CONSTRUCTION OF FRAME OF LARGE AERODROME
The general plan for the large aerodrome was never a matter of
uncertainty. At the time when the first general designs were made
there had been in the history of mankind only one type of machine,
that of the steam-driven Langley models, which had proved capable
of flight for any considerable distance. Furthermore, the selection
of this type had been the result not of sudden fancy or of purely
theoretical consideration, but of years of the most careful
experimentation, in the course of which nearly every conceivable
style of machine had been tested with some form of power. It would
have been worse than folly, therefore, if the one clear path had been
left to seek some unknown way.
It was fully realized from the first, however, that the increase in
size alone would make necessary in the design for the large aerodrome
a great many modifications from the designs of the steam-driven
models. It was not possible here, as in nearly every other kind of
structure, simply to magnify uniformly the parts and proportions of
the small machine in order to obtain a successful large one. This is
particularly true in the case of the aerodrome, because the rapid
increase of weight in the larger structure is out of all proportion
to the increase in strength, while it is very desirable that the
more expensive machine which is designed to carry a human being
shall be relatively even stronger than the easily replaced model.
This problem of increasing size without sacrificing strength and
stability, it was known from the beginning, would be encountered in
a particularly difficult form in designing the frame of the large
machine, and was to be solved not by the discovery of some new and
wonderfully strong material, but by improvements both in the general
plan and the details of the machine. Here, as is often the case, it
was not the large changes in the design but the improvements in small
and sometimes seemingly unimportant details which demanded the most
careful consideration and, as a whole, contributed most to the final
result. For this reason, as well as because the large changes, when
pointed out, are usually easily understood, the present chapter is
for the most part a description of the improvement of details.
From the experience gained in the construction of the frames of the
several steam-driven models, it was decided that the frame for the
large aerodrome must consist essentially of two principal parts.
First, a rigid backbone was required, extending from the point of
attachment of the front wings to the point of attachment of the rear
wings; and this backbone, for convenience designated [p165] the
“main frame,” must support the second principal part, the “transverse
frame,” which formed a cross with the main frame, and at the ends
of which the propellers were mounted. While it was necessary that
this transverse frame should have considerable rigidity and strength
in a vertical direction, yet its main strength and stiffness was
required in the horizontal plane for withstanding the thrust of the
propellers. It had been possible to construct the frames of the later
steam-driven models stiff enough, and at the same time light enough,
by the use of properly proportioned steel tubing, but calculation
very soon showed that in order to secure sufficient rigidity for the
frame of the large aerodrome and at the same time keep the weight
within the permissible limit, it would be necessary to depend very
largely on guy-wires and to use tubing only for forming the struts
against which the guy-wires should act. But this obviously introduced
a new series of problems. The extensive system of guy-wires necessary
would add materially to the head resistance of the aerodrome, and
this might conceivably be so great as to require more propulsive
power than would be required for a frame heavier but unincumbered by
the head resistance of the wires. It became necessary to consider
these problems, but no data were accessible from which the head
resistance could be computed with any confidence. The coefficient
of resistance for a cylindrical body moving through the air in a
direction perpendicular to its length may in general be taken as
one-half that of a flat body of the same cross-section; but it was
thought very certain that, owing to the fact that tightly stretched
wires are in constant vibration when the aerodrome is in the air,
the resistance of the wires must be considerably greater than would
be calculated from treating them as cylinders having a coefficient
of 0.5. Unfortunately, no data on the resistance of vibrating wires
were at hand. Before proceeding with the designs for the guying
of the frame, therefore, the following brief series of tests was
made in November, 1898, on the whirling table, in order to learn
approximately the resistance that the proposed system of guy-wires
for the large aerodrome would offer:
MEASUREMENTS OF THE RESISTANCE OF GUY-WIRES, USING FRAME ATTACHED TO
“BALANCE.”
RESISTANCE OF FRAME WITHOUT WIRES.
Frame consists of: 4 tubes, 1 cm. diameter, 14.5 cm long; 2 tubes, 1
cm. diameter, 41 cm. long; 2 tubes, 1 cm. diameter, 101 cm. long.
Revolutions of Velocity of Resistance. Calculated resistance
turn-table per frame. Feet Grammes. of frame.
minute. per minute. ‹r.› Grammes.
6.75 608 11.5 14.2
9.75 877 34.0 29.6
12.0 1080 51.8 44.8
16.35 1475 97.0 83.8
19.75 1775 134.0 121.3
22.7 2045 168.0 161.2
25.5 2290 205.0 202.0
RESISTANCE OF FRAME WITH 1ST SET OF WIRES.
First set of wires: 16 wires, 0.6 mm. diameter, 102 cm. long; 6
wires, 0.6 mm. diameter, 42 cm. long.
Revolutions Velocity of Resistance Resistance Calculated
of turn-table wires. Feet of frame of wires. resistance
per minute. per minute. and wires. R_1−r=r_1. of wires.
Grammes. Grammes.
R_1.
9.75 877 47.5 13.5 8.88
12.0 1080 73.5 21.7 13.47
13.75 1237 93.5 25.5 17.65
17.25 1550 144.0 37.0 27.7
20.25 1822 187.0 45.5 38.4
22.50 2025 216.0 47.5 47.4
22.875 2060 225.0 52.0 49.0
24.56 2215 250.0 56.5 56.7
RESISTANCE OF FRAME WITH 2D SET OF WIRES.
Second set of wires: 15 wires, 1.2 mm. diameter, 102 cm. long; 2
wires, 1.2 mm. diameter, 42 cm. long.
Revolutions Velocity of Resistance Resistance Calculated
of turn-table wires. Feet of frame of wires. resistance
per minute. per minute. and wires. R_2−r=r_2. of wires.
Grammes. Grammes.
R_2.
9.25 833 54.0 26.5 15.35
9.35 841 55.0 27.0 15.4
11.3 1018 82.0 36.75 22.65
11.5 1035 82.0 35.25 23.4
13.0 1170 104.5 43.0 29.9
13.15 1185 105.0 42.5 30.60
16.7 1505 160.0 59.0 49.5
16.75 1510 160.0 58.0 49.9
19.5 1755 196.0 64.0 67.4
19.7 1770 203.0 69.5 68.5
21.60 1945 236.0 77.0 82.6
21.65 1950 237.0 77.75 83.2
21.75 1957 235.0 75.5 83.7
RESISTANCE OF FRAME WITH 3D SET OF WIRES.
Third set of wires: 15 wires, 2 mm. diameter, 102 cm. long; 2 wires,
2 mm. diameter, 42 cm. long.
Revolutions Velocity of Resistance Resistance Calculated
of turn-table wires. Feet of frame of wires. resistance
per minute. per minute. and wires. R_3−r=r_3. of wires.
Grammes. Grammes.
R_3.
9.25 833 65 37.5 25.2
11.55 1040 91 43.5 39.35
11.55 1040 101 53.5 39.35
15.25 1375 160 75.0 68.7
18.1 1630 203 86.5 96.6
19.25 1735 221 92.0 109.5
19.25 1735 216 87.0 109.5
20.63 1860 237 90.5 125.8
The last column of these tables is calculated for a coefficient of
form equal to 0.5, which has been found to be approximately correct
for a rigid cylindrical body.
These tables are not sufficiently extensive to determine accurately
the exact resistance that wires of various sizes will offer at given
velocities, or to serve as the basis for the deduction of formulæ,
and were not made for that purpose. However, from the above data, and
the curves plotted in Plate 44, it will be seen that some unexpected
results were obtained.
[Illustration: PL. 44
RESISTANCE OF WIRES AT GIVEN VELOCITIES]
[p167]
These results are fairly well summarized in the following general
statements: First, that the coefficient of resistance increases to
some degree as the size of the wire is decreased; second, that in
the case of wires of the size which it was expected to use, and at
approximately the soaring speed of the aerodrome, the resistance
is certainly not greater than 75 per cent, and more probably less
than 50 per cent of the resistance encountered by a flat surface of
the same projected area; third, that the coefficient of resistance
did not seem to be increased by the vibration of the wires. On
the contrary, it was noted during the experiments that when they
reached a speed which just caused them to “sing,” there was a marked
diminution in the resistance. This statement is made, however, with
some reserve, for it is probable that the singing of the wires was
due to vibration in the horizontal plane, and it is not definitely
known what the effect would be of vibration in the vertical plane.
To make the very extensive experiments necessary to determine these
propositions conclusively would have required much more time than
could at this period be spared from the actual constructional work
on the aerodrome. Nevertheless, the data did seem to indicate that
it was at least not unwise to employ the extensive system of guying
which had been planned in order to give the necessary strength to
the frame of the large aerodrome. This plan of construction was,
therefore, definitely adopted, and as a result of later experience
the system of guying was still further extended.
As the transverse frame had to be made comparatively rigid in order
to prevent undue binding of the bearings of the transmission and
propeller shafts, it was necessary to make it intrinsically stronger
and, therefore, heavier in proportion to its size than the main
frame. The main frame, although requiring great strength to enable
it to withstand the strains, both torsional and direct, which were
imposed upon it by the weights which it supported, did not need
excessive rigidity, and could, indeed, be distorted an appreciable
amount without danger of any serious effect on the action of the
wings or rudder; but even a small amount of distortion in the
transverse frame might easily cause such friction at the bearings of
the shafts as to absorb fifty per cent or more of the engine power.
In the photographs, Plates 45 to 48, which show the actual condition
of the frame on January 31 and February 1, 1900, the letters ‹A›,
‹B›, ‹C›, ‹D›, ‹E›, ‹F›, ‹G›, ‹H› and ‹I› designate parts of the main
frame, ‹A› and ‹H› being the rear and front midrods, respectively,
to which the wings were to be attached. ‹B› and ‹I› are curved
extensions of the starboard main tube, the port main tube being
exactly similar, and ‹C›, ‹D›, ‹E›, ‹F› and ‹G› are cross-tubes which
connect the midrods to the port and starboard tubes. ‹R› is the
front main tube of the transverse frame, the rear main tube being
exactly similar, and both being connected to the main tubes of the
main frame where they cross them. The ends of the main tubes of the
[p168] transverse frame are joined together by the “bed plates” ‹L›,
which are of I-beam section, and have mounted on their outer faces
the bearings which support the propeller shafts. At ‹V› are bevel
gears mounted on the propeller shafts, which are driven by co-acting
bevel gears, ‹M›, mounted on the outer ends of the transmission
shafts, ‹O›, the latter being at this point firmly supported in
bearings mounted on the inner faces of the bed plates and steadied
by the intermediate bearings, ‹N›. The two transmission shafts are
seen to be not in line, the rotary cylinder engine that was then
under construction requiring this arrangement. The bed plates, ‹L›,
are further stiffened by the brace tubes, ‹K›, and the transverse
frame is braced against the thrust of the propellers by the tubes
‹J›. The four tubes, ‹P›, unite at their upper ends to form what
was designated as the upper “pyramid,” and the wires, ‹S› and ‹T›,
radiate from its apex to the rear and front, respectively, of the
main frame. The lower “pyramid,” on the under side of the frame, also
has similar wires running fore and aft. The main portions of both
frames are further strengthened by their sub-frames, which merge
together, and the main tubes of the main frame are individually
stiffened in the vertical plane by a minor system of guying. The
scales shown in the photographs are calibrated in metres.
It is to be particularly noted that the midrod, which had heretofore
formed the backbone of the main frame, was now made to act merely as
a means of attaching the wings to the frame, the main strength of
the frame being furnished by the two parallel fifty millimetre tubes
which extended the entire length of the frame and which, reinforced
by the guy-wires, formed a truss not only more rigid transversely,
but also many times stronger in its ability to resist torsional
strains than could be secured by a single tube of equal weight. In
this plan of constructing the main frame, the pyramids constituted
a very important element, for with the guy-wires arranged as they
were it was impossible for any portion of the frame to experience
a stress which was not transmitted in some way to the pyramids. In
the frame, as here shown, these pyramids were formed of tubes 15 mm.
in diameter, 0.5 mm. thick, stiffened against buckling under the
end pressure by means of the cross-braces, which united them near
their midpoints. While the sole function of the upper pyramid was
to serve in the system of guying the frame, the lower pyramid not
only served a similar purpose, but also provided a means for holding
the aerodrome to the launching car in the process of launching
it, the clutch-hooks gripping around the short horizontal tube at
the apex of the pyramid and thus drawing the “bearing points” of
the machine firmly against the uprights on the car. In fact, the
particular arrangement of these pyramids was largely determined by
this necessity for providing means for holding the aerodrome to the
launching car, and the form which seemed best suited to the purpose
was duplicated on the upper side of the frame.
[Illustration: PL. 45
FRAME OF AERODROME A, JANUARY 31, 1900]
[Illustration: PL. 46
FRAME OF AERODROME A, JANUARY 31, 1900]
[Illustration: PL. 47
FRAME OF AERODROME A, FEBRUARY 1, 1900]
[Illustration: PL. 48
FRAME OF AERODROME A, FEBRUARY 1, 1900]
[p169]
The “bearing points” were not attached to the frame at the time these
photographs were taken, but are seen leaning against the scales in
the foreground of Plate 46. Their position on the frame will be more
clearly seen in later photographs, where it will be noted that they
were made use of in the more elaborate system of guying which was
adopted.
While, in general, the frame at this time seemed to be reasonably
stiff and strong, yet it was subjected to a very thorough test by
supporting it at different points and suspending from it weights to
represent the various parts, such as engine, aviator, wings, rudder
and so forth, the deflections which were produced by these weights
being carefully noted. It was further tested by subjecting it to
vibratory strains, such as it would be likely to meet in actual use.
After this the whole frame was tested against torsional strains, such
as would be caused by the wind twisting one set of wings more than
the other. As a result of these tests it was decided that the frame
should be strengthened as far as it was possible to do so without
greatly increasing the weight, which even now was found to be rapidly
increasing beyond what had been calculated as permissible. The main
guy-wires were replaced by heavier and stronger ones, and while
these were found to add somewhat to the stiffness of the frame, yet
something more seemed necessary to insure safety.
The delay in securing the engine, which had been contracted for
with a guarantee that it would be delivered in February, 1899, had
become so serious and had delayed the completion of the frame to
such an extent that the question of building an exact duplicate of
the large machine, but of one-quarter its linear dimensions was
being carefully considered at this time, and it was decided to make
no further changes in the guying of the large frame until after the
small one was built. On account of its smaller size changes could be
more readily and cheaply made on it, and the advantages of different
methods of guying could be just as well studied. Later, when this was
completed, it was found that, with the same system of guying that
had been used in the larger frame, the model was so very stiff that
it did not require any further strengthening, the smaller scale,
of course, accounting for the difference. What was thought to be
the best system to follow in strengthening the frame of the large
machine was, however, first tried on the smaller one, and it was
found that for a very slight increase in weight a very great increase
in strength could be obtained. This change in the system of guying
consisted essentially of building a “trestle” of tubing at a point on
the upper side, midway between the pyramid and the rear end of the
frame. One of the former sets of guy-wires which passed to the rear
of the frame was then replaced by a set which started at the foot of
the rear tubes of the upper pyramid, passed over and was fastened to
the trestle, and from there passed to the rear end of the frame at
the points where the longer guy-wires from the pyramid had formerly
been attached. The [p170] guy-wires on the lower side of the frame,
at the rear, were correspondingly changed so that the upper and lower
systems should be similar, the wires which started from the main
tubes at the foot of the pyramid passing to the bearing points, and
from there to the rear end of the frame.
In order to keep the main frame of the large aerodrome as short as
possible, it had originally been planned to make the distance between
the center of pressure of the front wings and the center of pressure
of the rear equal to five metres. When these same proportions were
followed in the quarter-size model, it was found that it brought the
rear wings so close to the propellers that their lifting effect was
certain to be interfered with by the blast of air created by the
slip of the propellers. It was therefore decided that all things
considered it would be best to increase this distance between the
wings, even though this involved an increase in weight, partly on
account of the increased amount of tubing, and still more on account
of the guy-wires which it would be necessary to add in order to make
up for the weakness due to increased length. The large aerodrome
frame was accordingly lengthened 2.5 feet (76.2 cm.), and the
guy-wire system was changed to that clearly shown by the photographs
of July 10, 1902, Plates 49, 50 and 51, the black cross-lines on the
background being 50 centimetres apart. From an inspection of these
photographs it will be seen that two sets of guy-wires were carried
from the upper and lower pyramids, respectively, towards the rear
of the frame, the first set being carried to the main tubes at the
foot of the “trestle” and the bearing points, and the second set to
these same main tubes at the second cross-tube. The sets of wires
which started from the feet of the pyramids were carried over the
“trestle” on the upper side and the bearing points on the lower side,
and both joined to the main tubes at the rear cross-tube. Additional
cross-guy-wires for stiffening the frame sideways were added in each
of the squares formed by the junction of the cross-tubes with the
main tubes. A secondary system of truss guy-wires running over short
guy-posts attached to the tubes of the main frame also contributed to
the strength and rigidity of the whole.
Although the pyramids had shown no signs of weakness, nevertheless,
because of increased strains due to the lengthening of the main
frame, it was thought advisable to make them stronger. Instead of the
15-mm. tubing, which had formerly been used, 25-mm. tubing of the
same thickness was therefore substituted, and additional cross-braces
were added, as will be seen from the photographs, and from the scale
drawings in Plates 52, 53 and 54, which show the aerodrome as it was
when completed. The numerals attached to these drawings refer to the
detail drawings shown in later plates.
[Illustration: PL. 49
GUY-WIRE SYSTEM, JULY 10, 1902]
[Illustration: PL. 50
GUY-WIRE SYSTEM, JULY 10, 1902]
[Illustration: PL. 51
GUY-WIRE SYSTEM, JULY 10, 1902]
[Illustration: PL. 52
SCALE DRAWING OF AERODROME A, END ELEVATION]
[Illustration: PL. 53
SCALE DRAWING OF AERODROME A, SIDE ELEVATION]
[Illustration: PL. 54
SCALE DRAWING OF AERODROME A, PLAN]
In order to secure the proper adjustment of the guy-wires, not only
of the frame but of many other parts, notably the wings, propellers
and rudder, it was necessary to use a large number of turn-buckles.
As almost every wire [p171] required at least one, and in some
cases two turn-buckles, the weight represented by this single item
rapidly became so formidable as to require serious attention. In
the construction of the models, it had been necessary to employ
some special turn-buckles in connecting the guy-wires of the wings
to their guy-posts in order to secure the minute adjustment of the
wires necessary to prevent the wings from being warped and distorted
by unequal and improper adjustment. These turn-buckles had been made
in the Institution shops, as the very lightest ones which could
be secured in the market were from ten to twenty times as heavy
as it was necessary for them to be to provide ample strength. In
the construction of the large aerodrome, however, the large number
required, and the desire to complete the machine at the earliest
moment, made it advisable to procure the turn-buckles, if possible,
from outside sources, and a very careful search was accordingly made
among the various dealers. After much delay some bronze turn-buckles
were secured which were very much stronger for their weight than any
others on the market, but upon testing them it was found that while
they weighed 45 grammes, their average breaking strength was only
593 pounds. Previous experience had shown that turn-buckles which
would not break under a less load than 750 pounds could certainly
be made to weigh not more than 18 grammes. As even at this time it
was realized that at least 100 turn-buckles would be necessary for
the entire machine, the excess weight which the heavy turn-buckles
would add was felt to be absolutely prohibitory, and the construction
of steel turn-buckles was immediately begun in the Institution
shops. These turn-buckles were at first made in several sizes, and
while some few were at first made “double ended,” most of them
were threaded at only one end, the other end being provided with
a swivel-hook, or eye. They were at first made of mild steel, the
swivel-hooks, in fact, being made of wire nails in order to utilize
the head of the nail as a shoulder without the expense of machining
rod steel of a size large enough to form the shoulder. It was found,
however, that the weak point of this type of turn-buckle was the
swivel end, and most of those which were then on hand were made
double ended by removing the hook, tapping a left-hand thread into
this end of the shank, and fitting a threaded eye-socket in it. The
guy-wires themselves were attached to the eyes of the turn-buckles
and to the fittings on the frame by twisting loops at the ends of the
wires, and although the very greatest difference in the strength of
a completed guy-wire may result from the way in which the loops are
twisted, yet, after much training, the workmen were taught to twist
these very uniformly, following the plan which can be best understood
by an inspection of the drawings in Plate 55 which show the loops
more clearly than they can be described. After the loops had been
properly twisted, soft solder was run all through the twist in order
to unite firmly the twists of the wire. Although special grades of
wire were found which showed very high tensile strength when the wire
was [p172] tested without having loops formed in its ends, yet it
appeared that the twisting of these high-grade wires so seriously
affected them that in the case of guy-wires with loops at the ends,
better final results could be obtained by using softer grades of
steel. The wire which was actually found best, after much experiment,
was a good grade of Bessemer steel of a medium hardness, which had
been “coppered” to prevent rusting. However, even with the softer
grades of steel wire, it was found that there were sometimes hard
spots in the wire which revealed themselves only upon test, and that
when a hard spot occurred in the twisted portion where the loop was
formed, the final strength of the completed guy-wire was sometimes
only twenty-five per cent of what it should be. The precaution was
then taken to subject each of the completed guys to a test strain at
least twenty-five per cent greater than it was calculated the wire
would have to stand in actual use, so that no accident from defective
wires would be likely to occur.
Later on, however, much trouble was caused by the loops in the
ends of some of the guy-wires slipping, owing to the giving way
of the solder which had been run through the joint, the amount of
slipping, while small, being sufficient to alter completely the
relative stresses on the various wires, thus causing distortion of
the framework itself. In order to avoid this difficulty a new method
was devised of attaching the guy-wires to the turn-buckles and to
the fittings by which they were carried to the frame. This method
consisted in threading the ends of the guy-wires so that they could
be inserted directly in the threaded ends of the turn-buckles.
The wires when connected in this way to the turn-buckles showed
absolutely no slip, and the entire system gained greatly in strength
thereby. The only disadvantage which was found in this new method of
attaching the guy-wires to their fittings, was that if the wire was
bent very close to the fitting, it would break in the screw thread
very easily. But since most of the guy-wires when once attached to
the machine are always tight, and in fact, under more or less strain,
there was in most cases no likelihood of the wires being endangered
by being bent close to the fittings. Since the screw threads,
which it was necessary to adopt in this new plan of connecting the
guy-wires, had to be very much finer than the threads which had been
used in the turn-buckles previously constructed, it was necessary to
make new turn-buckles, the others being too thin to permit of their
being bored out, bushed and re-threaded. The new turn-buckles were
made of a much higher grade of steel, and probably represent very
nearly the maximum of strength for the minimum of weight possible
without the use of some of the very much higher-grade steels which
have recently come on the market, but which are exceedingly expensive
to work. By means of this improved plan of attaching the wires,[42]
it [p173] was found possible to gain practically fifty per cent in
the strength of the entire system of guy-wires used on the frame.
Many small changes were from time to time made in the various small
fittings by which the guy-wires were attached to the frame, nearly
all of these fittings having been originally made of a very mild
grade of steel owing to the fact that it was so very much easier to
work. At the time these fittings were made it was constantly expected
that a trial of the aerodrome would be possible very soon, and it
seemed necessary to expedite the work as much as possible and avoid
the delay involved in using grades of steel that would have been
materially harder to work. As is always the case in work of this
kind, retrospect shows many instances where what was supposed to be
a short cut to results actually proved to be the longest path, but
the work as a whole was remarkably free from imperfect parts which
necessitated reconstruction.
In the construction of the frames of the models it had been customary
to fit the tubing accurately at the joints and to join it permanently
together by brazing, as this was not only the lightest form of joint
that could be made, but also the most expeditious method consistent
with securing a strength of the joint comparable with that of the
tubing itself. The construction of the frame by this method of
brazing the joints together permanently, offered, however, several
serious drawbacks: among them, that when a tube got injured it was
a considerable task to replace it, while the brazing of the new
tube in place required extreme care to prevent the frame from being
warped when completed, as the tube became longer while very hot and
contracted after the joint had set. Furthermore, the great heat
required destroyed to a considerable degree the desirable qualities
due to the tube being “cold drawn,” a reduction of strength of
something like 25 per cent being almost inevitable, even when the
brazing was most carefully done. It was, therefore, decided that in
the construction of the large machine all of the main joints should
be made by a system of “thimbles,” and it was planned at first to
make these thimbles by brazing short pieces of steel tubing into the
proper shapes and angles so that they would accurately fit the tubes
which were to be joined. The construction of the thimbles in this
manner, however, seemed to involve an excessive amount of work; and,
as it was found that very thin castings of aluminum-bronze could be
obtained, which would show a tensile strength very nearly as great as
steel, it was decided to make up patterns for the thimbles and cast
them of aluminum-bronze.
The aluminum-bronze castings were obtained and properly machined
to fit the tubes, but when it was attempted to “tin” the interior
walls of the thimbles it was found that the solder could not be made
to stick to the bronze. As a considerable amount of work had been
expended on the machine work of these thimbles much time and effort
was spent in attempting to devise “fluxes” [p174] and solders which
could be made to work with the aluminum-bronze, but the final result
was that the aluminum-bronze thimbles had to be abandoned. They were
replaced by similar castings of gun-metal of a slightly heavier
section, which at the time were thought to be very suitable for the
purpose.
But, in finally assembling the frame after the changes described
above had been made, steel thimbles, built up of short pieces of
tubing, as had originally been planned, were substituted for the
gun-metal thimbles. This change was made not only because of the
great increase in strength, but more particularly because many of the
gun-metal fittings had been imperfectly constructed, so that it was
extremely difficult to align the frame. The steel thimbles, which
were made in the Institution shops proved thoroughly satisfactory and
gave no trouble of any kind. Many of these thimbles and the method of
attaching the guy-wire fittings to them are shown in Plates 56 and
57, as well as in Plate 55.
TRANSVERSE FRAME
It will be recalled from the description of the models Nos. 5 and
6, in Part I, that the position of the line of thrust, with respect
to the positions of the center of pressure and center of gravity in
the vertical plane was, theoretically, very much better in No. 6
than in No. 5. In designing the large aerodrome, it was desired to
reproduce as nearly as possible the relative position of the line of
thrust with reference to the center of pressure and center of gravity
which existed in No. 6, but for constructional reasons it was found
impossible to do so. In fact it appeared that without seriously
complicating the construction of the frame it was impossible to
raise the line of thrust with respect to the center of gravity
materially higher than it was in No. 5. In No. 6 the line of thrust
was 12 centimetres above the midrod, this being effected by placing
the engines some distance from the boiler, and at the extreme ends
of the transverse frame where they were connected directly to the
propellers. In the case of the steam engine the weight of the engine
proper is a relatively small portion of the entire weight of the
power plant, and it is, therefore, possible to put the engine almost
anywhere without materially affecting the center of gravity. But
where a gas engine is used the engine itself constitutes the greater
part of the weight of the power plant, and any raising of the engine,
therefore, materially raises the center of gravity of the whole
machine. The line of thrust in the large aerodrome was, therefore,
practically in the plane of the main frame, and consequently very
little higher than the center of gravity.
[Illustration: PL. 55
FRAME FITTINGS AND GUY-WIRE ATTACHMENTS, ETC.]
[Illustration: PL. 56
FRAME FITTINGS AND GUY-WIRE ATTACHMENTS, ETC.]
[Illustration: PL. 57
FRAME FITTINGS AND GUY-WIRE ATTACHMENTS, ETC.]
The use of one engine to drive two propellers mounted at opposite
ends of the transverse frame, and in a direction perpendicular to
the crank shaft of the engine, necessitates the use of a pair of
bevel gears between each of the propeller shafts and the shafts by
which the power is conveyed to them from [p175] the engine
shaft. Since the efficient transmission of power through bevel gears
requires that they be very accurately placed with reference to each
other, and maintained very accurately in this position while they
are at work, it was necessary to make the transverse frame very
rigid, especially at its extreme ends. This was accomplished by
the use of what were called “propeller-shaft bed plates.” They are
designated by the numeral 27 in Plate 54, and are shown in detail
in Plate 58 as of a very deep I-beam section, having very narrow
flanges top and bottom, the web of the I-beam furnishing the strength
in a vertical direction, while sufficient stiffness laterally was
obtained from the flanges, assisted by the brace tubes, which acted
as struts between the bed plates and the main tubes of the transverse
frame. These struts, while very light, added enormously not only to
the lateral stiffness of the propeller bed plates, but furnished
for a minimum weight a maximum prevention against twisting of the
plates. The propeller-shaft bed plates were originally planned to
be made of sheet metal with the flanges brazed to the web. But at
the time that they were constructed the pressure of the work was so
great in the Institution shops that it was found necessary to have
some of the work done outside, and the parties who undertook the
construction of these bed plates were unwilling to attempt to braze
them up, and accordingly worked them from steel forgings made for
the purpose. The expense of this plan of construction proved large
and unnecessary, as both previous and later experience proved that
it was not only practicable to braze up bed plates more complicated
in their design than these, but that equal strength for equal
weight could thus be obtained for less than one-quarter the cost of
constructing them from solid forgings. Furthermore, where such parts
are made from the solid, changes which later tests prove advisable
can frequently not be carried out without very serious cost and
delay, while with the bed plates formed by brazing less hesitancy is
felt in removing parts which are brazed thereto and substituting new
parts, or even discarding the bed plates altogether and substituting
new ones. Particular emphasis is laid on this point for the reason
that much expense and delay would have been avoided had these very
expensive propeller-shaft bed plates been discarded as early as 1901
and replaced by others which would have permitted a considerable
strengthening of the ball-bearings, which, while strong enough to
stand even more power than they were originally designed for, were
far too weak to be safe when working under the greatly increased
stress due to the very much higher engine power which was later used.
Instead of discarding these bed plates then for new ones, they were
strengthened by brazing to them crescent-shaped pieces, as shown in
the drawings and photographs. This strengthening was made necessary
by the larger hole cut in the bed plates for the larger bevel gears.
The bed plates for the engine, which are later described, besides
other bed plates which were made for other purposes, were all
[p176] formed by the use of sheet metal and tubing properly brazed
together, and none of them ever gave any trouble.
In the early photographs of the aerodrome frame, especially that
of January 31, 1900, Plate 45, it will be noted that the two
transmission shafts, which extend from the propeller-shaft bed plates
towards the center, are not in line, the port transmission shaft
being at the center of the transverse frame, while the starboard
shaft is three inches to one side. This arrangement was necessary in
order to connect the shafts to the rotary cylinder engine which was
being constructed under contract, and which was almost momentarily
expected for more than a year after its original promise of delivery
on February 28, 1899. Later, when this engine was finally found to be
a failure, and the writer constructed the engine in the Institution
shops, the starboard transmission shaft was moved over to the center
line and the crank shaft of the engine, which was carried through on
the center line of the transverse frame, was then connected directly
to the inner ends of the transmission shafts.
These shafts, as well as the propeller shafts, were originally
constructed of steel tubing 1.5 inches in diameter and 1/16 of an
inch thick, but on account of the increased power of the large engine
it was found necessary to increase the thickness of the shafts to 1/8
of an inch. Difficulty was also found with the tubing of which the
shafts were made. This, though not exactly straight when received
from the factory, could be pretty accurately straightened in the
lathe by exercising proper care, but the moment any real strain was
put upon it in the transmission of power, it again went out of shape
and caused serious damage to the bearings by whirling, buckling, and
so forth. As the skin of the tubing is really the strongest part,
owing to the cold-drawing process to which it has been subjected,
great care was taken to secure shafts which were sufficiently
straight for use without machining, but it was finally found
impossible to rely on the unmachined shafts, and all the later shafts
for the aerodrome were made by getting tubing a sixty-fourth of an
inch thicker than was calculated to be necessary and turning off this
extra metal in a lathe.
[Illustration: PL. 58
BEDPLATE, GEARS, ETC.]
Suitable flanges and collars were brazed to the propeller shafts;
but, for convenience in assembling, the flanges by which the main
transmission shafts were connected to the crank shaft of the engine
were at first fastened to the shafts by screw-threads, the threads
being in the proper direction to cause the flanges to jam against
the shoulders of the shafts when the engine turned in its normal
direction. This method of fastening, however, caused serious trouble,
owing to the flanges jamming so tight that it became impossible to
unscrew them after they had once been used in driving the propellers.
The usual provisions of keys and key-ways adopted in general
engineering practice, where solid shafts are employed, were, of
course, out of the question, since the shaft would have to be greatly
increased in thickness throughout its entire length [p177] merely
to provide the extra metal at the small place in which the key-ways
were formed. Taper pins either sheared off or very soon stretched
the holes so badly as to leave the parts loose, and were otherwise
very unsatisfactory. The method finally adopted, which proved very
successful, was that of forming integral with the couplings shallow
internal tongues and grooves which fitted corresponding tongues and
grooves either in the exterior surface of the shafts or in collars
brazed to them at the proper point. The form of flange coupling, in
which bolts draw the two flanges tightly together, was also a source
of considerable trouble and delay, which was finally overcome by
forming shallow tongues and grooves in the faces of the flanges, the
tongues taking up the torsion and relieving the bolts which held the
flanges together of all strain except one of slight tension. The same
difficulties experienced in mounting the couplings on the shafts were
met with in connection with the gears, both on the propeller and
transmission shafts, and were finally obviated in a manner similar to
that described above.
The bevel gears originally constructed for transmitting the power
from the transmission shafts to the propeller shafts, were made of
case-hardened steel and were eight-pitch, twenty-five teeth, with
three-quarter inch width of face. The gears were very accurately
planed to give as perfect a form of tooth as possible, in order to
avoid loss of power in transmission, and although the manufacturer
who cut the teeth on them asserted at the time they were made
that they would not be capable of transmitting more than five
horse-power, yet they actually did transmit considerably more than
twelve horse-power on each set; but they were not strong enough
to transmit the full power of the large engine which was finally
used. The gears that were finally used were similarly constructed
of mild steel which was case hardened 1/64 of an inch deep after
they were finished, there being thirty-one teeth in the gear on the
transmission shaft and forty teeth in the one on the propeller shaft,
the teeth being eight-pitch, three-quarters of an inch face. These
light gears proved amply strong, and several times stood the strain
which they accidentally received when one of the propellers broke
while the engine was under full power, and thus threw the entire
fifty horse-power over on the other propeller, which was consequently
driven at a greatly increased speed.
Plain bronze bearings had been used throughout on the model
aerodromes, but in the construction of the large aerodrome
ball-bearings were used on all of the propeller and transmission
shafts, not only on account of the decreased loss through friction,
but also because ball-bearings can be built much lighter than solid
bronze ones, and, furthermore, do not present such great difficulties
in lubrication. However, owing to the limited size which it was
possible to secure for these bearings, because of their having been
originally designed for only twenty-four horse-power, and without any
margin for a later increase of the [p178] space in which they had
to be applied, they were never really large enough for the work they
had to do when transmitting the full power of the large engine. They
gave continual trouble, and were the source of delay which, while it
cannot be accurately measured, since there were often other causes,
yet might be conservatively estimated at not less than three or four
months. Such a delay, when reckoned in retrospect, can easily be seen
to have caused an expense which would have sufficed for almost any
change in the bearings, bed plates, etc., had the change been made
immediately after the bearings were found to give trouble. With the
better steel which it is now possible to obtain for the races of the
bearings, and with the high-grade balls now obtainable, the bearings
could be readily replaced without changing any other parts and still
be amply strong for the work.
PROPELLERS
Both the tests on the whirling-table and the actual results with the
models had shown that propellers which were true helices formed out
of wood were rather more efficient than those constructed by the use
of a hub in which were inserted wooden arms, forming a framing over
which cloth was tightly drawn. But the very great difference in the
cost of construction and the facility with which the latter type
could be repaired in case of damage—the wooden ones were practically
of no use if once they were much injured-—made it seem advisable to
construct all the propellers for the large aerodrome in the manner
just explained. Several pair of small propellers had been built on
this plan, some as early as 1895, and one very important advantage
had been found to be possessed by this type besides cheapness and
facility of repair. Wooden propellers of even so small a diameter as
one metre had been found to suffer a quite appreciable bending of the
blades, due to the thrust produced by them, even though the blades
had been made of considerable thickness. In planning a propeller 2.5
metres in diameter for the large aerodrome it was seen that in order
to make the blade sufficiently strong to withstand its own thrust it
would be necessary to make it inordinately thick, which, of course,
would mean a considerable increase in weight. In fact, it was seen
that the weight of the larger propellers would increase practically
as the cube of the diameter; which, for the 2.5-metre propeller,
would involve a weight of something over fifteen times the weight of
those one metre in diameter. The other type, which for convenience
we will call “canvas covered,” permitted the bending moment produced
on the blade by the thrust to be taken up by guy-wires running from
the corners of the blades to a central post projecting from the hub
of the propeller, and it was found that in this way a considerable
saving in weight could be effected. [p179]
In November, 1897, in order to obtain by actual test some data on
propellers, such as it was planned to use on the large aerodrome in
case it was later built, it was decided to construct one propeller
2.5 metres in diameter and 1.25-pitch ratio with two blades, each
covering the sector of 36 degrees on the projected circle. About
this same time an engine builder, who some years before had made
some experimental model engines in the Institution shops, proposed
to construct a gasoline engine for the proposed large aerodrome. As
past experience, not only with such engines but with all other forms
of explosive motors, had not been very reassuring it was thought
best to make brake tests of one of the heavier engines which he was
at this time building, and at the same time make tests with one of
these large propellers. A first series of tests was made at several
different speeds, and then a second series was made with the engine
driving the propeller at the same speeds. The engine varied so much,
however, in the power developed at any speed that the data obtained
were of little value. As it was also desired to learn just how much
thrust could be obtained from these propellers, when driven by a
given horse-power, a special hand car was fitted up to carry the
engine, which was connected to a shaft on which the propeller was
mounted. The propeller was raised above the floor of the car and
projected over the rear end of it so as to be as little disturbed
as possible by the deflection of the air currents caused by the
car. This car, with the engine and propeller, was tested on a track
near Mount Holly, N. J., in November, 1897, but the results were
very unsatisfactory. In the first place, the car with the engine
mounted on it was so very heavy and offered such a strong tractive
resistance that very little speed of propulsion could be obtained.
In the second place, the engine, which was said to have furnished
over six horse-power on Prony-brake tests, evidently did not furnish
anything like this amount of power at this time. And in the third
place, the propeller was evidently far too large to permit the engine
to run at the speed at which it would develop a reasonable amount of
power unless some reduction gearing were interposed between it and
the propeller. As the tests, for various reasons, had to be made at
a great distance from Washington, and the supervision of them had to
be entrusted by Mr. Langley to others, who either did not understand
or appreciate the value of obtaining accurate data, it was found
impracticable to continue them.
The large propeller used in these tests was built without special
regard to weight, since it was expected that it would be subjected
to rather rough usage under the very sudden strains produced by
the irregular working of the gas engine. Its hub was made of brass
tubing, the horns being brazed to rings which were slid over a
central tube, the rings being finally soldered to the tube after
the arms had been adjusted to the positions which would give the
blade the correct shape and dimensions. The wooden arms were 1.5
inches in [p180] diameter at the hub end, tapering to 1.25 inches
at the end of the blade. The blade was exceedingly stiff as regards
pressure produced by thrust, but it was found to be considerably
strengthened and made very much safer when guy-wires were added, in
the manner explained above. This general type of construction was
adhered to in all the future propellers for the aerodrome, though
slight modifications, both as to the size of the arms and the number
and position of the cross-pieces which formed the framing of the
blade, were adopted from time to time. A pair of heavy propellers,
2.5 metre, 1.25-pitch ratio, 36-degree blade, the hubs of which were
formed of brass castings, was, however, constructed for experimental
purposes, where weight was not an important factor.
When these propellers were designed, the calculations as to their
size and the horse-power which would be required to drive them at a
certain speed were based on the very incomplete data obtained from
the various propeller tests conducted during the preceding years.
When later calculations were made for them, on the data obtained in
the more accurate tests made in the summer of 1898, it was found
that the power of the engines with which it was proposed to equip
the aerodrome would not be sufficient to drive the propellers at
anything like the speed which the former calculations had shown would
be possible; and that, therefore, either the ratio of the gearing
between the propellers and the engine would have to be changed so
as to permit the engine to run at a very much higher speed than the
propellers, or that propellers, having either less pitch or a smaller
diameter, and possibly both, would have to be substituted for these
larger ones.
Since it was easier to change the propellers then to change the
gearing, a new set of propellers was designed which were of 2 metres
diameter, with a pitch ratio of unity, and with a width of blade of
only 30 degrees. It was calculated that 20 horse-power would drive
these two propellers at a speed of 640 R. P. M., when the aerodrome
was flying at a speed of 35 feet per second and the propellers were
slipping about 50 per cent, this being found to be about the speed
at which the engines might be expected to develop their maximum
power. As the larger propellers having the brass hubs were thought
to be excessively heavy, the hubs weighing 10.25 pounds each, and
as any change either in size, pitch, or width of blade necessitated
a new set of patterns in case the hubs were cast, it was decided
to construct the new hubs of steel tubing. The weight was further
reduced by decreasing the size of the wooden arms to 1-1/4 inch in
diameter at the hub, tapering to 1 inch at the end of the blade.
After the engine builder in New York had been unable to fulfil his
contract on the engine, and it had been condemned, propeller tests
were made with the experimental engine built in the Institution
shops. These tests showed: First, that the results which might be
expected from larger propellers could be very safely predicted by
extrapolation from the results of the propeller tests of 1898; [p181]
and, second, that in order to get a thrust which would equal fifty
per cent of the flying weight of the aerodrome it would be necessary
to use propellers larger than two metres in diameter unless a very
large surplus of power were provided. It was accordingly decided
to make a set of propellers intermediate between the two-metre,
unit-pitch ratio, thirty-degree blade ones, and the original ones
which were two and one-half metres, one and one-quarter-pitch ratio,
thirty-six-degree blade. A set was, therefore, designed two and
one-half metres in diameter, unit-pitch ratio, and thirty-degree
width of blade, the hubs being made of steel tubing brazed up in the
same manner as the two-metre ones, and the wooden arms of the blades
being one and three-eighths inches in diameter at the hub end, and
tapering to one inch at the end of the blade.
Later, when the larger engine was actually tested in the frame, the
inability of the original transmission and propeller shafts to stand
the extra strain caused by the engine starting up very suddenly at
times, together with the unsatisfactoriness of the screw-thread
method of fastening the gears and couplings to the shafts made it
necessary to provide new shafts, gears, couplings, etc. It was then
decided to change the ratio of gearing between the engine and the
propellers, which had been one to one, so that the engine might run
faster and, therefore, permit the use of larger propellers. For
constructional reasons the ratio chosen was thirty-one to forty,
thus making the engine run approximately one-third faster than the
propellers.
In the various tests made of the engine working in the frame there
were two or three instances in which the propellers were damaged
either by the sudden starting of the engine or by their not being
able to stand the strain to which they were subjected by the power
absorbed, but in every case such breakages were found to be due to
imperfections of the brazing in the joints. While, therefore, it
would have been desirable to make the propellers somewhat heavier,
yet since the total weight of the aerodrome had been growing so
very rapidly, it was felt that this need not be done, as a pair of
propellers which had stood quite severe service in shop tests might
reasonably be expected to stand the strain of actually propelling the
aerodrome through the air.
Nevertheless, when in the summer of 1903 the actual trials of the
large aerodrome were started, it was found that the very important
difference between a propeller working in a closed room and one
working in the open air had not been given due consideration.
Several sets of propellers, 2.5 metres in diameter, unit-pitch
ratio, 30-degree blade had been constructed and were on hand, in
order that no delays might be caused through a lack of such extra
parts. On September 9, 1903, when the aerodrome frame without the
wings was mounted on the launching car on top of the boat for some
trial runs with the engine to make sure that everything was again
in readiness, before the engine had made 500 revolutions, the
port propeller broke; and a few minutes [p182] later, when a new
propeller had been substituted for this and the engine was again
started up, the starboard propeller also broke. When, upon further
trials and replacements of propellers, all had been so thoroughly
demolished that there was not a complete set remaining, it was seen
very clearly that the strains produced on a propeller working in the
open air are very much greater than those produced in shop tests,
where the air is necessarily quiet. These open-air tests of the
propellers had demonstrated that their weakest point was where the
steel tubes which received the wooden arms of the blade terminated,
and that another, though not so serious, point of weakness was where
the steel arms were brazed to the central hub, the thin metal tending
to tear loose even before the brazed joint would give way. It was,
therefore, decided to construct immediately a new set of propellers
in which the steel arms should be made of much heavier tubing, that
is, a sixteenth of an inch thick at the end where it was brazed to
the central hub, and tapering in thickness to one-thirty-second of
an inch at the other end. These arms were further made twelve inches
long in place of being only three inches long as before. This added
length carried the steel out beyond the point where the first section
brace joined the three arms together, and where they were further
strengthened by having the cloth covering tightly stretched around
them. In order to utilize such of the hubs of the former propellers
as had not been seriously damaged when the propellers broke, it was
also decided to try the effect of merely adding an extra length
of tube to the short arms by means of a thimble slipped over and
brazed to the two parts, which would make these arms twelve inches
long. The construction of these propellers was pushed as rapidly as
possible; and after their completion no further trouble was at any
later time caused by insufficient strength of the propellers. Even
in the test of October 7, 1903, when the aerodrome came down in
the water at a speed of something like fifty miles an hour, and at
an angle of approximately forty-five degrees, no break occurred in
either propeller until, when the aerodrome was plunging through the
water, a blade of one propeller was broken by the terrific blow which
it received when it struck the water under the impulse of the engine
driving it at full speed. The severity of this blow is attested by
the fact that the shaft, which was of steel tubing one-eighth inch
thick, was twisted about ninety degrees.
This experience with propellers very strongly emphasizes the fact
that on any flying machine the strains which are apt to be met with
in the open air must be allowed for in the proportioning of the parts
of the machine. But since an indiscriminate increase of strength
in all the various parts of the machine would entail a prohibitory
weight, very careful judgment, based on experience, will have to be
exercised in deciding just where added strength must be employed, and
also where the “live strains” are not apt to exceed very appreciably
the calculation for statical conditions.
[Illustration: PL. 59
WING CLAMPS]
[p183]
Owing to Mr. Langley’s belief that the tests of the man-carrying
aerodrome must not only be made over the water, but that it was
necessary that the machine be launched from a car running on a
track at a considerable elevation in order to permit the machine
to drop a short distance after being launched in case it was not
quite up to soaring speed when launched, it was necessary that the
aerodrome be so constructed that it could be readily transported
to the launching track from the interior of the house-boat where
it was stored. This plan of storing the main body of the machine
in the interior of the boat and hoisting it to the launching track
just before attempting a flight (some of the difficulties of which
may be more clearly appreciated by an inspection of Plate 60), made
it necessary that the wings, tail and guy-posts be so constructed
as to be readily attachable to and detachable from the main frame,
and since the weather conditions are seldom suitable for a test
for more than a couple of hours at a time, it was necessary that
the mechanism employed for attaching these parts be so arranged
that the proper settings of the different parts could be quickly
obtained, and without requiring the exercise of judgment which past
experience had shown did not often manifest itself during the hurry
of the preparations for a test. While the wings, therefore, were
made removable, yet all of the sockets, guy-wires, etc., which were
loosened in removing them, were made with positive stops on them so
that each fitting that was to be tightened up in assembling could be
adjusted to its definitely determined position.
As all of the models had been constructed with these same parts
removable in order to permit them to be readily shipped back and
forth in the many trips which had been made with them from Washington
to Chopawamsic Island, the same details of arrangement were used
for attaching these parts on the large aerodrome, though the actual
fittings by which the parts were attached in the latter case became
more elaborate.
In the drawings, Plates 52, 53 and 54, the method of attaching the
wings to the frame is clearly shown. Each of the two main ribs of
each wing was secured to the midrod of the frame by a wing clamp,
shown in detail in Figs. 1, 2, 5, 6 and 7 of Plate 59. Figs. 1 and
2 show the clamp for the middle main rib of each pair of wings, and
Figs. 5 and 6 show the clamp for the main front rib, the latter being
so constructed that the wings could be rocked on the midrib clamp as
a pivot and secured at any angle of lift desired from 6-1/2 degrees
to 15 degrees. The horns on each clamp merely acted as receiving
sockets for the ends of the ribs, and were not in any way intended
to do anything more than merely hold the ends of the ribs in their
correct positions. The wings were fastened to the frame by the
guy-wires which ran from two points on each main rib to an upper and
a lower guy-post mounted on the midrod. The system of guy-wires for
the wings is clearly shown in Plates 52, 53 and 54, and [p184] in
Plate 61, which shows the aerodrome mounted on its launching car at
the rear end of the track, and with the front pair of wings in place
and all the guy-wires adjusted. The details of the guy-posts are
shown in Plate 62, where it will be noted that the lower guy-post was
of wood, with metal fittings, and was 2 metres long from the center
of the midrod to the bottom, while the upper guy-post was a steel
tube 109 centimetres long from the center of the midrod to its top.
The guy-wires from the middle rib of each of the pair of wings were
fastened to the fittings at the bottom of the lower guy-post, while
the wires from the front main rib were fastened to the fittings which
were brazed and riveted to the slidable collar, which was mounted
on the steel tube forming the cap on this guy-post. This collar was
made slidable to permit the angle of lift of the wings to be readily
changed without affecting the length of the guy-wires. This collar,
when once set for any particular angle of the wing, was prevented
from sliding by a taper pin (not shown) which passed through it and
the guy-post. In order to secure the wings more rigidly to the main
frame and thereby throw on it all torsional strains from the wings,
which it was specially designed to take, each of the middle main
ribs was secured to one of the main tubes of the main frame by an
auxiliary clamp at the point where this rib crossed the main tube.
These auxiliary clamps are clearly shown in Figs. 3 and 4 of Plate 59.
Projecting from the lower end of each of the lower guy-posts was a
five-sixteenth-inch steel rod about one inch long, as clearly seen
in Plate 62. Brazed to the side of this rod, in such a position that
it would project towards the rear of the aerodrome when the guy-post
was in position, was a small arm or bracket. When the guy-post was
in place with the aerodrome on the launching car, this pin was in a
slot formed in a metal cap on the top of the small folding upright
at the front or rear of the car, as seen in Fig. 1, Plate 63, while
Fig. 2 of Plate 63 shows the pin just being inserted into this slot
as the guy-wires of the guy-post are being fastened. This small arm
or bracket on this rod projected under the cap to prevent the rod
of the guy-post from being lifted out of the slot in the folding
upright, when the wind acting under the wings tended to lift the
aerodrome from the car. Particular attention is here called to this
apparently insignificant detail, for it was this arm or bracket on
this small rod of the front guy-post which, hanging in the cap on
top of the folding upright, caused the accident in the launching of
the aerodrome on October 7, 1903. Certain it is that but for the
accident due to this apparently insignificant detail, success would
have crowned the efforts of Mr. Langley, who above all men deserved
success in this field of work, which his labors had so greatly
enriched.
[Illustration: PL. 60
FIG. 1
FIG. 2
FIG. 3
HOISTING AERODROME TO LAUNCHING-TRACK]
[Illustration: PL. 61
AERODROME ON LAUNCHING-CAR; FRONT WINGS IN PLACE, GUY-WIRES ADJUSTED]
[Illustration: PL. 62
DETAILS OF GUY-POSTS]
[Illustration: PL. 63
FIG. 1.
FIG. 2.
GUY-POST AND PIN ON LAUNCHING CAR]
[p185]
AVIATOR’S CAR
In determining on a suitable car for the aviator various designs were
made, differing all the way from that in which the aviator occupied
a sitting position facing directly ahead and with practically no
freedom of movement, but was even strapped to the machine to avoid
the possibility of being thrown out, to the one finally adopted, in
which he was provided with the greatest freedom of movement, could
either stand or sit, as the occasion seemed to demand, and could
face in any direction for giving proper attention to any of the
multitudinous things which might at any time require his attention,
and could, if agile, even climb from the extreme front of the machine
to the rear. The wisdom of giving the aviator complete freedom
without hampering him in any way by provisions for preventing his
being thrown out of the machine was amply justified, as will later be
seen in the description of the tests of the machine, where freedom of
movement and agility prevented a fatal accident.
The aviator’s car was therefore designed to occupy the entire
available space between the engine and the front bearing points, and
between the two main tubes of the main frame, thus allowing him a
space of something like three feet by five feet. The car itself was
shaped like a flat-bottomed boat, the bottom being approximately
level with the bottom of the lower pyramid. It had a guard rail
of steel tubing eighteen inches above the floor, with a cloth
covering drawn over the frame to decrease the head resistance of the
appurtenances of the engine which were placed at the rear end of the
car. The car was supported by vertical wires passing from its bottom
up to the main frame, and was prevented from longitudinal or side
motion by being fastened at the front to the cross-rod connecting the
front bearing points, and at the rear to the lower pyramid. A light
wooden seat extended fore and aft of the car at a height of about two
feet from the floor, this seat resting on blocks of sponge rubber to
absorb some of the tremor which existed in the whole aerodrome when
the engine and propellers were working at high speed. The aviator
was thus free to stand, to sit sidewise or to straddle the seat,
and while the network of wires surrounding him prevented any great
possibility of his being thrown out, yet there was a comparatively
large opening between the guy-wires passing overhead which permitted
him to climb out of the machine.
In order to enable the aviator to know exactly how the engine was
operating, a tachometer, giving instantaneous readings of the number
of revolutions, was connected by a suitable gear to one of the
transmission shafts and placed where it could readily be seen.
During 1898 and 1899 considerable time and attention had been given
to designing an instrument to be carried by the aerodrome which would
automatically record the number of revolutions of the engine, the
velocity and direction [p186] of the wind relative to the machine,
the height of the aerodrome as shown by a specially sensitive aneroid
barometer, and the angle of the machine with the horizontal plane
of the earth. The construction of this instrument was undertaken by
a noted firm of instrument makers, but after many months of delay,
during which it was several times delivered as being complete,
only to be returned for further work, it was finally condemned as
unsatisfactory, and it was decided not to encumber the machine with
such a delicate apparatus, which, even if perfectly made, could not
be depended on to work properly when mounted on the aerodrome frame,
in which there was a constant, though minute, tremor due to the high
speed and power of the engine.
The completed frame, which is perhaps best shown in Plates 49, 50
and 51, and Plate 60, Figs. 1, 2 and 3, in spite of its size gave an
appearance of grace and strength which is inadequately represented
in the photographs. In making the designs for the large aerodrome no
data were available for use in calculating the strains that would
come on the different parts of the frame while in the air, and the
size and thickness of the tubes and the strength of the guy-wires
were consequently determined almost entirely by “rule of thumb,”
backed by experience with the models. Although the dimensions, shape,
and arrangement of most of the auxiliary parts of the machine were
considerably changed during the course of construction in accordance
with the indications of the exhaustive series of shop tests, the
fundamental features of the construction were practically unaltered,
but the changes in the guy-wire system and in the fittings by which
they were attached, made the frame as a whole several times as strong
as it was originally, and it was felt that the direction of further
improvements in it would be shown only by actual test of it in flight
where any weaknesses would be certain to manifest themselves.
It may be well to remark here that even with the data which were
later obtained, judgment based on experience proved after all to
be the safest guide for proportioning the strength of the various
parts. It can be assumed that a live stress will produce a strain
ten times as great as that due to a static stress on the part when
the machine is stationary. For greater safety, it would be still
better to assume a strain twenty times as great. If one is building
bridges, houses, and similar structures, where weight is not a prime
consideration, it would be criminal negligence to fail to provide a
sufficient “factor of safety,” or what in many instances may be more
properly termed a “factor of ignorance,” while at the present time
the insistence on large factors of safety in machines intended to
fly would so enormously increase the weight that, before one-half
the necessary parts were provided, the weight would be many times
what could possibly be supported in the air. Later, no doubt, as
experience is gained in properly handling the machine in the air,
increased strength entailing [p187] increased weight may be added
in proportion to the skill acquired; and there is no doubt that man
will acquire this skill with marvelous rapidity, approaching, if not
equaling, that exhibited by him in the use of the bicycle, which,
when first ridden, requires not only all of the rider’s skill but
that of a couple of assistants, but when once mastered requires
hardly more thought for its proper manipulation than even the act of
walking involves, the balancing and guiding being done intuitively
merely by the motion of the body and with practically no exertion.
[p188]