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Aviation History
1938
1938 - 0515.PDF
February 24, 1938 Supplement to ™E AIRCRAFT• ENGINEER No. 145. XVI\ ENGINEERING SECTION February 24, 1938 WOODEN BLADES FOR V.P. AIRSCREWS Interesting Developments Now Taking Place May Have Far-reaching Effects (Illustrated by " Flight " photographs)F OR a great number of years the wooden airscrew held undisputed sway. During the pioneer days of flying wood was the ideal material for airscrews as well as for aeroplanes, its low cost and the ease with which it could be shaped making it particularly attractive for experimental work and for construction in small quantities. In those days flying was done in fine weather only, and thus it did not matter that wooden airscrews were apt to suffer severely from the effects of flying in rain or hail. As the art of flying developed, aircraft ventured out in less favourable weather conditions, and the wooden airscrew was apt to suffer damage. Also, it was realised that its relatively thick blade sections were not quite as efficient as would be blades of thinner section, such as could be manufactured of metal. Successful metal airscrews were introduced and gave excellent results in service. The wooden airscrew, in spite of fabric covering and/or var- nished surfaces, could not compete from the point of view of durability, and its popularity began to wane. The next stage came when the speed range of aircraft had increased to such an extent that airscrews with con- trollable pitch were necessary if a good take-off were to be combined with high efficiency at operational speed. In view of the good service given by the fixed-pitch metal air- screw, it was natural that when the variable-pitch airscrew came to be developed it should have metal blades. Of variable-pitch airscrews with metal blades there are now several types in use and giving good service. In the meantime, the champions of the wooden airscrew had not been idle, and they had succeeded in discovering protective coverings which would withstand the effects of rain and hail, not to mention prolonged operation in humid atmospheres, frequent spraying by sea-water, such as is associated with seaplane work, and so forth. One method in particular was evolved in Germany by the Schwarz firm of Berlin. The British rights in this method were secured by the Airscrew Co., Ltd., of Weybridge, and fixed-pitch wooden airscrews finished by the Schwarz pro- ,cess are now used very extensively in the machines of the Royal Air Force as well as in British civil aircraft. The process is well known, and consists in covering the wooden blades with a layer of nitrocellulose, the material being applied in sheet form and forced into the pores of the wood under air pressure in autoclaves. It was inevitable that when the variable-pitch airscrew The first stage. Impregnating a sheet of wood with syn- thetic resin. Six blocks of wood being compressed under heat in the hydraulic press. Actually eight blocks can be compressed at a time.
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