GENERAL CONSIDERATIONS
In the development of man-carrying flying machines two well-defined
paths are open. First: Starting with gliding machines, in which
gravity furnishes the motive power, the operator may by practice
acquire sufficient skill in controlling them to warrant the addition
of propelling mechanism, and individual skill in control may be
gradually replaced by automatic controlling mechanism. Second: From
self-propelled models, possessing automatic-equilibrium controlling
mechanism, and of a sufficient size to furnish determinative data,
one may, by proper modification in size and construction, progress
to an automatically controlled man-carrying machine in which, for
ideal conditions, no especial skill on the part of the operator is
required. Each method has its advantages.
After concluding his earlier and purely physical researches, the
results of which were embodied in “Experiments in Aerodynamics,” Mr.
Langley was so firmly convinced of the practicability of mechanical
flight that he undertook the construction of the model aerodromes
in order to demonstrate it. It is very doubtful if at any time,
prior to the successful flights of the models in 1896, he seriously
contemplated the construction of man-carrying machines. His object
in developing the models was not, therefore, to furnish a prototype
for a large machine, but merely to demonstrate the feasibility of
mechanical flight; and this he did. This is shown very clearly by the
closing remark of the article he published in 1897, describing the
flights of the models. “I have now brought to a close the portion of
the work which seemed to be specially mine—the demonstration of the
practicability of mechanical flight-—and for the next stage, which is
the commercial and practical development of the idea, it is probable
that the world may look to others.”[37] When he later undertook
the construction of the large machine for the War Department it
was natural that, with the inspiring sight of the models in flight
still fresh in his mind, he determined to use as a prototype these
successful machines, which were the only things of human construction
that had ever really flown for any considerable distance.
Not being an engineer, and realizing that to pass from the
construction of models to that of man-carrying machines involved the
solution of many engineering problems, Mr. Langley, in the spring of
1898, sought the advice of Dr. R. H. Thurston, who had from the first
manifested the deepest interest in his [p129] work in aerodromics.
On the recommendation of Dr. Thurston he engaged the services of the
writer, who assumed charge of the work in June, 1898.
While the method of “cut and try” had brought success in the models,
and was perhaps the only method by which they could have been
successfully developed, it was thought that, with these models as a
basis of design, much time would be saved by making an analytical
study of them as engineering structures, and from the data thus
obtained the proper proportions for the parts of the larger machine
could be calculated.
Such an analytical study, however, revealed very little from which to
make calculations as to the strength necessary for the various parts
of the large machine, but it did show very clearly that most of the
parts were working under stresses generally far above the elastic
limit of the materials, and in many cases the ultimate breaking
strength was closely approached. Such a condition was the natural
outcome of the method by which these models had been developed—all
the various parts having been built at first of the least possible
weight and, when they proved too weak, strengthened until they would
withstand the stresses imposed on them. It is extremely doubtful if
previous calculations as to the strength necessary would have been
of any assistance, in fact it is probable that it would have been a
distinct disadvantage and would have resulted in the machines being
entirely too heavy for flight.
The exact strength which had been incorporated in the frames of
the models was as unknown as was the exact amount of the stresses
which they has been made to withstand. Their static strength was
easily determined by calculation, but the stresses due to the live
loads were incapable of exact determination from the available
data, for stresses produce strains, which in turn generally cause
distortions accompanied by greatly increased stresses. While exact
data were, therefore, lacking as to stresses and strengths in many
of the important parts, yet the models furnished most important
illustrations of unusual strength for minimum weight, and a careful
study of them showed many ways in which increased strength could be
obtained with decreased weight which could hardly have been devised
without these concrete examples.
It was, however, by no means possible to build the large aerodrome
within the permissible limits of weight by simply increasing the
various parts of the models according to some predetermined function
of the size of the whole.
The fundamental difficulty is that inevitably, by the laws of
geometry, which are mere expressions of the properties of space,
if a solid of any form is magnified, the weight increases as the
cube, while the surface increases only as the square, of the linear
dimensions. Successive generations of physicists and mathematicians
pointed out that while this “law of the cube” is of advantage in
the construction of balloons, yet it is a stumbling block that will
prevent man [p130] from ever building a dynamic flying machine
sufficiently large to carry even one human being.[38]
However, since strength is a function of material and form rather
than weight, it is possible by selecting proper materials and
adopting suitable structural forms to evade to a certain extent
this “law of the cube.” The whole history of structural science has
therefore been a series of attempts to find stronger and lighter
material and to discover methods of so modifying form as to dispense
with all parts of a structure that do not contribute to its strength.
So in aerodromics the structural problem has been that of finding
materials and forms best suited to the purpose for which they are
required, for it does not always follow that either the form or
the material best suited for one scale of construction is the most
advantageous to employ on a different scale. Nor is even the form
or material which gives the greatest strength for the least weight
necessarily the best to employ. For the structural problem must
necessarily be co-ordinated with those of balancing, propelling, and
transporting, and each must, therefore, have its proper attention in
the design of the whole machine.
Many of the general considerations of the design of an aerodrome
sufficiently large to transport a man were determined during the
spring and summer of 1898, when the first actual drawings (Plate
32, Figs. 1, 2 and 3) of the proposed machine were made. Starting
with the assumption that the Models Nos. 5 and 6 were capable of
transporting a load of approximately ten pounds more than their
weight, it was seen that, since the supporting surface of any
aerodrome would increase approximately as the square of the linear
dimensions, in order to carry a man the aerodrome would need to be
approximately four times the linear dimensions of these models.
Calculations based on the results accomplished in the construction
of the models indicated that such an aerodrome would need to be
equipped with engines developing 24 horse-power. The best that could
reasonably be hoped for was that these engines would not weigh over
200 pounds, and, therefore, allowing 40 pounds for fuel and fuel
tanks, it became necessary to bring the weight of frame, supporting
surfaces, tail, rudder, propellers and every other accessory within
250 pounds, if the total weight of the machine, including 150 pounds
for the aeronaut, was not to exceed 640 pounds, or 16 times the
combined weight of the model and its load of 10 pounds. Although the
problem of constructing the frame, wings and all other parts within
the limit of 250 pounds seemed indeed formidable, it was believed
that the greatest obstacle in the production of such a machine would
be that of securing a sufficiently light and powerful engine to
propel it.
[Illustration: PL. 32
DRAWINGS OF PROPOSED MAN-CARRYING AERODROME, 1898]
[p131]
A brief account has already been given of the attempts made by Mr.
Langley to secure a suitable gasoline engine for the large aerodrome,
but the difficulties encountered in the search have not perhaps been
sufficiently emphasized. At this time (1898) the automobile industry,
through which has come the development of the gasoline engine, was
in its infancy, and there were few builders either in the United
States or Europe who were attempting anything but rough and heavy
construction. Many of them were enthusiastic over the possibilities
of the internal combustion engine, and were ready to talk of devising
such an engine as the aerodrome would require, but few were willing
to guarantee any such definite results as were demanded. However,
the prospects of securing a suitable gasoline engine from a reliable
builder within a reasonable time seemed so strong that it was decided
early in 1898 to begin the construction of the frame on the general
plan which would probably be best adapted for use with a gasoline
engine, and in case it finally proved impossible to secure such an
engine, to construct later a steam plant which could be adapted to
this particular frame.
Some tentative work on the construction of the frame was accordingly
begun in the summer of 1898, some months before an engine builder was
found who seemed likely to be successful in furnishing the engines.
An extensive series of tests on propellers was also made at this time
for the immediate purpose of determining what form and size would
be best, since the dimensions of the transverse frame could not be
definitely settled until it was known how large the propellers would
need to be.
Preliminary designs were also begun for the wings, rudders, and
launching apparatus, but when the point was reached of actually
making the working drawings for these, it was seen that the change
in the scale of the work required many important modifications in
constructional details. As the models had flown successfully only
three times, and in each case under practically the same conditions,
it was felt that it would be unwise to make changes in important
details without first making a series of tests of the models in
flight to determine the effect of such changes. It was therefore
decided to completely overhaul Models Nos. 5 and 6, strengthening
them in many important parts and “tuning up” their power plants,
which had slightly deteriorated since they were last used in
November, 1896. When the work of preparing these models for further
experiments was begun it was thought that it would require at most
only a few weeks, but as it progressed it was found that certain
parts of the mechanical work on the engines had been so poorly
executed originally that it would be necessary to practically rebuild
the engines. The final result was that the power plants of both
aerodromes were entirely rebuilt, and they were not ready for actual
test in flight until the spring of 1899. [p132]
Much of the preliminary work necessary for the determination of
actual working plans was therefore completed in the summer and fall
of 1898, and when on December 12 a seemingly satisfactory contract
for the engines for the large aerodrome had been made it was thought
that rapid progress could be made on the constructional work after
January 1, 1899, when the allotment from the War Department would
become available.
[p133]