SHOP TESTS OF THE AERODROME
In June, 1902, after the proper adjustments of the carburetor and
other accessories of the engine had been accurately determined in the
tests on the testing frame, the engine was assembled in its proper
position in the aerodrome frame and connected to the propellers.
The aerodrome frame was then mounted directly on the floor of the
launching car, which was placed on a short track laid on the floor of
the shop, as previously described. A large spring balance, which had
been previously calibrated, was then connected between the car and an
upright fastened to the track, and tests were made to determine the
thrust developed when the engine drove the propellers at different
speeds. Upon finding that there was comparatively little vibration
when the engine was driving the propellers even at its maximum speed,
it was felt safe to raise the aerodrome from the floor of the car
and place it upon the uprights on which it would be supported in
launching it. Quite an extended series of tests was then made, and
although the uprights raised the aerodrome frame until the midrod
was practically 9 feet from the floor of the car, and in the tests
at maximum power the propellers developed an average thrust of 450
pounds, yet it was found that the clutch hook held the bearing
points of the frame so securely on the uprights of the car that all
fear that the aerodrome might break loose from the car during the
launching process was removed.
Upon the completion of these tests, which had proved most
satisfactory, the aerodrome frame was supported from the ceiling
of the shop by means of four short coil springs which reproduced
as nearly as possible the elastic or flexible suspension which the
aerodrome would have when supported by its wings in the air. These
springs were attached at the same points on the main frame of the
aerodrome at which the wings would be attached, thus permitting a
careful study of the amount of flexure and vibration which it would
undergo in actual flight. The most remarkable difference in the
nature of the vibration induced in the frame was found when the
aerodrome was thus supported by springs. When it was supported on
the rather unyielding launching car, the general tremor set up in
the frame by the engine and propellers was, while small, yet harsh,
the effect on a person standing in the aviator’s car being rather
unpleasant in the joints of the knees when experienced for several
minutes. When the frame was suspended by the springs it was found
that all this harshness of tremor disappeared, it being replaced by
a slight general and rapid tremor of the whole frame, which was not
at all unpleasant, and which [p252] had no tiring effect on one
standing in the aviator’s car. In fact, the vibration in the first
case resembled rather closely that of a motor vehicle supported on
wheels having metal tires, and in the second case a motor vehicle
supported on wheels having pneumatic tires.
As in these tests in the shop it was impossible to keep the engine
cool by circulating its cooling water through the radiator, since
there was no air current blowing across the latter to carry away
the heat, it was necessary to connect an extra water tank in the
cooling-water circuit. A tank holding about ten gallons was used, and
this sufficed for about ten minutes before the water was raised to
the boiling point.
During one of these tests when the frame was supported from the
springs, and while the engine was developing about fifty horse-power,
without any warning whatever, both propellers suddenly twisted off
from the flanges by which they were connected to the propeller
shafts, thus leaving the engine entirely unloaded. The propellers
both dropped quietly to the floor, making only about one or two turns
in falling the distance of approximately 10 feet, and the engine,
which had been running at about 850 R. P. M., immediately speeded up
to an exceedingly high speed, which, while not exactly known, since
the tachometer only read to 2000 R. P. M., yet from the deflection
produced on the tachometer needle must have been considerably higher
than this. Although the fly wheels, which were 33 inches in diameter,
with the aluminum rims and wire spokes, had been exceedingly well
made, yet it was not considered safe to run them at this speed, and
the engine was immediately shut down. At the moment, however, that
the engine had broken loose from its propellers and also momentarily
jumped to this exceedingly high speed there was absolutely no
vibration that could be noticed, the unloaded engine running as
smoothly as an electric motor. This showed very clearly that the
running balance of the engine was as near perfect as it would be
possible to get it, except with a seven-cylinder engine, which is
theoretically capable of more perfect balance. It was evident that
what small vibration there was in the frame while the engine was
developing its power was due almost entirely to the reverse torque,
and, of course, could never be entirely eliminated.
In the tests of the engine working in the frame, both while mounted
on the car and also when suspended from the springs, a great
amount of delay was caused from the fact that the ball-bearings on
the transmission and propeller shafts frequently went to pieces.
There were two reasons for this: In the first place, although
carefully selected balls were used, defective ones were continually
encountered. Even a slight defect in a single ball resulted in its
breaking under the rather severe test to which they were subjected,
and, as is well known, the breaking of one ball in a ball-bearing
usually results in the destruction of the whole bearing, especially
if the races are light. The second cause was that [p253] the whole
aerodrome had been originally designed with the expectation of using
a maximum of 24 horse-power, and as no margin had been left to
provide for possible increases in the size of the bearings, there
was no room to permit them to be increased without almost completely
reconstructing portions of the transverse frame. While in the end
it would have been cheaper to have reconstructed these portions
in order to put in larger bearings, yet, as is always the case in
experimental work of this kind, small changes which seem to hold out
hope of overcoming difficulties are usually followed, rather than
reconstructions which can be seen to involve considerable expense
and delay. After a number of minor changes had been made in the
bearings, they were finally able to stand up fairly well under the
severe strain to which they were subjected when the engine developed
its full power, and no further changes were made in them; a defective
race being, however, replaced by a new one as occasion demanded.
These tests demonstrated very clearly that at speeds of approximately
1000 revolutions per minute ball-bearings which are subjected to
considerable loads should be calculated with a considerable margin
of safety, as the yielding of the frame, which must necessarily be
far from rigid, causes more or less error in the alignment of the
shafts and bearings, and this introduces considerably increased
strains on the bearings. In the early tests before the bearings were
strengthened, the balls in some of the races were on a few occasions
ground to a very fine powder before it was discovered that they had
failed. Such a result, it will be understood, could and did occur in
the course of a very minute length of time.
In imitating as nearly as possible the conditions to which the
carburetor of the engine would be subjected during the period of
launching, numerous tests were made in which the engine was brought
to its maximum speed and, without changing the adjustment of the
mixture-controlling devices of the carburetor, sudden blasts of air
were turned on it from various directions, and these were continued
until the mixture-control devices were perfected to such a point
that gusts of thirty miles an hour suddenly directed from any point
against any portion of the apparatus would in no way effect the
speed and power of the engine. These tests were considered necessary
in view of the very sudden changes in conditions to which the
aerodrome would be subjected during its brief run down the launching
track, the conditions changing in approximately three seconds from
absolute quiescence of the aerodrome to a plunge through space at
thirty-five feet per second. An aviator would be more than occupied
with maintaining control of himself and of the aerodrome, which at
the moment of leaving the track might require considerable change
in the adjustment of the Pénaud tail, and he would, therefore, not
be able to make any adjustments of the engine-control devices.
This supposition was entirely confirmed in the actual tests of the
aerodrome which are to be later described, the rush down the track
being [p254] so very brief that the engine could not have been given
any attention by the aviator had it needed it, which fortunately it
did not.
It is hardly necessary to recount at any length the great
difficulties which was experienced in these tests of the engine in
the aerodrome frame before the shafts, bearings, propellers, and,
in fact, the frame itself were all properly co-ordinated so that
confidence could be felt that all of the parts would stand the
strains which were likely to come on them when the aerodrome was in
flight. These tests were really not tests of the engine itself, but
of the frame, shafts, and bearings. Suffice it to say that nearly a
year was consumed by the various breakages of the shafts, bearings,
and propellers before it was felt that all of these parts could
be depended on, and even then the weakness of the bearings above
referred to was fully recognized. Had some of the better-grade balls
and steels for the bearings, which have since that time come on the
market, been obtainable then, there would have been no difficulty
with these bearings. However, this same remark might be made with
reference to nearly all of the details of the aerodrome, for it was
the accessories, such as bearings for the transmission and propeller
shafts, spark plugs, coils, batteries, and a suitable carburetor for
the engine, that caused the chief delay after the main difficulty of
getting a suitable engine had been overcome.
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