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Aviation History
1919
1919 - 1421.PDF
OCTOBER 30, 1919 IZSSS AIRSHIP ENGINEERING PROGRESS IN THE UNITED STATES* By J. C. HUNSAKER, Eng. D., Commander, Construction Corps, U.S. Navy LIGHTER-THAN-AIR engineering is a very recent^development in the United States, and was not really taken up seriously until December of 1916, when the possibility of the United States becoming involved in submarine warfare began to be feared. The Navy Department had always had a mild interest in airships or dirigibles, and placed a contract in 1915 with a firm which had got hold of a German engineer, a Ger man mechanic, and an Austrian airship pilot. This contract called for a training ship of very moderate performance, but it was not delivered until two years later, and was so much over-weight and otherwise so unreliable that after a few short flights it was broken up as useless. This experience was dis couraging, as it was hoped from experience with this ship to get some idea of where the Navy ought to use airships. However, reports began to drift in of the use by the British Navy of small airships for anti-submarine work, coast patrol and dropping depth bombs. In December, 1916, the Depart ment asked the Bureau of Construction and Repair to design an airship for coast patrol and training work. The character istics were : Speed, 45 m.p.h. ; endurance, 12 hours at 35 m.p.h. ; radio for 150 miles ; crew of three ; provision for alighting on the water. At this time, barring the concern with the German talent, which had built the first rather unfortunate ship, there was no experience in modern airship construction in the country to draw on either for design or for knowledge of the special materials needed. As the United States still preserved diplomatic relations with Germany, no detailed information from abroad could be secured legitimately. Development of B-Class To meet the problem, a design was started which was later designated Type B. The only engines available having any' fair claim to reliability were the 100 b.h.p. Curtiss eight- cylinder and the 100 b.h.p. Hall-Scott four-cylinder types. Calculations based on the speed required from this horse power available and the load to be carried fixed the envelope volume, which was to be a minimum consistent with the characteristics demanded. Lacking experience, dependence had to be placed on theoretical and experimental investigations. The literature of the art was assembled from the Library of Congress and the Smithsonian Institution, and a deliberate attempt made to read it all. It looked a very imposing collection, but boiled down there appeared to be little or no practical in formation. The theory of ballooning was very complete, but it appeared that the theory of airship design had been developed under governmental control in France and Ger many, and the greater part had been kept confidential. For details of construction we had the benefit of an inspection of airships made in 1913 in France, England and Germany, and a good deal of information had been collected in spite of the restrictions. However, such information was now four years old. To determine a form of envelope of low resistance, a series of models was run in the bureau's wind tunnel and the best selected. The sizes of fin and control surfaces were also determined from wind tunnel tests. We had no practical design rules to follow, and it appeared that a speed of 45 m.p.h. for a small airship was so high that difficulties in steering might be anticipated. Accordingly, very careful experi mental and theoretical studies were made to make sure. The results on the trials showed that the calculations gave ample stablitity, and after the pilots had become more experienced it was found possible to cut down the vertical fin surface. The design of the car presented no difficulties, as we could follow aeroplane practice, using a standard aeroplane power- plant. To connect the car to the envelope or gas-bag was a different story, however. This is the most delicate part of the design, • From Aviation, U.S.A. and the point where we were most ignorant. The non-rigid airship keeps its form against the pressure of the air because it is held out full and taut by inner gas pressure. It now was necessary to suspend the car containing all the heavy weights from the fabrics of this gas-bag in such a manner as not to deform the bag or put undue local stresses in any part of it. The calculations involved in the design had to be devised from general naval architectural principles but were not very satisfactory on account of the indeterminate nature of the distribution of load between the various suspension cables. It was desired to use the lightest fabric that could be considered safe, but the envelope must remain fair and stiff, with no more than 1 in. of water pressure. The loads were arranged in what appeared a reasonable manner, and the curves of bending moment and shearing force drawn, considering the envelope as a beam. The calculation made was identical with that used in computing the longitudinal strength of vessels, and need not be described further. To verify the calculated strength of envelope fabrit and the Four views of a water model of B-class envelope under going test inclined at 30° and level trim. '423 G
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