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It will be recalled that in “Experiments in Aerodynamics” Mr. Langley made tests of the soaring speed, etc., of surfaces when superposed. In many of his experiments with rubber-driven models, he also employed superposed surfaces. During the summer of 1898 several forms of superposed surfaces, of a proper size for use on the steam-driven models Nos. 5 and 6, were constructed and were tested under as nearly as possible the same conditions as would exist when used on the aerodrome, by mounting the surfaces on the whirling-table and measuring their soaring speed, lift, drift, etc., to determine just what arrangement of surfaces gave the greatest lifting effect with the least resistance. Two of the forms which were tested are shown in Plate [37], Figs. 1 and 2, and Plates [64] and [65]. At the conclusion of these tests, it was decided to construct a set of surfaces on the plan shown in Plates [64] and [65], and to have them ready for use on either of the models Nos. 5 and 6. These surfaces were taken to Chopawamsic Island in April, 1899, when all of the other aerodromic material was first carried there. It was planned to make some tests with them to determine whether or not it would be best to use superposed surfaces on the large aerodrome or to follow the plan of “single-tier” ones, which had the great advantage of having already proved their worth in the successful flights of the models. On August 3, Aerodrome No. 5, equipped with these superposed surfaces, was launched. It will be noted from Data Sheet No. 11 that the superficial area of the superposed surfaces was considerably greater than that provided by the “single-tier” ones, and on the assumption of the same efficiency per unit of surface in both cases, the aerodrome should have soared at a less speed and required less power when using the superposed surfaces. The results obtained, however, were just the reverse, the aerodrome being unable to sustain itself when using the superposed surfaces, whereas with the “single-tier” ones it was evident that a slight excess weight might easily have been carried without preventing the aerodrome from soaring properly. While it was felt that these tests were not entirely conclusive as to the superior lifting power of the “single-tier” surfaces, yet as the engine builder was constantly promising, each time with increased emphasis, that he would within less than a fortnight deliver the engine for the large aerodrome, and that it would develop even more power than the specifications called for, it was deemed best to cease the experiments with the models and concentrate all effort on the completion of the large aerodrome frame and the construction of a set of “single-tier” supporting surfaces for it. It was recognized from the first that the “single-tier” supporting surfaces lacked the rigidity which could be secured by the truss construction afforded by the superposed plan, yet these models, which were the only machines in the history of the world that had ever flown successfully, had been equipped with “single-tier” surfaces; and the experience so dearly bought during the long [p154] years of development of these models had taught the very valuable lesson that in work of this kind where we have no margin on anything, but everything has to be calculated on the “knife-edge” basis, it is an exceedingly unwise thing to introduce any modification from what has been proved to be satisfactory, unless such modification is absolutely necessary.

The principal object in building the one-eighth size model of the large aerodrome, as mentioned in the first part of this chapter, was to determine by actual experiment whether the new form of “underneath” launching apparatus, which had just been designed, was likely to prove as satisfactory as the original “overhead” type, which had been used in the successful flights of the models in 1896. Yet after it was completed this aerodrome was found so very strong and stiff, even though roughly constructed by merely tying the joints of the tubing together with wires and soldering over the joints, that it was decided to equip it with power, if a suitable form of power could be found which could be easily applied. Just at this time liquid air as a motive power was attracting considerable attention all over the country, and attempts were made to procure a small power plant for operation by liquid air. After devoting considerable time to the matter it was found impossible to do anything with it just at that time, as the liquified air could not be obtained in Washington, and one of the chief experimenters in New York, who had been given a commission to make certain experiments at his plant, so continuously delayed beginning them that it was found necessary to give up the idea.

However, after the completion of the tests of the launching apparatus some experiments were made in flying the model as a kite. For this purpose a mast twenty feet high was constructed and so arranged that it could be mounted at the center of a small power launch. The model aerodrome was flown by a cord connected to it by a bridle, the cord passing over a swivel pulley on top of the mast and down into the boat, whence it could be played out or hauled in as occasion required. By heading the launch into the wind it was possible to secure sufficient relative velocity to cause the model to support itself and a number of tests were made in this way. It was found that when the bridle was attached at the point at which the propellers would deliver their thrust, had they been in use and driven by power, the model flew exceedingly well, maintaining its equilibrium even during very strong gusts. Owing to the rolling produced by waves from the large boats which were continually passing in the part of the river where these tests were made, the power launch was often in danger of being upset by its tall mast; and finally, when the tests were just reaching the point where accurate information was being obtained on the balancing of the model, a sudden rolling of the boat caused the mast to snap off while the model was in the air. Before it could be picked up from the water a passing boat had swamped it and it was lost in the river. [p155]

Although the model was, as has been said, rudely constructed and, therefore, did not represent a serious loss, yet the pressure of the more important construction work for the large machine prohibited the construction of another rough model for continuing these kite experiments, which it was felt could not at best be more than approximate indications of the general stability of the machine under practical conditions.

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CHAPTER IV
HOUSE-BOAT AND LAUNCHING APPARATUS

The use of a house-boat seemed to Mr. Langley so indispensable in former years in making open-air tests of the models that he decided from the outset, though advised by the writer against doing so, to use the same plan on a much larger scale in connection with the large aerodrome. Aside from its supposed utility as a convenient and apparently safe place from which to launch the aerodrome, the house-boat was valuable as a portable workshop for making necessary repairs and as a temporary storehouse for the apparatus, thereby saving much packing and unpacking. It also provided sleeping quarters for the workmen.

It was early seen that this plan would require a boat at least 60 by 40 feet, which could be built only at a large initial cost. But as the experience with models had so firmly convinced Mr. Langley that it was necessary not only that the aerodrome be launched over the water, but also at a considerable height above it, and from a station that commanded all points of the compass, he decided to adopt this plan for the large aerodrome, and designs for such a boat were accordingly made in the latter part of 1898.