THE QUARTER-SIZE MODEL AERODROME
Owing to the very considerable changes which constructional reasons
necessitated in the relative positions of the center of pressure,
center of gravity, and line of thrust from those which theoretical
considerations pointed to as being best, it was decided in January,
1900, to build a one-quarter-size model of the large aerodrome, if
a suitable engine capable of furnishing something like one and a
half horse-power could be procured without delay. It was hoped that
it might be possible to construct this model immediately without
seriously interfering with the progress of the work on the large
machine, and that some tests in free flight could then be made with
it, which would give very much more reliable data from which to
determine the balancing of the large aerodrome than had been obtained
from the tests of the steam-driven models Nos. 5 and 6. A factor of
uncertainty would still remain, due to the difference in size between
the large machine and the model, which could be determined only by
actual trial of the large machine itself; but by making the model
an exact duplicate, on a smaller scale of the large machine, very
valuable results could be obtained. Tests of it in free flight would
involve, even with the probable attendant breakages, a comparatively
small expenditure of time and money. A search was immediately begun
for an engine builder who would undertake to furnish a suitable
engine for this model. The specifications called for an engine
developing one and a half horse-power on the Prony brake for five
minutes without diminution in power caused by over heating. While
it was desired if possible to get an engine which would come within
the given weight and develop the required power for a longer time
than five minutes, it was foreseen that the construction of a
multiple-cylinder engine of so small a power made it necessary to
resort to the air-cooled type, and that such an engine would be
doing exceedingly well to develop its maximum power continuously
for as much as five minutes. The only engine builder who could be
found willing to undertake the construction of such an engine was
the one already engaged in the construction of the larger engine.
As this builder was already twelve months behind in the delivery of
the large engine, it was felt that it would be unwise to give it
to him, both because the work on it might still further delay him
in the completion of the large one, and also because he was still
having troubles with the large one, which it was not certain he would
ever be able to overcome. After further consideration of the matter,
however, it seemed so important to have a model which was an exact
duplicate of the large machine for the [p227] making of tests, which
might prevent not only serious damage but possibly fatal accidents,
that upon the assurance of the engine builder that the undertaking
of the small engine would in no way interfere with the completion
of the large one, a contract was entered into on February 23, 1900,
which specified that the engine should be delivered by April 1, with
a penalty for any delay beyond that date.
The frame for this quarter-size model was immediately begun and
extra workmen were employed for work on it in order that its
construction should in no way delay the completion of the large
machine. The decision to construct this quarter-size model of the
large aerodrome had been made on the assumption that, since it was
to be one-sixteenth the weight of the large machine, and therefore
much heavier in comparison to its size than the steam models Nos. 5
and 6, it would, therefore, not need to be so carefully constructed
in order to obtain sufficient strength. But when construction was
actually begun it was found not only that the simpler and less
expensive methods which it had been proposed to use in joining its
frame together resulted in a weak construction, but also that the
time consumed in tinkering up the imperfections in the joints more
than counterbalanced the extra time which would have been required to
make the joints in the best manner from the beginning. Before going
very far it was therefore decided to make the joints by following
the same process which had been developed in the construction of
the previous models. The frame was accordingly built in the most
substantial manner, and when guyed by a system of guy-wires similar
to that employed for the large machine it was found to be exceedingly
stiff, in fact very much stronger and stiffer than the frame of any
of the preceding models.
In originally planning the model the intention was to make all its
linear dimensions exactly one-fourth those of the large aerodrome.
Before the designs were completed, however, it was seen from the
previous experience with the steam-driven models that instead of the
62.5-cm. propellers, which a strict adherence to the quarter-size
plan would demand, it would be necessary to use propellers which were
at least one metre in diameter. Moreover, as the small engine would
be more than one-fourth the size of the engine under construction
for the large aerodrome, a departure from the scale in the case of
the transverse frame would be necessary. The designs were therefore
altered so as to admit of using the larger propellers, and the tubes
which formed the front of the transverse frame were bent, as shown in
the plan photograph, Plate 70, in order to give a large enough space
for properly mounting the engine.
The frame with these modifications was completed in June, 1900, but
no engine was ready for it, as the builder had failed to fulfill
his contract for either the large or the small engine, although
several trips to New York had been made to expedite their successful
completion. [p228]
Soon after this it became certain that the engines for both
aerodromes would have to be constructed in the shops of the
Institution, and owing to the greater importance of the experimental
engine for the large aerodrome, all the facilities of the shops were
devoted to the early completion of it. In November, 1900, however,
it was seen that the experimental engine alone would not furnish
sufficient power for the large aerodrome, and that a duplicate
of it would have to be built or a new and larger engine designed
and constructed, and that therefore it would be impossible to get
the first tests of the large aerodrome in free flight before the
following summer. It was therefore decided that it would be best
to suspend work temporarily on the large aerodrome and its engine,
and put all the workmen who could possibly be employed on the
construction of the small engine, so that it would be ready in time
to permit some tests of the quarter-size model to be made during the
following spring.
In order to expedite its construction as much as possible, the
attempt was made to utilize all the available parts from the small
engine which had been undertaken by the engine builder in New York.
The cylinders, which it had been expected would be kept cool by
their rotation around the crank pin, were not well adapted for use
as stationary cylinders, since they were not provided with radiating
ribs, but it was hoped that by using them an engine could be very
quickly constructed which would keep cool long enough to enable some
short flights to be made with the model.
The work on this small engine was pushed forward very rapidly, so
that within a short time it was sufficiently complete to allow some
power tests to be made with it. In the first of these tests the
attempt was made to measure the power by means of the Prony brake,
but as the engine had no fly wheel the fluctuations in speed during
each revolution were so great as to make it impossible to obtain
readings of any value. When it was attempted to remedy this by
putting a fly wheel on either side of the crank shaft of the engine,
it was found that the sudden starting of the engine caused such
severe strains in the crank shaft, which had been built strong enough
for driving the propellers but not for suddenly starting fly wheels
having considerable inertia, as to make it unsafe to continue the use
of fly wheels. As without them the Prony brake could not be used, it
was decided to build a small water-absorption dynamometer on the same
principle as the larger ones which were under construction for the
large engine. As this larger dynamometer has already been described,
it is only necessary to add that the small one consisted of twelve
rotating plates and twelve stator plates twelve inches in diameter.
In order to avoid the construction of a special and elaborate testing
frame for mounting the engine and the dynamometer exactly in line
with each other, it was attempted to connect them by means of a
universal joint. This “short cut” also proved the “long way around.”
The strains set up in the universal joint by the sudden starting of
[p229] the engine caused so much trouble on account of the inertia
of the rotating plates of the dynamometer that the time lost in
keeping the universal joint in working order during the tests more
than counterbalanced the extra time which would have been required to
construct a special wooden frame on which the dynamometer and engine
could have been mounted in line with each other so that the crank
shaft of the engine could have been directly connected to the shaft
of the dynamometer.
Much time was also lost in the effort to construct an apparatus
by which a record could be obtained of the power actually used in
propelling the aerodrome. Various methods were in use by which
the thrust of the propellers could be more or less satisfactorily
measured while the aerodrome was at rest, but it was desired to
know just how much power the aerodrome consumed while in actual
free flight. Such a record it was hoped to obtain from a device
incorporated in the propeller shafts. This thrust-measuring device
consisted essentially of a propeller shaft made in two sections, one
section telescoping the other for a short distance. On the section
of the shaft to which the propeller was attached there was mounted
a drum, having in its circumference two long slots diametrically
opposite. To the other section of the shaft a disc was fastened with
two diametrically opposite rollers mounted on its periphery, which
fitted the slots in the drum of the other section. A compression
spring was interposed between the disc and the drum, and the outside
of the drum was so arranged that a strip of paper could be wound
around and fastened to it which would serve as a chronograph sheet. A
pencil was fastened to the frame, and, since the drum was connected
to the section of the shaft to which the propeller was attached and
which therefore moved to and from the frame under the action of the
propeller thrust, a record of the actual thrust of the propeller at
any particular moment could be obtained by simply pressing the pencil
up against the paper on the drum and calculating the thrust from
the calibration of the compression spring. Since the thrust would
naturally be greater when the propellers were revolving in a moored
condition, during the few moments after the engine was started up and
before the aerodrome was launched, it was necessary to provide means
for having the pencil point held away from the chronograph sheet
until the aerodrome was launched, and then have the point come to
bear on the sheet. This was accomplished by having the point held off
by a small trigger arrangement which was to be released just at the
moment that the aerodrome left the launching car. A set of propeller
shafts embodying this thrust-recording device was constructed, but
when they were actually tested on the aerodrome many difficulties
were encountered which had not been anticipated. In the first place
the gasoline engine for the model was started up (or “cranked over”)
by turning the propellers by hand. A gasoline engine never starts
slowly, and on account of this suddenness of starting causes a very
great strain in any [p230] shafting by which it is connected to any
driven mechanism. The inertia of the driven mechanism, even though it
be apparently small, becomes a most serious matter when an attempt is
made to start up very suddenly. This effect is very much intensified
if the driven mechanism is connected to the engine through even one
pair of gears, for there is always a certain amount of back-lash
between the teeth of the gears, and the effect of this back-lash
is still further intensified when the driven mechanism is turned
over by hand in order to start the engine, as this takes up the
back-lash on one side of the gears, and the moment the engine starts
permits a free movement until it suddenly takes up the back-lash and
strikes the other side of the gear teeth with a blow. The effect
of this sudden starting of the engine proved most disastrous to
the thrust-recording devices, and, although they were considerably
strengthened, it was found after a short time that in order to make
them strong enough to withstand the shock of the sudden starting of
the engine it would be necessary to make them inordinately heavy.
It was therefore decided to abandon all attempts to incorporate the
thrust-recording device on this quarter-size model, but it was hoped
to install it later on one of the steam-driven models, where the
engine starts so slowly that there would be no need for excessive
strength in it.
The engine for the quarter model when reconstructed with stationary
instead of rotating cylinders was found in the shop tests referred
to above to develop when working at its best between 1-1/2 and 2
horse-power, as measured by the absorption dynamometers. However,
it was impossible to maintain this power steadily for more than
30 seconds. In the first place, the same difficulties (heretofore
described) that were met with in securing a suitable carburetor for
the experimental engine were experienced at the same time in the
development of the small engine. In the second place, as the engine
had no cooling apparatus of any kind, it was found that it could not
be tested in the shop for more than 30 seconds owing to premature
explosions. It was hoped, however, that by having everything ready
for a flight before starting the engine, it might be possible to
launch the aerodrome before the cylinders began to heat seriously,
and that the greatly increased cooling effect due to the motion of
the aerodrome through the air would permit the engine to develop
sufficient power to secure a flight that would show whether or not
the balancing was correct, as the final disposition of some of the
accessories on the large aerodrome could not be so well settled until
it was known just how the calculated balancing of this new model
corresponded with the actual balancing necessary for flight.
On account of Mr. Langley’s reliance on the generally sound theory
that where a successful method of conducting an experiment has been
found only after a long series of failures it is best not to change
to some unknown and untried plan, it was impossible, especially
where failure in the test might involve a fatal accident, to get him
to deviate from his original plan of [p231] launching the large
aerodrome from the top of the house-boat. He apparently realized as
well as anyone, that in many respects the making of the test from the
top of the house-boat had many serious drawbacks, but he emphasized
and impressed on the writer the importance of following as far as
possible in the construction and test of the large machine, the
plans which had brought success with the models. Believing, however,
that there was probably a better method of launching the aerodrome
than from the top of the house-boat, and that it would be well to
prepare before hand as far as possible for following some other
plan of launching immediately after a first successful test had
been obtained from the top of the boat, Mr. Langley had constructed
some floats which were arranged to be attached to the launching car
of the quarter-size model so that the car could be converted into
a catamaran raft. It was not believed that this crude arrangement
would suffice for a complete launching apparatus, since the power of
the aerodrome propellers would not be great enough to force the raft
through the water at a sufficiently high speed; still it was thought
that by having the launching car arranged in this way the model might
be allowed to drive the raft rapidly through the water and thus give
some idea as to what would be necessary, in a more complete launching
apparatus, to obviate the danger of the drag of the raft causing the
model to plunge over headlong into the water. The launching car with
these floats attached to it, and with the quarter-size model mounted
on the car, is clearly shown in Plates 73 and 74.
While the results obtained with superposed wings in the tests
of models Nos. 5 and 6 in the summer of 1899 indicated that the
“single-tier” surfaces were much more efficient, still, as has been
already stated, the great advantages of the superposed surfaces, so
far as strength of construction is concerned, was fully realized
at all times. As a result of these tests it was decided to use the
“single-tier” surfaces in the first test of the large machine in
order to insure as far as possible the best conditions. However,
it was from the beginning planned to construct superposed surfaces
for use in the later tests of the large machine; and, in order to
obtain more reliable data on such surfaces than had been obtained in
the tests of the models in the summer of 1899, a set of superposed
surfaces for the quarter-size model were constructed during the
winter of 1900–1901. The quarter-size model, equipped with these
surfaces, is shown in Plates 75 and 76, where the model is seen
mounted on its launching car, which is attached to the floats
heretofore referred to. It was originally planned not to employ
guy-posts when using the superposed surfaces, but after the latter
had been constructed and attached to the frame, it was found that
they would have to be made with rigid joints instead of hinged joints
if the guy-posts were omitted. As the hinged joints, however, were
already made, and permitted the surfaces to be folded up so as to
occupy a much smaller [p232] space in shipping them, it was decided
to retain the hinged form of construction and use the guy-posts as
shown in the above plates.
[Illustration: PL. 70
PLAN VIEW OF QUARTER-SIZE MODEL AERODROME, JUNE 1, 1900]
[Illustration: PL. 71
PLAN VIEW OF QUARTER-SIZE MODEL AERODROME]
[Illustration: PL. 72
END, SIDE, AND THREE-QUARTER ELEVATION OF QUARTER-SIZE MODEL
AERODROME]
[Illustration: PL. 73
LAUNCHING-CAR WITH FLOATS]
[Illustration: PL. 74
LAUNCHING-CAR WITH FLOATS]
[Illustration: PL. 75
QUARTER-SIZE MODEL AERODROME EQUIPPED WITH SUPERPOSED SURFACES, JUNE
11, 1901, SIDE VIEW]
[Illustration: PL. 76
QUARTER-SIZE MODEL AERODROME EQUIPPED WITH SUPERPOSED SURFACES, JUNE
11, 1901. END VIEW]
[Illustration: PL. 77
CYLINDERS OF ENGINE OF QUARTER-SIZE MODEL AERODROME]
After much delay, due to various causes, the quarter-size model,
as shown in plan, end elevation, side elevation and three-quarter
elevation in Plates 71 and 72, respectively, was taken down the river
in June, 1901, in order to make some tests with it from the small
house-boat, which had been previously moved to the middle of the river
opposite Widewater, Va. A test of it in free flight was made on June
18, its condition at this time being shown by Data Sheet No. 12 in
Appendix. The launching apparatus worked perfectly and the aerodrome
started off on an absolutely even keel, dropping only a few inches
immediately upon leaving the launching apparatus, and continuing
straight ahead directly into the light wind of something less than
2 miles an hour. After it had gone only about 100 feet, however, it
began to descend slowly, but still maintained a perfectly even balance,
and finally touched the water about 150 feet from the house-boat,
having been in the air between 4 and 5 seconds. It was immediately
recovered, and as soon as the wings could be dried out another test
was made, as it was thought probable that the wind had interfered with
the carburetor to such an extent that the engine had not received the
proper mixture of gas. Upon this second test the launching apparatus
again worked perfectly and the aerodrome again flew straight ahead on
a perfectly even keel, and at a uniform height from the water until
it had gone about 300 feet, when it again began to descend slowly
and finally touched the water about 350 feet from the house-boat,
having been in the air about 10 seconds. While the tests were very
disappointing, owing to the extreme brevity of the flights, yet they
showed conclusively that the balancing of the aerodrome was correct,
at least as far as motion in a straight line and in a quiet atmosphere
was concerned. As one and a half horse-power, which was felt to be
the very minimum which would successfully propel the aerodrome, was
furnished by the engine only when working at its very best, and as the
change in conditions from a quiet state to a velocity of something
like 40 feet per second evidently caused a considerable drop in the
power because of the change in the gaseous mixture which the carburetor
furnished to the engine, it was decided not to make any further test
of the aerodrome until the engine cylinders could be reconstructed
so as to provide more effective means for cooling it, and thereby a
reasonable margin of power above that actually necessary. The aerodrome
was accordingly returned to Washington for the purpose of making new
cylinders for the engine. In constructing these new cylinders the
old cylinder heads from the previous cylinders were used in order to
expedite their completion. This proved in the end to be a very great
mistake, though at the time it seemed probable that the use of them
would save much delay and considerable expense. The new cylinders were
constructed of steel tubing originally one-half inch thick, which
[p233] was machined to the form clearly shown in the photograph,
Plate 77, where it will be seen that thin radiating ribs spaced
one-quarter inch apart were formed integral with the cylinder, the
combustion chambers or heads being screwed on and brazed to the
cylinders. After much delay the new cylinders were completed, and upon
test it was found that while the radiating ribs assisted very greatly
in keeping the engine cool, yet the valves were so small that the gas
was not able to get in and out of the cylinders rapidly enough to
permit the engine to furnish its full power. Even at this stage it
would have been better either to have made new cylinder heads with
larger valves or to have made entirely new cylinders and cylinder
heads, but in the effort to economize time and money it seemed best to
try to overcome part of the defect by adding an auxiliary inlet valve.
This was constructed, and upon test it was found that, although the
engine developed 3.2 horse-power on the Prony brake at 1800 R. P. M.,
and even maintained 5.1 horse-power on the brake for a few seconds when
running at 3000 R. P. M., the ports leading from the valve chamber to
the cylinders were so small that they became heated after the engine
had run for 2 minutes and premature ignition occurred, which, of
course, immediately and very greatly reduced the power developed.
It was decided, however, in view of the tests in which the engine had
developed 3.2 horse-power at 1800 R. P. M., that there was sufficient
margin of power to enable it to propel the quarter-size model, even
if it was not working at its best. After concluding the Prony-brake
tests on the engine, it was mounted in its proper position in the
aerodrome frame and connected to the propeller shafts. Some pendulum
tests were then made, showing an average lift of approximately 57
per cent of the total flying weight. But it was found that the
propeller and transmission shafts and their bearings would not stand
the strain due to the increased power of the engine. Newer and
stronger shafts and bearings were, therefore, constructed and further
pendulum tests were made. It was then found that the transverse frame
which supported the shafts and bearings was too weak, and this was
strengthened by substituting newer and thicker tubing where it seemed
necessary.
These changes and repairs were all completed by October, 1901, and
the quarter-size model was at last, after months of delay, felt to be
in a condition which justified the expectation that its next flight
would be entirely successful. In view of the much more important
work on the large aerodrome which demanded immediate attention the
quarter-size model in this completed condition was put aside. Nothing
more was done with it until April, 1903, when some shop tests were
made preliminary to taking it to Quantico, where, on August 8, it
made a successful flight, which is described in Chapter XII.
[p234]