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Aviation History
1943
1943 - 2245.PDF
Flight, September 16th, 1943. Advt. I. EVEN i CAN UNDERSTAND—11. Q. How is an aeroplane propelled ? A In dealing with the subject of Induced Dragearlier in this series, it was shown that the lift on a wing was obtained by making it push air downwards, and thatthe power used in pushing this air could be greater or less to sustain a given weight depending on whether a smallermass was pushed at a higher speed or a greater mass at a lower speed. FIG. I Power used in propelling aeroplane Power wasted in propelling mass of air backwards The same considerations apply in propelling an aero-plane. The necessary thrust is obtained by pushing a mass of air backwards, a large mass with a low speed being amore efficient way of getting a given amount of thrust than the converse. This is a simple way of looking at it. In most cases othersources of loss occur such as eddies, noise and rotation of the slip, which do not contribute to the thrust, but do tothe losses. It has been looked at in this general way so that the explanation will apply to any method of propulsion. The propeller is a convenient implement for laying holdof a large mass of the passing air, pushing it back and getting the thrust. It is not, however, the only method ofdoing this. Anything on the aeroplane which projects some- thing backwards, such as a tail gun, an exhaust pipe, or evena ducted radiator, will make its contribution to the thrust. The engine exhaust, whose thrust is not great enoughto count much at low speed, becomes important at high speeds. Moreover, there is some flexibility in this mode ofpropulsion, for the changes can be rung on the amount of back pressure put on the engine (reducing its B.H.P., butincreasing the speed of the exhaust) by reducing the orifice of the exhaust pipe so that the best compromise is obtainedto suit the speed. A point of difference from the ordinary propeller which gives a push to the passing air is that theair is stopped in its backwards path inside the engine and has to have its original speed restored before a net gain inthrust is obtained. MEASUREMENT OF THRUST EXERTED AND ENERGY WASTED . FIG.. 2 Thrus* exerted Is proportional to mass of slip per second multiplied by its velocity ^_ Energy wasted is proportional to mass of slip per second multiplied by square of its velocity. Useful power exerced is proportional to thrust multiplied by. speed of aeroplane. The aeroplane differs in this way from the locomotiveor motor-car, both of which push backwards at their tracks in getting their drive. They do not, however, move theirtracks appreciably, because their tracks are anchored to the vast mass of the earth—which illustrates the need for makingthe mass of air acted upon as large as possible. The greater the mass acted upon the less wasted power will be carriedaway by it. Thus the power that the aeroplane has to exert in orderto get a given thrust is not all usefully employed ; part is employed in propelling the aeroplane forwards and part inpropelling a mass of air backwards. Measuring the masses and movements of the air enablesus to work out how the power has been used. If we also know the speed of the aeroplane then the whole propulsivepicture is revealed. The observer can work out :— (1) The thrust exerted by the propeller ; this is pro-portional to the mass per second of the slipstream multiplied by its velocity relative to the surround-ing air (action and reaction being equal and opposite). (2) The energy wasted by setting this mass of air inmotion ; this is proportional to the mass per second multiplied by the square of its velocity. (3) The useful power exerted ; this is proportionalto the thrust (obtained in (1) above) multiplied by the speed of the aeroplane, as discussed inNo. 10 of this series of articles. The efficiency of the propelling device (useful power gotout as a fraction of power put in), is simply the thrust power as a fraction of the total power (which is represented by theslip loss plus the thrust power). The object of this note has been to give a general ideaof propulsion, however it may be obtained, so that pro- pellers, exhaust pipes, ducted radiators or anything else whichthrows stuff backwards can come into the picture. PROJECTING ANYTHING BACKWARDS GIVES THE AEROPLANE SOME FORWARD THRUST Bullets from tail gun give forward thrust Engine exhaust gives forward thrust FIG 3 Ducted cooling air gives forward thrust compensating for the drag of the radiator either wholly or partially This is one of a series of articles on technicalities sponsored by The de Havilland Aircraft Co., Ltd., in the beliefthat they will prove of interest and use to students and others in the Services and the aircraft industry.
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