failed, a pump driven from the engine
shaft was designed and used. In the early experiments various types
of pumps were tried in which the valves were opened and closed
automatically by the pressure of the water. It was found, however,
that with the mixture of steam and water to be handled, the valves
could not be depended upon to open and close properly at the high
speeds at which is was necessary to run the engine. In Aerodrome
No. 5, therefore, a double-acting pump with a mechanically operated
valve was used. The pump, shown in detail in Plate 26A, is driven
from a shaft connected with the main engine shaft by a spur gear
and pinion, which rotates at half the speed of the engine shaft.
The pump itself consists of two barrels, the main barrel having a
diameter of 23 mm. with a piston stroke of 20 mm. The outer shell of
the barrel is made of aluminum bronze and is lined with a cast-iron
bushing 1.25 mm. in thickness. The piston has a length of 14 mm. and
is formed of an aluminum disc and center, having a follower plate
of the same material with two cast-iron split rings sprung in. The
water is received into and delivered from the valve cylinder, which
is 18 mm. in diameter and also lined with a cast-iron bushing 1.25
mm. thick. The aluminum bronze shells of both cylinders are 0.75 mm.
in thickness. The valve is a simple piston valve 35 mm. long with
bearing faces 4 mm. long at each end. The water is taken from the
bottom of the separator and led to the center of the valve chest of
the pump by a copper pipe 1 cm. outside diameter. The ports [p115]
leading from the valve to the main cylinder are 3 mm. wide and 34
mm. apart over their openings. It will thus be seen that when the
valve is in its central position, as it should be at the beginning
of the piston stroke, both ports are covered with a lap of 0.5 mm.
inside and out, so that the valve has to move 0.5 mm. before suction
or discharge can take place. As the valve is moving most rapidly at
this point, it opens and both functions begin before the piston has
advanced perceptibly. The delivery is made at the ends of the valve
cylinder through two copper pipes of 1 cm. diameter that unite into
a single pipe before reaching the boiler. The throw of the valve is
14 mm. so that the ports are uncovered and held wide open for the
greater portion of the stroke of the piston, and begin to close only
when the latter approaches the end of its stroke. In this way perfect
freedom is given to the flow of the water and all choking is avoided.
As the engine has been run at a speed of more than 688 revolutions
per minute, the pump must have made at least 344 strokes in the same
time, thus displacing 166.2 cc. of water. The diameter of the piston
rod and valve stem is 3 mm. and they pass through stuffing boxes with
glands of the ordinary type for packing. This pump served its purpose
admirably, and with it it was possible to maintain a continuous
circulation of water through the two coils of the boiler.
The third element in the steam-generating system is the boiler
proper[36] (Plates 25 and 26A), which consists of two coils of copper
pipe, having an outside diameter of 10 mm., each coil being formed of
21 turns each 75 mm. in diameter upon the outside and spaced 7.5 mm.
apart, so that the total axial length of each coil is 36 cm.
The water is delivered to the front end of the right-hand coil, and,
first passing through this, crosses over at the rear of the boiler to
the left-hand coil, returning through it to the front whence it is
led to and delivered into the top of the separator. Here the steam
and water are separated, the former going through the separator and
thence to the engine, while the unevaporated water falls to the
bottom to be again taken into the pumps and sent through the coils.
In order that the draft of the burner and the gases of combustion
might not be dissipated, it was necessary to sheathe the boiler. The
method of doing this is shown in Plate 25. It will be seen that the
front half of the boiler is wrapped in a sheet of mica through which
the coils can be faintly seen. This, in turn, is held at the extreme
front end by a strip of thin sheet-iron, ‹O›. Over the back end the
stack ‹Q›, made of very thin sheet-iron, is slipped. This has an
oblong cross-section at the lower end where it goes over the boiler;
it is provided with a hole through which the midrod passes, and
terminates in a circular opening of about 10 cm. diameter. [p116]
The engine, which is clearly shown in the dimensional drawing, Plate
26B, is of the plain slide-valve type, using a piston valve and solid
piston, without packing rings. The cylinder is formed of a piece of
steel tubing 35 mm. outside diameter, with flanges 47 mm. in diameter
and 2.25 mm. thick brazed to each end, to which the cylinder heads
are attached by small machine screws. Inside this cylinder is a thin
cast-iron bushing in order to obtain a better rubbing surface for the
piston. The cross-head is a small piece of aluminum bronze, running
on round guides that also serve as cylinder braces. There are also
four hollow braces, 5 mm. in diameter, running from the back cylinder
head to a corrugated steel bed-plate, that stands vertically and
reaches from one side rod of the frame of the hull to the other, and
to which are bolted the bearings of the main shaft. The connecting
rod has the cross-section of a four-rayed star and drives a crank
in the center of the shaft. The following are some of the principal
dimensions of the engine:
millimetres.
Inside diameter of cylinder 33
Stroke of piston 70
Length of cylinder inside 88
Length of piston 11
Clearance at each end 0.5
Diameter of piston rod 5
Length of cross-head 17.5
Diameter of guides 4.5
Distance from center to center of guides 26
Length of guides 110
Length of wrist-pin bearing 8.5
Length of connecting rod 150
Ratio of connecting rod to stroke 2-1/7 to 1
Length of crank pin 10
Diameter of main shaft 8
Length of main bearings 25
Distance from center of cylinder to center of valve stem 35
Length of valve 72
Width of ports 2
Outside lap of valve 4
Inside lap of valve 3
Lead of valve 0
Travel of valve 13
Cut-off from beginning of stroke 57
Exhaust opens End of stroke
Exhaust closes on return stroke 48
Diameter of valve stem 4.5
Diameter of eccentric 36
Width of eccentric 4
Width of crank arm 4
The weights were nearly as follows:
grammes.
Engine 464
Pump and pump shafts 231
Gasoline tank and valves 178
Burners 360
Boilers, frames holding boilers, and mica covers over boilers 651
Separator, steam gauge and pipe for engine 540
Exhaust pipe 143
Smoke stack 342
In all, 2909 grammes, or 6.4 pounds.
[Illustration: PL. 26A
DIMENSIONED DRAWING OF BOILER COILS, BURNERS, PUMP, NEEDLE VALVE AND
THRUST BEARING]
[Illustration: PL. 26B
DIMENSIONED DRAWING OF ENGINE NO. 5]
[Illustration: PL. 27A
SIDE AND END ELEVATIONS OF AERODROME NO. 5, MAY 11, 1896]
[Illustration: PL. 27B
AERODROME NO. 5 PLAN VIEW. OCTOBER 24, 1896]
[p117]
These weights are those determined in December, 1896, when some
slight changes had been made from the conditions existing at the time
of the flight by this aerodrome on May 6. Previous to that time, with
a pressure of 130 pounds, between 1.1 and 1.25 horse-power was given
on the Prony brake. At the actual time of flight the pressure was
about 115 pounds, and the actual power very near 1 horse-power.
The valve stem was pivoted to the center of the valve partly because
this was the lightest connection that could be made, and partly to
allow the valve perfect freedom of adjustment upon the seat. Many
parts, such as guides, braces, crank-pins, wrist-pin and shafts are
hollow. The steam is taken in at the front end of the steam chest,
and the exhaust taken out of the center, whence it is led back to
the stack and by means of a forked exhaust pipe discharged in such
a way as to assist the draught of each coil of the boilers. Like
the cylinder the steam chest is made of a piece of steel tubing, 20
mm. diameter on the outside, with an inside diameter of 19 mm., and
is fitted with a cast-iron bushing 0.5 mm. thick, making the inside
diameter of the steam chest 18 mm. It, too, has flanges brazed to the
ends, to which the heads are held by small machine screws.
The shaft for conveying the power to the propeller shafts extends
across the machine from side to side; it is hollow, being 8 mm.
outside diameter, with a hole 5 mm. diameter through the center.
It is formed of five sections: the middle section, containing the
crank, has a length of 110 mm. and is connected at either end,
by flanged couplings, to lengths 320 mm. long, which are in turn
extended by the end sections having a length of 230 mm. In addition
to the four main bearings that are bolted to the pressed-steel
bed-plate already mentioned, there are two bearings on the outer
framework on each side. At the outer end of each shaft there is keyed
thereto a bevel gear with an outside diameter of 27 mm. and having 28
teeth. This gear meshes with one of 35 teeth upon a shaft at right
angles to the main shaft and parallel to the axis of the aerodrome.
These two shafts, one on either arm, serve to carry and transmit the
power to the propellers. They are 192 mm. long, 8 mm. in diameter,
and are provided with three bearings that are brazed to a corrugated
steel plate forming the end of the outrigger portion of the frame.
These shafts are also hollow, having an axial hole 4 mm. in diameter
drilled through them. The propeller seat has a length of 43 mm. and
the propeller is held in position by a collar 25 mm. in diameter at
the front end, from which there project two dowel-pins that fit into
corresponding holes in the hubs of the propellers, which are held up
against the collar by a smaller one screwed into the back end of the
shaft. The thrust of the collar is taken up by a pin screwed into
the end of the forward box and acting as a step against which the
shaft bears, the arrangement being clearly shown by the accompanying
drawing, Plate 26A. [p118]
This, then, comprises the motive power equipment of the aerodrome,
and, to recapitulate, it includes the storage, automatic feeding and
regulation of the fuel; the storage, circulation and evaporation of
the water; the engine to convert the expansive power of the steam
into mechanical work; and the shafting for the transmission of the
energy developed by the engine to the propellers.
The propellers were made with the greatest care. Those used in the
successful trials were 1 metre in diameter, with an actual axial
pitch of 1.25 metres. They were made of white pine, glued together in
strips 7 mm. thick. The hub had a length of 45 mm. and a thickness
or diameter of 25 mm. At the outer edge the blade had a width of 315
mm. and a thickness of 2 mm. These propellers were most accurately
balanced and tested in every particular; each propeller blade was
balanced in weight with its mate and the pitch measured at every
point along the radius to insure its constancy; finally the two
propellers of the pair to be used together were balanced with each
other so that there would be no disturbance in the equilibrium of
the machine. As will be noted from the foregoing description of the
machinery, the propellers ran in opposite directions, as they were
made right- and left-hand screws. The weight of each propeller was
362 grammes.
We now turn again to take up the details of the construction of the
framework by which this propelling machinery is carried. The whole
aerodrome, as clearly shown in the photographs, Plates 27A and 27B,
is built about and dependent from one main backbone or midrod, which
extends well forward of all of the machinery and aft beyond all other
parts. This rod, as well as all other portions of the framework,
is of steel tubing. The midrod, being largest, is 20 mm outside
diameter, with a thickness of 0.5 mm. It is to this midrod that the
wings are directly attached, and from it the hull containing the
machinery is suspended.
The plan outline of the hull skeleton is similar to that of the
deck of a vessel. The steel tubing, 0.5 mm. thick, of which it is
formed, has an outside diameter of 15 mm. from the front end to
the cross-framing used to carry the propellers, back of which the
diameter is decreased to 10 mm.
The midrod makes a slight angle with this frame, the vertical
distance between the centers of the tubing being 73 mm. at the front
and 67 mm. at the back. The tube, corresponding to the keel of a
vessel, is braced to the upper tubes by light U-shaped ribs and by
two 8-mm. tubes forming a V brace on a line with the back end of the
guides of the engine. At the extreme front and back there is a direct
vertical connection to the midrod.
The propeller shafts are 1.23 m. from center to center, and are
carried on a special cross-framing, partaking, as already stated, of
the character of an outrigger on a row-boat. (See Plate 27B.) The
rear rods, which are of 10 mm. steel tubing, start from the front end
of the rear bearings of the propeller shaft and [p119] extend across
from side to side. The top rod is brazed to the side pieces of the
hull and the bottom rod to the keel. They are connected by a vertical
strut of 8-mm. tubing at a distance of 265 mm. inside of each
propeller shaft. At the front end of the propeller shaft two more
rods run across the frame. The lower is similar and parallel to the
back rod already described, while the upper is bowed to the front,
as shown in the plan view of the frame (Plate 30). In order to take
the forward thrust of the propeller a second cross-brace is inserted,
which runs from the rear bearing of the propeller shaft to a point
just in advance of the front head of the cylinder, and is brazed to
the two upper tubes of the cross-frame as well as to the upper tubes
of the main framing of the hull. The outer ends of the tubes of the
cross-framing are brazed to a thin, stamped steel plate which firmly
binds them together, while at the same time it forms a base for
attaching the bearings of the propeller shaft. This end plate has a
thickness of one millimetre.
In addition to the framing proper there are two guy-posts which fit
into the sockets ‹CC›, and over which truss wires are drawn, as shown
in the side view in Plate 27A. These posts have a length of 730 mm.
from the lower edge of the socket, and are capped at their lower
extremity by a light steel ferrule whose outside diameter is 10 mm.
From the drawing of the wings of No. 5, shown in Plate 17, it will
be seen that they are formed of two pine rods 15 mm. in diameter at
the inner ends, tapering to a half circle of the same diameter at
the tips. These rods are connected by eleven spruce ribs measuring
8 mm.×3 mm., and curved, as shown in the side elevation, these, in
turn, being covered by a light white silk drawn so tightly as to
present a smooth, even surface. The total length of the wing is 2
metres, and the width over all is 805 mm. Vertical stiffness is
obtained in the wings by a series of guy-wires, which pass over light
struts resting upon the main rods. These main rods are inserted and
held in the wing clamps ‹A› and ‹B›, Fig. 16, and make an angle of
150° with each other. As is the case with all other essential details
of the aerodrome, a great deal of time and attention was given to the
designing of the wing clamps before a satisfactory arrangement was
secured.
To enable it to control the aerodrome in both directions, the
tail-rudder, Plate 27A, has both a horizontal and a vertical surface,
the approximate dimensions of which are, length 115 cm. (3.8 feet),
maximum width 64 cm. (2.1 feet), giving each quarter section an area
of about 0.64 sq. m. (6.9 sq. ft.). It is given the proper angle and
degree of elasticity in a vertical direction by the flat hickory
spring, which fits into the clamp ‹N›, and attaches the rudder to the
frame.
The only other attachments of the aerodrome are the reel, float,
and counter. They have nothing whatever to do with the flying of
the machine, and are [p120] merely safety appliances to insure its
recovery from the water. The reel consists of a light spool on which
a fine cord is wound, one end of which is attached to a light float
that detaches itself and lies upon the surface of the water when the
machine sinks, while the other end is fastened to the spool that goes
down with the aerodrome. The “float” is a light copper vessel with
conical ends which is firmly fastened to the midrod, and which is
intended to so lower the specific gravity of the whole machine that
it will not sink. The cylindrical portion of this float has a length
of 250 mm. and a diameter of 170 mm., one cone having a length of 65
mm. and the other and front one a length of 140 mm., which makes the
total length of the float 375 mm. It is made of very thin copper, and
served in the successful trials not only as a float to sustain the
machine on the surface of the water, but also as a weight by which
the center of gravity was so adjusted that flight was possible.
The counter records the number of revolutions of the propellers
after launching. It is a small dial counter, reading to 10,000, with
a special attachment which prevents any record being made of the
revolutions of the propellers, until the actual moment of launching,
when a piece on the launching apparatus throws the counter in gear at
the instant that the aerodrome leaps into the air.
DESCRIPTION OF AERODROME NO. 6
Aerodrome No. 6, it will be remembered, was the outgrowth of a number
of changes made in No. 4 during the fall of 1895 and the early part
of 1896. In this reconstruction the aim was to lighten the whole
machine on account of the smaller engines in either No. 4 or No. 5.
The modifications from No. 4 were so radical and the differences that
exist between Nos. 5 and 6 are so considerable as to demand careful
attention.
As regards general appearance the frame of Aerodrome No. 6 resembles
that of No. 5 in consisting of a single continuous midrod of steel
tubing, 20 mm. in diameter, 0.5 mm. thick, immediately beneath which
the hull containing the machinery is situated. In reconstructing
the framework after the tests in January, 1896, had shown it to be
dangerously weak, especially against torsion, it was decided to make
the hull only strong enough to carry its contents and to attach it to
the stronger midrod in such a way that all torsional strains would
be taken up by it, whereas in No. 5 the hull structure must bear a
large proportion of such strains. It was therefore built throughout
of 8-mm. tubing, 0.3 mm. thick, and was rigidly attached to the
midrod by braces at the front and rear, and also at the cross-frame.
The hull was also made narrower (except at the rear, where it was
widened to contain the boiler) and shorter than the hull of No. 5--an
advantageous change made possible by the fact that the engines were
not contained in the hull, but mounted on the transverse frame.
[Illustration: PL. 28
STEEL FRAME OF AERODROME NO. 6, ON LAUNCHING CAR]
[p121]
In No. 5, as described above, a single engine mounted at the
front end of the hull communicated its power through transmission
shafts and gearing to the propellers, which were necessarily in
the same plane. This brought the line of thrust very nearly in the
same plane as the center of gravity of the aerodrome, a condition
tending to promote instability of longitudinal equilibrium. In No.
6, however, the use of two engines situated on the transverse frame
and communicating their power directly to the propellers, made it
possible to raise the transverse frame 12 cm. above the hull, and
thus raise the line of thrust to a position intermediate between the
center of pressure and the center of gravity, without materially
affecting the latter. As a result of this change Aerodrome No. 6
was rendered much more stable and made steadier flights with fewer
undulations than No. 5.
The engines in use on No. 6 were the small engines described above
in connection with No. 4. The cylinders were of steel tubing 2.8
cm. in diameter, with a 5-cm. stroke, each cylinder thus having a
capacity of 30.8 cc. They were lined with a thin cast-iron bushing
and cast-iron rings were sprung in the piston head so as to give as
smooth a rubbing surface and as perfect action as possible. As in the
engine of No. 5 a plain sliding valve of the piston type was used,
cut-off being approximately at one-half, though the ports were so
small that it was difficult to determine it with any great accuracy.
No packing was used, but the parts were carefully ground so as to
give a perfect fit.
These engines, as is most clearly shown in Plate 30, were mounted
symmetrically on either side of the cross-frame and were connected
directly to the propeller shafts. In order to insure that the
propellers would run at the same rate, there was provided a
synchronizing shaft, ‹T›, in Plate 30, having on each end a bevel
gear, which intermeshed with similar gears on the propeller shafts.
Steam for the cylinders was conveyed from the separator through the
pipes ‹LL›.
The steam-generating apparatus for No. 6 was exactly like that
already described in connection with No. 5, the only difference being
in the more compact arrangement in the case of No. 6. The relative
location of the apparatus in the two models is clearly shown in
Plates 28, 29B, and 30, the corresponding parts being similarly
labeled, so that a separate description for No. 6 is superfluous.
The wings used on No. 6 were somewhat smaller than those of No. 5,
and differed from them in having the front mainrib bent to a quadrant
at its outer extremity and continued as the outer rib of the wing.
The degree of curvature of the wings was also somewhat less, being
one-eighteenth for No. 6 and one-twelfth for No. 5. The four wings
were of the same size and had a total area of 54 sq. ft. On account
of the shortened hull of No. 6 they were allowed a much greater range
of adjustment, which rendered it much easier to bring the ‹CP› into
the proper relative position to the ‹CG› than was the ease with No.
5. [p122]
The Pénaud rudder for No. 6 was similar to that for No. 5, the two in
fact being interchangeable, and was similarly attached to the frame.
The reel, float, counter, and all other accessories were identical
for the two machines.
To sum up the comparative features of these two successful
steam-driven models: Aerodrome No. 6 was both lighter and frailer
than No. 5, and required much more delicate adjustment, but when the
correct adjustments had been made its flying qualities were superior,
as regards both speed and stability.
[Illustration: PL. 29A
PLAN VIEW OF AERODROME NO. 6. OCTOBER 23, 1896]
[Illustration: PL. 29B
SIDE ELEVATION OF AERODROME NO. 6. OCTOBER 23, 1896.]
[Illustration: PL. 30
PLAN VIEW OF STEEL FRAMES AND POWER PLANTS OF AERODROMES NOS. 5, 6]
[Illustration: PL. 31
DETAILS OF AERODROME NO. 5]
FOOTNOTES.
[2] “Experiments in Aerodynamics,” ‹Smithsonian Contributions to
Knowledge›, Vol. 27, 1891.
[3] This chapter was written almost entirely by Mr. Langley in 1897.
[4] 1897.
[5] In this statement, of course, no account is taken of the
“internal work of the wind.”
[6] Ten years prior to 1897.
[7] Communication to the French Academy. Extract from the Comptes
Rendus of the Sessions of the Academy of Sciences, Vol. 122, Session
of May 26, 1896.
(Translation.)
A Description of Mechanical Flight. By S. P. Langley.
In a communication which I addressed to the Academy in
July, 1891, I remarked that the results of experimental
investigation had shown the possibility of constructing
machines which could give such a horizontal velocity to
bodies resembling in shape inclined planes, and more than
a thousand times heavier than air, that these could be
sustained on this element.
While I have elsewhere remarked that surfaces other than
planes might give better results, and that absolutely
horizontal flight, which is so desirable in theory, is
hardly realizable in practice, so far as I know there
has never been constructed, up to the present time, any
heavy aerodrome, or so-called flying-machine, which can
keep itself freely in the air by its own force more than
a few seconds, the difficulties encountered in absolutely
free flight being, for many reasons, immeasurably greater
than those experienced when the flight is controlled
by the body’s pressing upward against a horizontal
track, or whirling-arm. No one is unaware that many
experimenters have been engaged in trying to execute free
mechanical flight, and although the demonstration which I
furnished in 1891 [“Experiments in Aerodynamics,” 1891]
of its theoretical possibility with means then at our
disposition, seemed conclusive, so long a time has elapsed
without practical results, that it might be doubted
whether these theoretical conditions are to be realized.
I have thought it well, then, to occupy myself with the
construction of an aerodrome with which I might put my
previous conclusions to the test of experiment.
The Academy will, perhaps, find it interesting to read the
narrative given here by an eye-witness, who is well known
to it. I am led to present it not only by the request
with which he honors me, but by the apprehension that my
administrative duties may put a stop to these researches,
so that it seems to me advisable to announce the degree in
which I have already succeeded, although this success be
not as complete as I should like to make it.
The experiments took place on a bay of the Potomac River,
some distance below Washington. The aerodrome was built
chiefly of steel, though lighter material entered into the
construction, so that its density as a whole was a little
below unity. No gas whatever entered into the construction
of the machine, and the absolute weight, independent
of fuel and water, was about 11 kilos (24 pounds). The
width of the supporting surfaces was about 4 metres (13
feet), and the power was furnished by an extremely light
engine of approximately one horse-power. There was no
one to direct it on board, and the means for keeping it
automatically in horizontal flight were not complete. It
is important to remark that the small dimensions of the
machine did not allow it to include any apparatus for
condensing the steam, so that it could only carry water
enough for a very brief course--a drawback which would not
be encountered in one of a larger construction.
It is also to be noted that the speed estimated by
Mr. Bell was that obtained in a continuous ascending
flight, and much less than would have been attained in a
horizontal course.
On Mechanical Flight. Letter of Mr. Alexander Graham Bell
to Mr. Langley.
Washington, May 6, 1896.
I am quite aware that you are not desirous of publication
until you have attained more complete success in obtaining
horizontal flight under an automatic direction, but it
seems that what I have been privileged to see to-day marks
such a great progress on everything ever before done in
this way, that the news of it should be made public, and
I am happy to give my own testimony on the results of two
trials which I have witnessed to-day by your invitation,
hoping that you will kindly consent to making it known.
For the first trial, the apparatus, chiefly constructed
of steel and driven by a steam engine, was launched from
a boat at a height of about 20 feet from the water. Under
the impulse of its engines alone, it advanced against the
wind and while drifting little, and slowly ascending with
a remarkably uniform motion, it described curves of about
100 metres in diameter; till at a height in the air which
I estimate at about 25 metres (82 feet), the revolutions
of the screws ceased for want of steam, as I understood,
and the apparatus descended gently and sank into the
water, which it reached in a minute and a half from the
start. It was not damaged, and was immediately ready for
another flight.
In the second trial it repeated in nearly every respect
the action of the first, and with an identical result.
It rose smoothly in great curves until it approached a
prominent wooded promontory, which it crossed at a height
of 8 to 10 metres above the tops of the highest trees,
upon the exhaustion of the steam descending slowly into
the bay, where it settled in a minute and thirty-one
seconds from the start. You have an instantaneous
photograph of it, which I took just after the launch. [See
plates 20, 21, and 22 of present work.]
From the extent of the curves which it described, which
I estimated with other persons, from measurements which
I took, and from the number of revolutions of the
propellers, as recorded by the automatic counter which I
consulted, I estimate the absolute length of each course
to be over half an English mile, or, more exactly a little
over 900 metres (2953 feet).
The duration of flight during the second trial was one
minute and thirty-one seconds, and the average velocity
between twenty and twenty-five miles an hour, or, let us
say 10 metres a second, in a course which was constantly
ascending. I was extremely impressed by the easy regular
course of each trial, and by the fact that the apparatus
descended each time with such smoothness and gentleness as
to render any jar or danger out of the question.
It seemed to me that no one could have witnessed these
experiments without being convinced that the possibility
of mechanical flight had been demonstrated.
[8] It is desirable that the reader should be acquainted with the
contents of this treatise, and of another by me, entitled “The
Internal Work of the Wind,” both published by the Smithsonian
Institution. A knowledge of these works is not absolutely necessary,
but of advantage in connection with what follows.
[9] “Experiments in Aerodynamics,” p. 107.
[10] Chapter VIII.
[11] His device for obtaining automatic equilibrium is found in
connection with the description of his “Aeroplane Auto Moteur,” in
“L’Aeronaute” for January, 1872.
[12] I have never obtained so good a result as this with any rubber
motor. S. P. Langley.
[13] One pound of twisted rubber appears, from my experiments, to be
capable of momentarily yielding nearly 600 foot-pounds of energy, but
this effect is attained only by twisting it too far. It will be safer
to take at most 300 foot-pounds, and as the strain must be taken up
by a tube or frame weighing at least as much as the rubber, we have
approximately 0.0091 as the horse-power for one minute, or 0.091
horse-power for six seconds as the maximum effect, in continuous
work, of a pound of ‹twisted› rubber strands. The longitudinal pull
of the rubber is much greater, but it is difficult to employ it in
this way for models, owing to the great relative weight of the tube
or frame needed to bear the bending strain. In either form, rubber is
far more effective for the weight than any steel spring (see later
chapter on Available Motors).
[14] The aerodrome is sustained by the upward pressure of the
air, which must be replaceable by the resultant pressure at some
particular point, designated by ‹CP›.
[15] See Century Magazine, October, 1891.
[16] Subsequent observations indicate that the maximum velocity of
horizontal flight must have been about 10 metres per second.
[17] Observers following de Lucy have long since called attention to
the fact that as the scale of Nature’s flying things increases, the
size of the sustaining surfaces diminishes relatively to the weight
sustained. M. Harting (Aeronautical Society, 1870) has shown that the
relation √area/∛weight is surprisingly constant when bats varying
in weight as much as 250 times are the subject of the experiment,
and later observations by Marey have not materially affected the
statement. As to the muscular power which Nature has imparted with
the greater or lesser weight, this varies, decreasing very rapidly
as the weight increases. The same remark may be made apparently with
at least approximate truth, with regard to the soaring bird, and the
important inference is that if there be any analogy between the bird
and the aerodrome, as the scale of the construction of the latter
increases, it may be reasonably anticipated that the size of the
sustaining surfaces will relatively diminish rather than increase.
We may conveniently use M. Harting’s formula in the form ‹a› =
‹n›^2‹w›^{2/3} = ‹l›^2/‹m›^2 where ‹a› = area in sq. cm., ‹w› the
weight in grammes, ‹l› the length of the wing in cm., ‹n› and ‹m›
constants derived from observation.
[18] A singular fact connected with the stretching of rubber is
that the extension is not only not directly proportional to the
power producing it, but that up to a certain limit it increases more
rapidly than the power, and after this the relation becomes for a
time more nearly constant, and after this again the extension becomes
less and less in proportion.
In other words, if a curve be constructed whose abscissae represent
extensions, and ordinates the corresponding weights, it will show
a reverse curvature, one portion being concave toward the axis of
abscissae, the other convex.
[19] The following table taken from “Experiments in Aerodynamics,”
p. 107, gives the data for soaring of 30×4.8 inch planes, weight 500
grammes.
-------+-----------------------+-----------------+--------------------+
| | | Weight with planes |
| Soaring speed | Work expended | of like form that |
Angle | ‹V›. | per minute. | 1 horse-power will |
with | | |drive through the |
horizon| | |air at velocity ‹V›.|
α. +-----------+-----------+---------+-------+----------+---------+
| Metres | Feet |Kilogram-| Foot- | Kilo- | Pounds. |
|per second.|per second.| metres. |pounds.| grammes. | |
-------+-----------+-----------+---------+-------+----------+---------+
45° | 11.2 | 36.7 | 336 | 2,434 | 6.8 | 15 |
30 | 10.6 | 34.8 | 175 | 1,268 | 13.0 | 29 |
15 | 11.2 | 36.7 | 86 | 623 | 26.5 | 58 |
10 | 12.4 | 40.7 | 65 | 474 | 34.8 | 77 |
5 | 15.2 | 49.8 | 41 | 297 | 55.5 | 122 |
2 | 20.0 | 65.6 | 24 | 174 | 95.0 | 209 |
-------+-----------+-----------+---------+-------+----------+---------+
The relations shown in the above table hold true only in case of
planes supporting about 1.1 pounds to each square foot of sustaining
area. For a different proportion of area to weight, other conditions
would obtain.
[20] This pressure per unit of area varies with the area itself, but
in a degree which is negligible for our immediate purpose.
[21] See “Internal Work of the Wind”; also Revue de L’Aeronautique,
3^e Livraison, 1893.
[22] More recent experiments under my direction by Mr. Huffaker give
similar results, but confirm my earlier and cruder observations that
the curve, used alone, for small angles, is much more unstable than
the plane.
[23] As stated in the Preface, Part III has not yet been prepared for
publication.
[24] According to Wellner (“Zeitschrift für Luftschiffahrt,”
Beilage, 1893), in a curved surface with 1/12 rise, if the angle of
inclination of the chord of the surface be α, and the angle between
the direction of resultant air pressure and the normal to the
direction of motion be β, then β<α and the soaring speed is
‹V› = √(‹P›/‹K›×(1/(‹F›(α)×cos β)))
while the efficiency is
‹W›/‹R› = Weight/Resistance = tan β
The following were derived from experiments in the wind:
α = −3° 0° +3° 6° 9° 12°
‹F›(α) = 0.20 0.80 0.75 0.90 1.00 1.05
Tan β = 0.01 0.02 0.03 0.04 0.10 0.17
so that according to him, a curved surface shows finite soaring
speeds when the angle of inclination is 0° or even slightly negative.
[25] The following formulæ proposed by Mr. Chas. M. Manly show how
the center of pressure may be moved any desired distance either
forward or backward without in any way affecting the center of
gravity, and by merely moving the front and rear wings the same
amounts but in opposite directions, the total movement of each wing
being in either case five times the amount that is desired to move
the mean ‹CP›_1, and the direction of movement of the front wing
determining the direction of movement of ‹CP›_1.
In Figure 7, ‹CP›_{fw} and ‹CP›_{rw} are the centers of pressure of
the front and rear wings respectively; the weights of the wings,
which are assumed to be equal and concentrated at their centers of
figure, are represented by ‹w›, ‹w›, and ‹a› is the distance of the
center of pressure in either wing from its center of figure. The
original mean center of pressure of the aerodrome is ‹CP›_1, ‹W› is
the weight of the aerodrome, supposed to be concentrated at ‹CG›_1,
while ‹m› is the distance from ‹CP›_{rw} to ‹CG›_1.
Now, if we have assumed that the rear wing, being of the same size
as the front one, has a lifting effect of only 0.66, and on this
assumption is calculated the proper relative positions of the front
and rear wings to cause the ‹CP›_1 to come directly over the ‹CG›_1,
and upon testing the aerodrome find that it is too heavy in front
and, therefore, wish to move the center of pressure forward an
amount, say ‹b›, without affecting the center of gravity, we can
calculate the proper relative positions of the front and rear wings
in the following manner. While the aerodrome as a whole is balanced
at the point ‹CG›_1, the weight of the wings is not balanced around
this point, for the rear wing, owing to its decreased lifting effect,
is proportionately farther from ‹CP›_1 than the front wing. In order,
therefore, to avoid moving the center of gravity of the machine as
a whole, any movement of the wings must be made in such a way as to
cause the difference between the weight of the rear wing multiplied
by its distance from ‹CG›_1 and the weight of the front multiplied by
its distance from ‹CG›_1 to equal a constant: that is,
‹w›(‹m› + ‹a›)−‹w›(0.66‹m›−‹a›) = constant,
and
0.33‹w›‹m› + 2‹w›‹a› = constant.
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
[Illustration: FIG. 9.
FIGS. 7–9. Diagrams Illustrating formulæ for moving C. P. without
disturbing C. G.]
If now the wings be moved so that ‹CP›_1 is moved forward a distance
‹b›, we may indicate the distance from ‹CG›_1 to the new ‹CP›_{rw}
by ‹z›, and equating the difference between the weight of the rear
wing multiplied by its new distance from ‹CG›_1 and the weight of the
front wing multiplied by its new distance from ‹CG›_1 and making this
difference equal to the constant difference, we can calculate ‹z› in
terms of ‹m› and ‹b›, as follows:
Fig. 8,
‹w›(‹a› + ‹z›)−‹w›(0.66(‹z› + ‹b›) + ‹b›−‹a›) =
0.33‹w›‹m› + 2‹w›‹a›,
∴ ‹z› = ‹m› + 5‹b›.
Knowing ‹z›, we readily find that the new distance from ‹CP›_{fw} to
‹CG›_1 equals:
0.66(‹z› + ‹b›) + ‹b› = 0.66‹m› + 5‹b›.
In a similar manner we may calculate the proper relative positions of
the front and rear wings when we wish to move the center of pressure
backward a distance, ‹b›, from the original ‹CP›_1 without changing
the position of ‹CG›_1. From Fig. 7, we have as before:
‹w›(‹m› + ‹a›)−‹w›(0.66‹m›−‹a›) = constant,
0.33‹w›‹m› + 2‹w›‹a› = constant.
Fig. 9,
‹w›(‹z›_1 + ‹a›)−‹w›(0.66(‹z›_1−‹b›)−‹b›−‹a›) =
0.33‹w›‹m› + 2‹w›‹a›.
∴ ‹z›_1 = ‹m›−5‹b›.
Similarly we have for the new distance from ‹CP›_{fw} to ‹CG›_1:
0.66(‹z›_1−‹b›)−‹b› = 0.66‹m›−5‹b›.
[26] It is to be remembered that these aerodromes were under
incessant modifications, No. 4 for instance, presenting successive
changes which made of it in reality a number of different machines,
one merging by constant alterations into the other, though it still
went under the same name. After 1895 the type of the models remained
relatively constant, but during the first five years of the work,
constructions equal to the original building of at least eight or ten
independent aerodromes were made.
[27] Chapter V.
[28] “Pocketing” is a form of distortion in which the canvas or silk
bags locally in numerous places between the cross-ribs.
[29] The site of these experiments, which was 30 miles below
Washington, has been described. The writer is designated by the
initial “L”; Dr. Barus, who several times assisted, by the letter
“B”; Mr Reed, carpenter, by “R”; Mr Maltby, machinist, by “M”; and
Mr. Gaertner, instrument maker, by “G.”
[30] Weights and dimensions are here given in approximate pounds and
feet.
[31] “Experiments in Aerodynamics.”
[32] On the data of “Aerodynamics,” a plane having 1.8 sq. ft. of
surface per pound, and advancing at an angle of 20°, would soar at a
speed of 24.1 ft. per second.
[33] It will be remembered that the purely theoretical conclusions
just cited apply to the power delivered in direct thrust, but that of
the above actual H. P. an indefinite amount was lost in friction and
slip of propellers.
[34] It may be observed that at this time the position of the ‹CP›
was calculated on the assumption that the pressure for flight
surfaces was proportional to the areas, without also allowing for
the fact that the following surfaces, like the tail, were under the
“lee” of the wind and so far less efficient. It follows, then, that
the value ‹CP›−‹CG› was not really 0, as was assumed, but something
considerable.
[35] Very exact accuracy in these minute details is indispensable to
the efficient working of the engines.
[36] The reader who may care to note the evolution of this boiler,
by trial and error, will find a portion of the many discarded types
shown in Plate 13.
[p123]
PART II. 1897 TO 1903
BY CHARLES M. MANLY
Assistant in Charge of Experiments
CHAPTER I
INTRODUCTORY
Although in 1896 Mr. Langley had made the firm resolution not to
undertake the construction of a large man-carrying machine, as he
realized that his multitudinous administrative duties left him
practically no time available for original research, yet the longing
to take the final great step of actually transporting a human being
through the air, which the successful flights of the models had now
for the first time in the history of the world actually proved to be
possible, soon became irresistible.
Ten years of almost disheartening difficulties, a full appreciation
of which can hardly be gained from the preceding description,
had already been spent in demonstrating that mechanical flight
was practicable, and Mr. Langley thoroughly realized that the
construction of a large aerodrome would involve as great, if not
even greater difficulties. Nevertheless, his indomitable will,
which balked at no obstacle, however great it might seem, prevailed
against the advice of his close friends and associates, and even
that of his physician, who had counselled him that a resumption of
concentrated thought and vigorous endeavor would materially shorten
his life, which had already passed three score years. Only a few were
privileged to come into close contact with him in his daily work,
and thereby catch the inspiration of his unwavering persistence, his
ceaseless perseverance, his plain inability to submit to defeat; but
no one who has read the record of his astronomical expedition to Mt.
Whitney, or the story of his development of the Bolometer, or the
preceding chapters of this history of his years of patient work in
the development of the flying machine, can have failed to obtain some
appreciation of this most striking feature of his character. Having
once determined on the accomplishment of a definite object, no amount
of difficulty that might arise deterred him from pushing on until in
some way and by some means he had succeeded; and no one appreciated
better than he that if the thin edge of the right wedge can be
inserted under an obstacle, that obstacle can be removed, no matter
how formidable it may seem.
The undertaking of the construction of a large aerodrome was very
largely influenced by President McKinley, who had become impressed
with the great [p124] possibilities of a flying machine as an engine
of war. When he found that Mr. Langley was willing to devote his
own time to the development of a machine, provided the Government
would furnish the funds for the actual construction and tests of it,
he appointed a joint board, consisting of Army and Navy officers,
to investigate and report on the plans with which Mr. Langley had
achieved success with the models. The report of this joint board of
Army and Navy officers being favorable, the Board of Ordnance and
Fortification of the War Department, at the direction of President
McKinley, requested Mr. Langley to undertake the construction and
test of a machine, which, while not expected to be a practical war
machine, might finally lead to the development of such an engine of
war. In this connection it is interesting to read a letter which Mr.
Langley addressed to the Board of Ordnance and Fortification at the
time he undertook this work.
SMITHSONIAN INSTITUTION, December 12, 1898.
‹The Board of Ordnance and Fortification, War Department›.
GENTLEMEN: In response to your invitation, I repeat what I had the
honor to say to the Board--that I am willing, with the consent of
the Regents of this Institution, to undertake for the Government the
further investigation of the subject of the construction of a flying
machine on a scale capable of carrying a man, the investigation to
include the construction, development and test of such a machine
under conditions left as far as practicable in my discretion, it
being understood that my services are given to the Government in
such time as may not be occupied by the business of the Institution,
and without charge.
I have reason to believe that the cost of the construction will come
within the sum of $50,000.00, and that not more than one-half of
that will be called for in the coming year.
I entirely agree with what I understand to be the wish of the Board
that privacy be observed with regard to the work, and only when it
reaches a successful completion shall I wish to make public the fact
of its success.
I attach to this a memorandum of my understanding of some points of
detail in order to be sure that it is also the understanding of the
Board, and I am, gentlemen,
With much respect,
Your obedient servant,
S. P. LANGLEY.
MEMORANDUM
ATTACHED TO MY LETTER OF THIS DATE TO THE BOARD OF ORDNANCE AND
FORTIFICATION
While stating that I have, so far as I know, an exclusive right of
property in the results of the experiments in aerodromics which
I have conducted heretofore and am now conducting, and while
understanding that this property and all rights connected with it,
whether patentable or otherwise, will remain mine unqualifiedly, I
am glad to place these results, without charge, at the service of
the Board of Ordnance and Fortification for the special construction
at present proposed, which seems to me to be of National utility.
[p125]
I assume that no public statement will be made by the permission
of the Board until the work is terminated, but that I may publish
ultimately at my discretion a statement of any scientific work done
in this connection.
I understand that the exercise of this discretion includes the
ordering and purchase of all material by contract or in open market,
and the employment of any necessary help, without restriction, and
that, while I desire that no money shall pass through my hands,
itemized bills for each expenditure, made in proper form and
approved by me, will be paid by the Chief Signal Officer.
Much has already been spent at the Smithsonian Institution for the
purpose in question, in special apparatus, tools and experiments,
and in recent constructions now actually going on, which have
involved still more time than money, and which are essential for
experimental use in building the proposed machine; and since to
re-create all this independently would greatly defer progress, I
assume that my discretion includes the decision as to how far this
shall be used and paid for at the cost of this allotment (it being
understood that I have no personal property in any of the material
which might be transferred for the purpose of the work); and I also
assume that my discretion includes the decision as to where the work
shall be conducted--that is, whether in shops already constructed,
or in others to be elsewhere erected or rented, with the necessary
adjuncts, whether on land or water, and generally whatever is
necessary to the earliest attainment of the object desired by the
Board.
S. P. LANGLEY.
SMITHSONIAN INSTITUTION, WASHINGTON, D. C.,
December 12, 1898.
As is always the case in experimental work, especially in a field so
very new as was the field of aerodromics at the time that this larger
construction was undertaken, the “plant,” or shops and laboratories
required for the constructional and testing work, grew to a size
far beyond what seemed even remotely possible at the beginning of
the work; and even the mere administration involved in the carrying
on of this work proved to be no inconsiderable matter before it had
progressed very far.
The years of experiment with the models had demonstrated clearly that
the greatest difficulty in the development of the aerodrome was the
construction of a suitable power generator, which should combine the
elements of extreme lightness and unusual power with a fair degree
of durability. Although remarkably good results had been secured in
the case of the models through the use of steam, it was realized
from the first that not only would the development of a steam-power
plant for a large man-carrying aerodrome present difficulties of a
constructional nature, but that such a steam plant would necessarily
be so fragile and delicate as to make it a constant menace to the
machine which it was to propel. The solution of the difficulty, it
was believed, was to be found in the use of an internal combustion
engine; but Mr. Langley had had very little experience with such
engines, and was averse therefore to undertaking the construction of
a large aerodrome until he had assurance that a suitable gasoline
engine could be secured. Before making an agreement to attempt the
work for the War [p126] Department, he had, therefore, made a search
for a reliable builder who would undertake to construct a gasoline
engine of not less than 12 horse-power to weigh not exceeding 100
pounds, and what then seemed a safe contract had been entered into
with such a builder to supply one engine which would meet these
requirements.
Almost immediately before the Board of Ordnance and Fortification
had officially placed the work in Mr. Langley’s hands and had made
an allotment of fifty thousand dollars to meet the expenses thereof,
it was found that the engine builder could not be depended on, and
that it would, therefore, be necessary to find one who was more
reliable and more experienced in the construction of light engines.
After a most extended search for the best builder to undertake this
work, a contract was entered into on December 12, 1898, with Mr. S.
M. Balzer, an engine builder in New York City. He was to furnish a
twelve-horse-power engine to weigh not more then 100 pounds, and
delivery of it was to be made on or before February 28, 1899. With
this great problem of the engine apparently provided for, every
facility of the Institution shops was pressed to the utmost limit in
order to have the frame, supporting surfaces, launching apparatus,
and other accessories ready as soon as possible after the delivery of
the engine. It was expected from the first that more power would be
necessary than this one engine would furnish, and provision had been
made in the contract that a duplicate engine should be constructed
immediately after the completion of this first one. From past
experience, however, it was not likely that the correct balancing of
the aerodrome could be determined from ‹a priori› calculation based
on the results obtained with the models, and it was, therefore,
expected that the aerodrome would have to be launched several times
before a successful flight could be obtained. In view of this it was
planned to make a test of the machine as soon as the first engine
was ready, with the expectation that, while the aerodrome would not
have sufficient power to fly, yet the test would furnish definite
data on the all-important question of balancing, and also determine
whether or not the launching apparatus would require modification. In
fact, Mr. Langley felt so apprehensive that the first, and possibly
the second test, would be unsuccessful that, in order to avoid the
possibility of a fatal accident, it was planned that a dummy should
be used to represent the weight of the man in these preliminary tests.
This plan, however, was not carried out. In 1903, when the large
aerodrome was finally completed, so much time had been lost that the
writer proposed to assume the risks of such an accident and to guide
the machine in its first test, in the hope of avoiding a disaster,
with the consequent delay of months for repairs, which the presence
of a controlling hand capable of correcting any inaccuracies of
balancing rendered far less likely to occur. To this proposal Mr.
Langley assented with great reluctance, as he fully realized the
danger involved. [p127]
Particular attention is called to the above facts, which clearly show
that while a certain degree of success in the initial tests was later
hoped for, yet from the beginning it had been felt rather certain
that several tests would have to be made before final success would
be achieved.
To those experienced in scientific experiments this realization of
the probability of several tests being necessary before success could
reasonably be expected does not seem strange, for the record of past
experience contains very few examples of epoch-making inventions
springing full fledged from the hand of their maker and proving a
success on the first test.
The two experiments made in the fall of 1903, in which the aerodrome
was each time so damaged in the process of launching that its ability
to fly was never really tested, should therefore be considered merely
as the first of a series which it had been expected would need to
be made before success would be achieved. Further tests were made
impossible at the time on account of the lack of funds, the expense
of such work being unusually heavy.
While the lack of funds, therefore, was the real cause of the
temporary suspension of the work, yet an influence which does not
often enter into scientific work—the unjust criticism of a hostile
press—was directly responsible for the lack of funds. It seems very
certain that had it not been for this criticism of the press the
funds would have been readily forthcoming for continuing the work to
the point of success.
[p128]