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Aviation History
1961
1961 - 1294.PDF
396 FLIGHT, 7 September 1961 VTOL Propulsion A PAPER BY A ROLLS-ROYCE ENGINEER THE September issue of the Journal of the Royal AeronauticalSociety contains a paper, Powerplcmts for Vertical Take-offAircraft, by Mr G. L. Wilde, AFRAes, chief of preliminary design Rolls-Royce Ltd, Derby. The following is an abbreviated version of his text, which forms a useful companion to other papers on the subject due to be read before the Anglo-American Aeronau- tical Conference next week. Rolls-Royce studies, and experience with the Short SC.l, show that two types of VTOL aircraft can be developed in the immediate future All the engineering problems of their powerplants can be solved using data which is now available or the results of current tests and research. . One of the types is a short/medium-range strike aircraft. It would cruise at high subsonic speed at sea level, carrying a military load equal to about 10 per cent of its gross weight. If supersonic per- formance at altitude were required this could be provided with reheat on the propulsion engine, but the aircraft weight would be greater The other type, which could have either VTOL or STOL capability would be a tactical transport of about 100,0001b take-off weight It would carry a payload of 20,000 to 30,0001b for ranges of about 500 n.m., cruising at speeds of 300 to 400kt at heights up to 20,000ft. Both types of VTOL strike aircraft rely on jet-lift engines.The cranked delta has four lift engines in the centre and a single efficient by-pass engine for propulsion. The second type has a wingof higher aspect ratio; the four lift engines are installed in pairs at the front and rear of a central weapons bay. Propulsion is bytwo turbojets in the wing roots; jet deflectors could give additional lift during take-off. The aircraft are stabilized during the hover andtransition by bleed-air jets regulated by an autostabilizer. The cranked-delta is really a refinement of the SC.l design,with lighter lift engines and a much more efficient propulsion engine. Although the second configuration is new, the layout presents noereat difficulties. The pitching moment resulting from an engine failure will be greater than with the cranked-delta, and the aircraftwill thus need a greater installed lift-thrust/weight ratio. The propulsion engines will be lighter than the by-pass engine of theother aircraft, but this will be offset by their higher fuel consumption. It is often claimed that there will be a big saving in installedthrust if aircraft are designed for STOL rather than VTOL perform- ance This depends on the definition of STOL adopted. If it istaken to mean that the aircraft must reach a height of 50ft in a horizontal distance of 500ft, the thrust/weight ratio needed is1 05 For STOL performance of this type it is necessary to deflect the thrust during take-off at low forward speed, when the aircraftaerodynamic controls will be inadequate. It is much better to design the aircraft from the start for VTOL operation, when a thrust/weight ratio of 1.2 is necessary. Installed thrust is not a great deal more than for the STOL, and if the aircraft is designed at the outsetfor VTOL its take-off and landing will be safer. This does not mean, of course, that such an aircraft is limited toVTOL If a strip is available, the VTOL can roll forward at up to, say 30kt before opening up its lift engines; this will prevent debrisingestion If no prepared ground is available, the STOL cannot operate, but the VTOL can take-off vertically.If a definite requirement is stated for an STOL strike aircraft, a choice must be made between the use of a single engine with jetdeflection or a composite powerplant; in both cases a thrust/weight ratio of 1.05 is needed. If a single-engined layout is chosen, thepowerplant will have too high a thrust for the cruise and must be designed with a medium pressure ratio to keep the specific weightlow and with a high by-pass ratio to reduce consumption. The engine must be throttled to about 12 per cent thrust for cruiseat M0 45, giving a poor specific consumption. If the composite powerplant is chosen, both types of engine can be designed withoutcompromise. Aircraft of these two types have been compared when both aredesigned to meet the same low-level strike role. The aircraft with Tactical strike aircraft ofswept-wing configuration, with four lift engines and two propulsion engines with jet deflection Tactical strike aircraft of delta configuration, with four 6,0001b lift turbojets and a single ll,000lb by-pass propulsion engine. The /otter is not provided with jet deflection, but can be equipped with reheat composite powerplant will be much smaller for any given mission; and if the composite type has a by-pass propulsion engine, giving a good specific consumption, the single-engined aircraft will have a take-off weight more than double. One multiple-engine approach is to provide all the lift thrust by lightweight booster or lift engines. Although this has a greater installed thrust and engine weight, it does not require jet deflectors, which are likely to be heavy and bulky; and it avoids the design problem of placing the propulsion engines so that their deflected thrust line passes close to the e.g. With both composite types the lift-engine thrust can be tilted by up to ±30° from the vertical with only small weight penalties; in this way a large horizontal thrust component can be provided to assist acceleration or de- celeration. In estimating weight for the low-level strike mission a delta planform of low aspect ratio was chosen. High-lift devices were omitted, and a lightweight undercarriage was assumed. This led to the assumption that a structure weight of 18-20 per cent was possible, compared with 28-30 per cent for the STOL. Compared with the STOL aircraft a 50 per cent increase in propulsive thrust/ weight ratio has been assumed. This is because it is necessary to maintain a high "g" capability in a steady level turn at high sub- sonic speed. The conclusion from these Rolls-Royce studies is that sufficient booster or lift thrust can be installed to achieve vertical take-off and landing without the need for deflection of the propulsive thrust, and that a VTOL aircraft of this type need only be very little heavier than an STOL aircraft with a composite powerplant which has been designed for the same mission.
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