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Aviation History
1942
1942 - 1837.PDF
SEPTEMBER 3RD, 1942 FLIGHT Advertisements i. EVEN I CAN UNDERSTAND—3. Q. What is Compressibility? A. # It is an aerodynamic phenomenon which has troubled propeller designers for many years. Now it is just beginning to worry designers of high-speed aeroplanes ! The speed of sound* is the speed at which pressure disturbances are transmitted through the air. If a body, such as an aerofoil, is travelling at a speed much lower than this, then it can " warn " the air ahead of it that it is coming. The air then flows round the body with relatively gentle changes in pressure and is never appreciably altered in density in the process. of an aerofoil). The outward and visible sign of all this is the formation on the body of a stationary compression wave, or shock wave. (The word " visible " above is no figure of speech—the waves can be photographed by a suitable process). TURBULENT FLOW SHOCK WAVE \ v£ T]5^^^fflIMllIi^ DIRECTION OF AIRFLOW I.OCAL VELOCITY JUST HERE EQUALS SOUND VELOCITY VELOCITY LESS THAN SOUND VELOCITY When the speed of the body approaches the speed of sound, this process is very much changed, and the nature- of the flow alters considerably. Sudden changes of density and pressure take place, with accompanying drastic increase in drag and decrease in lift (in the case The speed at which the type of flow changes is termed the " critical " speed, and is normally appreciably lower than actual sound velocity. This is because even when the body itself is travelling below sound velocity, the local air velocity relative to it may at some points have reached this value. It will readily be appreciated that the tip speed of a propeller must be limited by the above considerations, otherwise its efficiency is drastically impaired. Other ways of minimising the effect of compressibility on propellers are the use of the thinnest possible blades, and large blade area to keep the angle of attack of the sections low. * 1120 ft./sec. or 764 m.p.h. at sea level. Decreases to 974 ft./sec. or 664 m.p.h. at 35,000 ft., above which it remains constant. Z UJ U o u o < Q CRITICAL SPEED Z UJ U O u •1 1 1 r FORWARD VELOCITY RATIO SOUND VELOCITY RATIO 1 r FORWARD VELOCITY SOUND VELOCITY THIS IS CALLED MACH NUMBER- 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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