CHAPTER XI

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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