Langley Memoir on Mechanical Flight, Parts I and II / Smithsonian Contributions to Knowledge, Volume 27 Number 3, Publication 1948, 1911

by S. P. Langley; Charles M. Manly

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 112 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.