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Aviation History
1919
1919 - 1451.PDF
NOVEMBER 6, 1919 AIRSHIP ENGINEERING PROGRESS IN THE UNITED STATES By J. C. HUNSAKER, Eng. D., Commander, Construction Corps, U.S. Navy (Continued from page 1427.) Balloon Fabrics FABRICS for balloons and airships usually consist of cotton cloth coated with rubber. The requirements are high strength, light weight, low diffusion, water resistance and durability. The latter is the most important desideratum, and from extended observations it would appear that the life of a rubberised fabric varies inversely with the intensity and time of exposure to sunlight. Attempts to devise an accelerated test using artificial ultra violet light, have led to the conclusion that, while such light causes deterioration, the greatest rate of deterioration occurs on exposure to direct sunlight accompanied by high humidity and temperature. The Bureau arranged at the beginning of this work to have the Bureau of Standards put in testing facilities. At first; the Bureau of Standards was used as a check on the fabric-testing apparatus at contractors' works, but very soon the chemists of the Bureau of Standards began to contribute valuable suggestions for improvement, and at the present time are conducting research to develop better material. A study of British and other foreign practice in fabrc manufacture, combined with the results of extended exposure tests carried out under various climatic conditions with fabric made experimentally in this country for the purpose, has now led to the adoption of what is believed to be a marked improvement in rubberised fabric. The cloth is very carefully inspected after manufacture, both before and after such treatments as desizing and washing. All slubs and imperfect spots are marked so that they may be cut out before rubberising. In the rubber factories the cloth is first passed through spreading machines, where thin coats of dough (rubber cut with absorbed naphtha and similar volatiles) are applied. Carefully selected pure up-river Para gum is used for this purpose, and there is added only a very minor percentage of sulphur and litharge without the usual organic cure accelerators previously used. This thin rubber solution fills up the interstices of the weave. Much heavier rubber dough is then applied on the spreading machines as the process proceeds. After 20 to 25 coats are spread and dried, a continuous, gas-tight film is pro duced . The gas-tightness depends, however, upon such factors as the thickness of the rubber proofing and the count of the cloth: high count cloth and heavy proofing giving the minimum diffusion. For instance, with two-ply BB cloth having a gas film of from 3 J to 4 ozs., there is obtained very low diffusion. Added weights of proofing applied to higher count cloth would probably produce but slightly better results than are obtained with the above construction. Two plies of the treated cloth are stuck together by means of roll-ply machines. The fabric is then wound on drums, wrapped and steam-cured at .carefully controlled temperatures and pressures. Over-vulcanisation results in a fabric that ages rapidly, becoming brittle and stiff upon exposure. Examination of such fabrics upon exposure invariably shows high diffusion and rapid rise in acetone extract (indicating great oxidation). An acrid smell is always observed, and the rubber between the plies is hard, and shows a lack of its original adhesive pro perties. Under-vulcanisation is, therefore, the lesser evil, since exposure to the air and sun causes a certain degree of auto-vulcanisation. Yellow and red organic dyestuffs soluble in naphtha are often added to the proofing gum. These dyestuffs become thoroughly incorporated throughout the rubber solution, and act as light screens in preventing the admission of sunlight to the rubber proofing. Yellow cadmium sulphide pigment is also used for this purpose. The exterior surface of the fabric is coated with pure gum rubber dough containing usually a substantial proportion of litharge, which has been found to act as a screen or filter in preventing the admission of light rays to the rubber proofing. To the same exterior dough coating a quantity of finely divided aluminium powder is added, and then spread as the outer facing. The principle of this is to provide a continuous, light-reflecting coat. Fabrics made with such a coating have proved to be efficient in this respect, remaining usually 200 to 30° Fahr. lower in temperature than similar fabrics without the aluminium facing. This aluminium coat is usually applied either with a spreader or a calico printing roll. In the latter case a larger number of coats are usually necessary on account of the thin character of the solution required for successful printing. "" The inner or gas side of the fabric is coated with from i to 1 oz. of pure rubber, which helps to keep the cloth fungus and moisture-proof, reduces diffusion, and makes a good sticking coat for successful taping. The exterior and interior seam tape is made fairly wide (2 ins. or 3 ins.), and contains a substantial amount of proofing. The exterior tape has an aluminium facing. Both tapes are now cut on the bias so that stretch may occur. The seams in the envelopes of non-rigid airships constructed abroad are cemented with a pure gum cement consisting of solvent and fine Para-rubber only. A cement of this character is entirely satisfactory where the temperatures to which the envelope is exposed rarely reach 95 ° ; but it was found that with the high temperatures prevailing in the United States during the summer this cement could not be depended on. General softening occurred, and the strength of the seam was reduced to the strength of the sewing, which is far below the strength of the fabrics used in making the balloon. One contractor, to overcome this difficulty, intro duced a certain amount of resin into the cement which acted as a hardening agent. Even this was apparently insufficient, for when heated in an oven to a temperature of above ioo° Fahr., the seams made with this cement showed a tendency to slip on account of the softening of the cement. Another contractor who had accepted the European practice of using a strictly pure gum cement encountered great difficulties with the earlier envelopes on account of the softening of cement. He investigated this problem very thoroughly, and finally developed a method of applying the pure gum cement with a semi-curing solution, with the result that the seams apparently hold up to temperatures of 1300 Fahr. without softening. This practice is considered to be an important improvement. The B class ships had envelopes made of doubled fabric, with the threads of the " bias ply " at 45 deg. to the threads The nose of a C-type envelope, showing the stiffening wood battens and finger-patch suspension attachments. 1453
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