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Aviation History
1957
1957 - 1020.PDF
110 AERO ENGINES 1967 . . . crews can remain awake; this is yet another case of man's creationsoutstripping his own frailty. The U.S.A.F. and its contractors, such as Lockheed's Georgia Division, are intensively investigat-ing crew-behaviour in future nuclear-powered aircraft by simu- lating missions lasting 120 hr (five days). Notes on nuclear power-plants actually under development are published on pages 135 and 138. At present nuclear propulsive systems are necessarilylarge, complex and exceedingly weighty, particularly when they have to be shielded to protect human beings, or pans of an air-frame, from intense radiation. It is not immediately evident how this handicap can be overcome. Basic gas turbines, however, aremore amenable to being scaled up and down than any other type of prime-mover, and it is a fair prediction to assert that engineerswill eventually find a way to produce a small nuclear powerplant (using "small in its relative sense). Certainly, one can todaydiscern among engine-manufacturers a strong tendency for the evolution of baby versions of established engines, and also forcompletely new gas turbines of extremely simple and light con- struction capable of giving thrusts of the order of 2,000 to5,000 lb. One of the chief reasons for this is the omnipotent square /cubelaw which helps to explain why we shall never see a mosquito as big as an elephant. If, while preserving geometrical similarity,one were to make an animal twice as big (in linear dimensions) as it was previously, the monster would probably be unable tostand up; it would be four times as strong—but eight times as heavy.It is for this reason that a small engine should have a better thrust /weight ratio than a similarly designed large one. What-ever else may be done to reduce weight, big engines cannot avoid thick metal sections, big cast casings and heavy forged parts wherestress is high. To a much greater extent it is possible to make a small engine out of sheet-metal, and thus both reduce weightand cost. Few engines of truly lightweight conception have yet appeared. One of the first was the Fairchild J44, but this unitwas perhaps planned too early, when the full potentialities of the technique were not appreciated, and its brochure figures are notreally outstanding. Probably the first engine capable of showing what could beachieved was the Rolls-Royce R.B.82, or Soar, which was designed six years ago. For every pound of weight the Soar gave nearlyseven pounds of thrust, and its successors are substantially better. In the U.S.A. General Electric have a turbojet with a thrust/weight ratio of ten, and even this is by no means the limit. More- over, these are not short-life engines intended for vertical lift orthe propulsion of drones. At a recent meeting of the Society of Automotive Engineers apaper was presented by Curtiss-Wright's Turbomotor Division in which that company's design-thinking for future small turbo-jets was lucidly outlined. A proposed turbojet rated at 2,000 pounds' thrust, for propulsion at Mach numbers of the order of2.5, was described in some detail. There would be only two bearings, one immediately forward of the turbine and the otherbehind the compressor. The latter would only require four or five stages, and so could be very short and rigid and capable ofbeing overhung at the front. In the absence of anything better, the nose bullet could be mounted on the compressor itself, andallowed to rotate. The geometry of the compressor was unusual, in that short, stubby blades were to be mounted on a large-diameter hub, the r.p.m. being high and the first stage bemg transonic.Even at Mach 2.5 it was considered that turbine-inlet tempera- ture need not exceed 1,200 deg K, and the suggested turbinewould have one, uncooled stage. Altogether, the dry weight of the engine was calculated at the reasonable figure of 170 lb, givinga thrust/weight ratio of almost 12:1. A value of 15:1 was sug- gested as a target for immediate future development.A secondary advantage of such engines is that they could be employed in clusters, and their low capital cost and ease ofmaintenance should keep their overhaul costs down to those of larger engines used singly. It would certainly be possible toemploy four or more small engines in a single nacelle, all driving a common accessory group and perhaps even fed with fuel from acommon system employing one pump and one set of controls. The latter arrangement, however, might be frowned on by theauthorities responsible for certification of the aircraft concerned. An offshoot of this idea, but employing a final shaft-drive, hasbeen suggested in France. Three small turbojets—the Viper was actually cited—are mounted in a group within a common nacelle,and their jet-pipes are coupled to a volute casing surrounding a Although perhaps not directly connected with the development of aero- engines for 1967, this photograph is of dramatic quality and illustrates a possible method of airline take-off in 1987. The subject actually shown is a two-stage vehicle which was fired from Patrick A.F.B. two months ago as part of the earth-satellite-vehicle programme. FLIGHT, 26 July 1957 single turbine rotor of very large diameter, which is coupled toa propeller. The advantages are that multi-engine reliability is obtained in a very flexible powerplant, that the large-diameterturbine can extract full power while turning at very moderate r.pjn. and so eases reduction-gear problems, and that the "fluid-drive" character of the completely free power-turbine system would permit a fixed-pitch propeller to be used. The overallpowerplant weight was given as 3,090 lb (3,900 lb with cowling, propeller and all installational requirements) and the shaft horse-power as 6,800. Returning to the theme of small turbojets, there still remains agreat deal of life in the concept of the jet-flap, pioneered by workers at the British N.G.T.E., and by others in France. Onlylast week Fairchild were given a $218,000 contract to study a configuration employing this principle, and many other firmsand government agencies are formulating plans. Essentially, the jet-flap aircraft has a fixed wing propelled by a jet issuing from athin slit along the extreme trailing edge. The jet effectively separates the flows above and below the wing and thus makessome of the most fundamental dictates of traditional aerofoil theory inapplicable. It would, of course, be possible to achievepartial jet-flappery with present turbojets, but the ideal solution —from the viewpoint of the aerodynamicist, if not from that ofthe operator—would be to employ small turbojets disposed right across the span. It may, in fact, be possible to employ engineslying spanwise within the wing-^-or to adopt some entirely new method of jet-generation. Possibly the entire wing could beemployed as a two-dimensional ramjet. The suggestion just made may well be quite impracticable, butone may rest assured that it will not be dismissed as such until it has been tried, exhaustively evaluated and proved to be notworthwhile. It is never a good policy to discourage enthusiastic promoters of new ideas, but amateur inventors the world overmight be surprised to see the suggestions which are even now being studied by such competent groups as the NACA, theR.A.E., O.N.E.R.A.—and, of course, Soviet workers at TsAGI. It is frequently held that the day in which an inventor could reallybe said to invent are over, and that everything has already been done—or at least started to be done. When one regards the flight-propulsion scene it certainly looks like it. Even electro-gravities, or the negation of gravity by electrostatic or other means, is alreadyin the "hardware" stage. But that will not lead to a powerplant by 1967 ... W. T. G.
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