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Aviation History
1944
1944 - 0119.PDF
JANUARY 20TH, 1944 Advertisements i. EVEN I CAN UNDERSTAND—16. Q. What are the principal features of jet propulsion? A, In the past few years there have appeared many descriptions of inventors' ideas or apparatus for making use of jet propulsion for aircraft, or perhaps it would be more correct to say for eliminating the ordinary propeller where it has already been shown that forward thrust in fluid-borne vehicles is obtained by throwing stuff (generally some of the fluid) backwards. Whether this is done by means of a propeller or a gas turbine (as proposed in these inventions) makes no difference in kind so far as the propulsion is concerned. It makes a difference only in the quantities involved, the efficiency obtainable, and so on. It has already been said (No. 11 of this series of articles) that a better efficiency is got by squirting back a large mass of air slowly than a small mass quickly—remembering that the same thrust (mass x velocity) can be obtained either way. It merely costs more in power to get the thrust from the small mass thrown back at high speed. Talking of " high " and " low " speeds is good enough only for comparative purposes, but it must be explained what is meant by such terms. Is 15 m.p.h. a high or a low speed ? It would be very high indeed for the slipstream of a marine propeller, but very low for that of a jet-propelled aircraft. The first thing is to fix a datum and a good one to take is the speed of the vehicle through the fluid. Using this, one can say that the efficiency of propulsion is limited by the ratio of the speed of the slipstream to the speed of the vehicle, both relative to the fluid, and thereby put a measure on " high " and " low." Thus the higher is the forward speed of the vehicle through the fluid in relation to the backward speed of its slipstream through the fluid the higher the efficiency. The limiting efficiency of propulsion 2 may in fact be denoted by r =7 where R is the ratio of I + K the speed of the slipstream backwards through the fluid to the speed of the vehicle forward through the fluid. A graph can thus be made, Fig. 1, which shows how the maximum possible efficiency of propulsion depends on this ratio. Modern clean propeller-driven aeroplane 99 % (Fig. 2) Hypothetical jet-propelled aeroplane 66 % (Fig. 3) If this graph were 11 ft. 6 in. wide, a jet - propelled barge could be shown to have an efficiency limit of 0.2 % (Fig. 4) as its slipstream, Fig. 2. If a jet be imagined propelling an aeroplane at 500 m.p.h., Fig. 3, by ejecting air at 1,000 m.p.h. (i.e., 500 m.p.h. relative to the air) the ratio R is 1.0 and the limiting efficiency 66 per cent. This is rather low because the aeroplane is not going any faster than its slipstream. It is evident that very low drag' and high speed are needed to get a good return from jet propulsion and bring it somewhere near the best end of the graph, Fig. 1. Fig. 2—Modern clean propeller-driven aeroplane 400 m.p.h. ip 10 m.p.h. To go to the other extreme one might picture a canal barge, Fig. 4, being propelled at 1 m.p.h. by the thousand- 999 2 miles-an-hour air jet. Here R = — and efficiency = -^rj = 0.002 = 0.2 per cent. Not good. Fig. 3—Hypothetical jet-driven aeroplane 500 mp.h. Slip 500 mp.h A word of warning is necessary about the term efficiency as used above. Account is taken only of the fluid jJfc is successfully projected straight back—none is taken of el^s, noise, air motions other than straight back, or heat thrown away in the jet or for that matter the ordinary engine exhaust. The business of making the jet may be quite a wasteful one. It is important to realise that the efficiency as defined above is not necessarily attainable. It is merely the limit beyond which it is impossible to go however economically the jet or slipstream is produced, un'^ss the ratio of slip speed to forward speed can be reduced. FORWARD SPEED An aeroplane making use of jet propulsion (taking this to mean a concentrated high-speed jet of air) must be very fast if it is to make good use of its jet. A modern aeroplane with propellers, with a very high limit to its efficiency— something like 99 per cent.—might be going 40 times as fast Fig- 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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