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Aviation History
1955
1955 - 0513.PDF
22 April 1955 513 MIXED POWER Engines of Different Species in Combination: Mr. M. J. Brennan s R.Ae.S. Lecture THE chief designer of Saunders-Roe, Ltd., Mr. M. J.Brennan, B.Sc, M.I.Mech.E., F.R.Ae.S., gave a RoyalAeronautical Society "main" lecture before the Glasgow Branch on April 14th. Entitled Mixed Power Units in Modern Aircraft, his paper outlined the propulsion requirements of various types of aircraft, discussed the manner in which these had been met in the past, reviewed the basic characteristics of a variety of types of aero engine and concluded by studying in some detail the problems posed by three widely differing types of military aircraft: the long-range jet bomber, the helicopter, and the supersonic intercepter. Taking a broad view of the whole field of providing power foraircraft propulsion, Mr. Brennan pointed out that one of the most significant features was the variety of basically differentunits now available to designers. For fighters, the liquid-cooled piston-engine had almost completely given way to the turbojet,although the air-cooled engine was still widely used in all types of aircraft. Reference had also to be made to the American WrightTurbo-Compound and the Napier Nomad, the latter a compound dicsel engine with a phenomenally low specific consumption.The lecturer then outlined the various developments of gas turbines since the introduction of the simple turbojet in theearly 1940s. Today, this already wide variety of engines was being supplemented by the ramjet, the pulse-jet and the rocket. From the point of view of the aircraft designer the mostsignificant characteristics of a powerplant were its specific weight (installed weight divided by thrust or horsepower), the specificfuel consumption (fuel burnt per lb-thrust or h.p. per hour) and specific frontal area (frontal area divided by h.p. or thrust). Allthree of these qualities, clearly, varied according to flight speed and altitude. Another complicating factor was that the output ofpiston engines was measured in horsepower whereas thrust was the criterion in all jet-propulsion engines; thrust could beapproximately converted to horsepower by multiplying by V/300,* where V was true airspeed in m.p.h. Typical examples[shown on the following pages] of these three basic characteristics had been drawn for each basic type of aero engine. They were,said Mr. Brennan, applicable to reasonably modern units, although the quantitative data were very general and in some casespossibly pessimistic. The lecturer went on to outline the salient features of each figure; generally speaking, it was always possibleto achieve higher specific output at the expense of efficiency and, * Allowing for airscrew T] = 82 per cent. conversely, higher efficiency at the expense of an increase incomplexity. If each particular type of aircraft had to meet the needs of onlyone condition of flight—as far as the power requirement was concerned—there would be only one choice of powerplant for theoptimum design. In practice, of course, every aircraft had to meet both take-off and powered flight conditions, and the lattermight in addition be broken down into several phases requiring differing or conflicting types of power. The remainder of Mr.Brennan's lecture was devoted to an examination of the power requirements of the three types of military aircraft alreadymentioned. The Long-Range Bomber.—The principal flight conditions tobe satisfied by the power units were, in this case, the take-off and the maximum-range cruise; there was, however, a tendencyto sacrifice range to gain cruising speed, so variation of the two latter parameters had been considered. It could be shown thatthe range was equal to f , L V[dWrange= j •&_ - - J w where L = lift, D = drag, V=t.a.s., c = specific consumption andW=weight. It could be seen that for any given full load the only variables were V,L/D and c. It could be shown that themaximum range which could be attained by a typical jet bomber (in which the weight of basic aircraft, engines and fuel was afixed proportion of take-off weight), reached a maximum at a cruising Mach number of slightly over 0.9, dropped rapidly to lessthan half this peak as soon as Mach 1 was exceeded, and then rose again almost to its former maximum at a cruising Machnumber of about 2.5. The trend of development was still towards ever-higher thrust/weight ratios and higher take-off wing loadings. Generally speaking, an increase of 10 per cent in take-off weight allowed anincrease in maximum range of some 8 per cent; the take-off weight was normally limited by the take-off speed, and this well-known design procedure applied wiui equal force to supersonic aircraft, since it could reasonably be expected that the high thrustrequired for supersonic flight would ease the take-off case. It could be remarked that it might be desirable to design for acruising Mach number as high as six, and rough calculations indicated that an optimum range of 2,500 nautical miles mightbe achieved with this sort of performance using ramjet power. Consequently, if it was again found desirable to sacrifice rangefor speed, future bombers might be powered by a mixture of 6000 5000 PROPELLER DRIVEN A/C ROCKET BOOSTED TO. 10% OVERLOAD+ FUELWT.-50%\ UNASSISTED TO. 10% OVERLOAD BASIC AIRCRAFT ENCINE WT + FUELWT .507o A.U.W. IMPROVEMENT OF RANGE FOR 10% INCREASE OF A.U.W RETAINING SAME TO. PERFORMANCE 2000 1000 10 20 CRUISING MACH NUMBER MODIFIED PULSEJETMODIFIELDRAMJET RADIAL PISTON y ENCINE(MECHANICAL DRIVE) CAS TURBINE (MECHANICAL DRIVE) ROCKET STARTER / (SHORT PERIOD ONLY) CURVES APPLY FOR SEA LEVEL ISA CONDITIONS 20 30 40 DURATION (MINUTES) 60 fig. /. Variation of range with Mach number for turbojet and airscrew-driven aircraft. The basic aircraft in each case is assumed to engine-plus-fuel weight equal to half the all-up weight. Fig. 2 (right). Characteristics of helicopter propulsive systems.
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