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Aviation History
1942
1942 - 2043.PDF
OCTOBER IST, 1942 FLIGHT Advertisements i. EVEN I CAN UNDERSTAND—5. Q. Why are feathering propellers now considered a necessity on multi-engined aircraft? A When the engine which normally drives a propeller fails while the aeroplane is still in flight, the propeller starts to act as a windmill and produces drag instead of thrust. The amount of drag it produces depends largely on the pitch in which it is windmilling—the coarser the pitch the lower the drag. (On all installations where it is not possible to feather the propeller should be put into positive coarse pitch immediately after engine failure). The least drag of all is produced when the blades of the propeller are turned edge-on to the direction of flight, i.e., when they are feathered. As the pitch is coarsened, the windmilling r.p.m. are reduced, until when the blades are feathered the propeller finally ceases to rotate at all. Feathering is done by pressing a button and stops the propeller in a few seconds. It may surprise some people to learn that the drag of a feathered propeller is not drasti cally lower than that of a propeller windmilling in a very coarse pitch (although it is much lower than that in a fine pitch). Indeed, the greatest advantages of a feathering propeller lie in the fact that it does not rotate. This prevents further structural damage to an engine if the cause of its failure was some internal breakage, and avoids possible vibrations and heavy impulses which may tear an engine out of its mountings. Also the feathered propeller creates less disturbance to the airflow over the wings and tail, and hence may result in a lower aircraft drag with correspondingly better one- engine performance (speed, ceiling, etc.) and better control. Engine damaged but propeller windmilling :— 1. Further damage to engine is likely. 2. Vibration or heavy impulses may break engine mounting. 3. Disturbed airflow and drag on one side adversely affect performance and control. Propeller of dead engine feathered:— 1. Further damage to engine avoided. 2. Vibrations or heavy impulses avoided. 3(a). Less disturbed airflow over wing and tail on that side, (b). Less propeller drag on that side. This is one of a series of articles on technicalities sponsored by The de Havilland Aircraft Co., Ltd., in the belief that they will prove of interest and use to students and others in the Services and the aircraft industry.
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