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Aviation History
1960
1960 - 0010.PDF
10 FLIGHT, 1 January 1960 TOMORROW'S AERO ENGINES . . . able and more economical jet aircraft, powered by ducted-fanengines. It must be reaffirmed that a modern jet aircraft is more economical, and will lead to lower fares than the slowerpropeller-driven machines. t Suitable engines for these airliners are not yet available anda great opportunity faces the British aircraft industry in this field. It must be emphasised that Government support—eitherin a direct development contribution or in orders for the RAF— is essential to make the commercial risk justifiable. The prize isbig, though it is unlikely that the history of the Viscount will be repeated, for in that case competition was virtually absent. The two greatest engine companies in the USA are alreadyoffering ducted-fan engines, a type which is certain to take over from the straight jet in the next decade. If we wish to stay in therace, the correct technical choice of both aircraft and engines must be made, and we must apply all our effort in this direction. Engines for Supersonic Flight. For engines designed forsupersonic flight, perhaps the most signficant factor is the high air pressure generated by the forward speed alone. At M2, apressure ratio of 6:1 can be obtained from a suitable air intake, and this increases to more than 20 :1 at M3. Compression ratiosof this order are sufficient to eliminate the necessity for the com- pressor and turbine, resulting in the concept of the ramjet. Atapproximately M2.6, the ramjet would have a similar specific con- sumption to a modern turbine engine of 12 :1 pressure ratio;and above this speed the ramjet is superior. This is amply demonstrated by plotting specific fuel con-sumptions of turbojets and ramjets at varying Mach numbers. Additionally, one can examine the specific consumption of acombined turbojet /ramjet unit, since it is necessary to provide an auxiliary means of accelerating the aircraft up to speed. A more detailed examination of a transport aircraft (Fig. 7),capable of cruising at 2,000 m.p.h. with a transatlantic range, shows the chief advantage which a combination of ramjetand turbojet engines has over a similar machine powered solely by turbojets is the saving in weight which the ramjets offer.The aircraft with combination engines has a payload of 8.6 per cent of the gross weight, and that with turbojets only 4 per cent.The difference is almost entirely due to the difference in engine weight. With an aircraft of, say, 350,0001b (a trifle more thanthe Boeing 707 Intercontinental) a useful payload of 30,0001b can be obtained with an aircraft having a combination of ramjetand turbojet engines, with a cruising speed of 2,000 m.p.h. and a still-air range of 4,000 n.m. Ramjets have so far been used only for missile propulsion,but their light weight, simplicity and inherent reliability must bring them more and more into the picture as the speed of trans-port aircraft increases—as it assuredly will—to three or four times the speed of sound. It is incidentally worth noting that,because of the vast airflow required to propel an airliner at M3 at 70,000ft, the size of intake ducts leads to a configuration(Fig. 7) with ample space for fuel and payload. Flight at Hypersonic Speeds. So far we have reached 2,000nxp.h. and 100,000ft, and to go further the ramjet is the only Fig. 6. Bearing a superficial resemblance to the Hawker P. 1127, this VTOL strike machine is supported at full speed by tiny wings possible propulsion unit. In the hypersonic flight region (i.e.,above about 3,000kt or M5.0) there is a broadly defined corridor in which normal flight should be possible. This corridor isbounded on the one hand by an aerodynamic limitation, where at very high altitudes the air pressure developed by the forwardspeed is insufficient to maintain flight, and by a skin temperature limitation on the other hand defining the speed above whichthe temperature of the aircraft skin will be too high for struc- tural materials—say, l,200°K. If we restrict our considerationto speeds up to 5,000 m.p.h., then suitable flight altitudes lie between 100,000 and 200,000ft. A ramjet engine designed for internal supersonic combustionhas a converging passage forming a supersonic diffuser in which air is compressed, fuel being injected just upstream of thethroat. The fuel readily ignites because the air temperature due to compression will be several thousand degrees Kelvin. Down-stream the combustion is stabilized by a standing detonation- wave in the supersonic flow which occurs in the divergingnozzle. This type of engine suffers from the difficulty that its internal surfaces are exposed to very high temperatures. Aninteresting suggestion to alleviate the problem of skin temperature is for an external hypersonic ramjet, in which the air is com-pressed by the curved undersurface and the fuel is injected so that combustion takes place adjacent to a downward- and rear-ward-facing surface, thus providing lift and thrust. The overall thermal efficiency which might be expected froman air-breathing engine varies from some 40 per cent at M2 through a figure of nearly 60 per cent at M4, to a peak of over65 per cent at M7. This increase in overall thermal efficiency with rise in Mach number tends to offset the fall in aircraft lift/dragratio which occurs during passage through the transonic region. As a result, the range which may be obtained from a family of air-craft, all using the same percentage of fuel during flight and designed to have the best L/D ratio appropriate to their cruisingMach number, has a maximum at M0.8, falls rather sharply to a minimum at Ml.5 and then rises steadily as the Mach numberincreases to over M5. This indicates that the speed range between Ml and M2 should be avoided, and justifies looking to a speedin excess of M2 for the first supersonic airliner. Fig. 7. Dr. Hooker presented this supersonic airliner as a promising configuration; such transports were described in our issue of June 19 last 'If PASSENGER COMPARTMENTS ENGINE SAY CREW
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