It seemed so desirable to obtain as soon as possible a first test in actual flight of the large machine that the writer offered to put this engine in the aerodrome frame and make a test with it if the machine were launched over the water, but with the launching track mounted directly on the river bank. However, Mr. Langley felt it so necessary to make the initial test from the top of the house-boat and at an elevation of 30 feet or more that he would not consent to this, and as the engine at its best did not develop quite 24 horse-power, which had been calculated as the minimum which should be provided, it was thought unwise to attempt to make the first test from the top of the house-boat until the aerodrome had been provided with engines that could be depended on to develop continuously not less than 24 horse-power.

It then became necessary either to build a duplicate engine and use both of them in the aerodrome, the original plan as already explained having been to have two engines developing the 24 horse-power together; or, second, to construct an entirely new engine large enough to furnish a minimum of 24 horse-power and use this single engine.

As the construction and tests of this experimental engine had shown many places in which the weight might be safely reduced, the writer decided to construct an entirely new and larger single engine, and thereby avoid the extra weight and difficulties which would be introduced by having to use synchronizing gears where two engines were used, it being impossible, of course, to run the two propellers from the two engines independently without risk of serious disaster. [p224]

It will be recalled that when the aerodrome was originally planned in 1898 it was proposed to have two engines of 12 horse-power each, and the contract for the single engine of 12 horse-power provided that a duplicate was to be supplied, if desired, immediately upon the completion and delivery of the first one. The calculations, both from the whirling-table tests and from the results with the steam-driven models in actual flight, indicated that 24 horse-power would be ample for the aerodrome, which it was then expected would not exceed 640 pounds in weight, with a supporting surface of 960 square feet. But it was found that the total weight of the machine was rapidly increasing on account of slight increases in the various details, which when added together made a considerable increase in weight. Furthermore, as it had been found difficult to keep all five of the cylinders of the experimental engine working uniformly, it was thought best to build this new engine sufficiently large to provide not only the extra power necessary because of the increased weight of the aerodrome, but also to provide for further inevitable increases in weight, and over and above all this, to provide also that the engine would furnish all the power necessary, even though one of its cylinders should absolutely fail to work and act as a dead load on the others. The writer accordingly designed this new engine to give 40 horse-power when all five of the cylinders were working, and 28 horse-power even though one cylinder should act as a dead load on the others.

The various materials for the construction of this engine were ordered early in December, 1900, with the promise of delivery not later than January, 1901. Owing to various causes, however, the major portion of the materials could not be obtained until late in the spring, and, in fact, a portion of them were not obtained until the summer of 1901. During this period of delay, however, the engine for the quarter-size model was completely reconstructed and further tests were made with the experimental engine in developing accessories, such as carburetors and spark coils.

The float-feed type of carburetor which was then coming into prominence in automobile work proved at that stage of its development to be totally unsuitable, as the slight but constant tremor of the aerodrome frame, when the engine was working at high speeds under a heavy load, caused the float to act as a pump and periodically flood the carburetor. This resulted in an irregularity of action of the engine which at times injured not only the transmission shafts, gears, and frame, but the engine itself by the serious pounding which occurred. A form was next tried in which the gasoline was fed in through the valve seat of a lightly loaded valve which raised whenever there was suction in the inlet pipe, the amount of gasoline fed being regulated by a pin valve. Later there were built several shapes and sizes of tanks filled with absorbent material, which was saturated with gasoline and the surplus drawn off before starting the engine. Some of these tanks were provided with a jacket through which [p225] a portion of the exhaust gases was passed in order to compensate for the cooling of the tank caused by the evaporation of the gasoline. As a result of these tests it was found that a type consisting essentially of a tank filled with small lumps of a porous cellular wood (tupelo wood) which was initially saturated with gasoline, and into which the gasoline was fed through a distributing pipe as rapidly as it was taken up by the air, which was sucked through it by the engine, gave the best results. Instead of jacketing this tank, the cooling effect due to evaporation was compensated by drawing the somewhat heated air from around the engine cylinders up through the loosely packed lumps of wood. When tested in the shop this type was found to give such a very uniform mixture that the engine ran as smoothly and regularly as an electric motor, the vibration in no way interfering with it, and even when the sudden change from a state of rest to one of rapid motion through the air was imitated by suddenly turning on the carburetor the blast of several large electric fans from various angles, it was found to have no appreciable effect on the running of the engine, thus indicating that the trouble which was experienced with the model aerodrome in the trials of 1901 was not likely to be repeated with the large aerodrome. Somewhat more than a dozen carburetors of various forms were constructed before this last type was devised, but this proved so satisfactory that there were never thereafter any carburetor troubles. In fact, as will later appear, a carburetor of this type kept the engine on the large aerodrome running at full power not only when the aerodrome was in a vertical position in the air, but also after it had turned completely over on its back.

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[p226]

CHAPTER IX
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.