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Aviation History
1957
1957 - 1296.PDF
384 Two more three-engined schemes. The upper "triple-cluster" suffers from difficult intake- ducting and presents the worst balance case. The lower scheme is an al- ternative to the three- engined layout on the previous page, for better middle-engine efficiency. THE SHORT-RANGE JET AIRLINER . . . It was really the take-off requirement that made the small engine sodifficult to achieve: the required cruising thrust, low weight and economy were attainable, but for a take-off power that would be only just withinthe temperature limitations of the engine. No one was keen on an engine which would be straining itself and running almost too hot at the verystart of its career. It was natural for designers and for B.E.A. to have a look at the three-engined scheme. There were many apparent advantages. Firstly, the engines, in the12,000 to 13,000 lb thrust bracket (the Rolls-Royce RB.141 and the Bristol Olympus 553 have both been unofficially mentioned), were not sodifficult to achieve, and were in fact feasible developments of engines already ruuning. There would be a big reserve of power, because theaeroplane would, like a twin-engined machine, be designed to single- engine performance requirements. Total installed powerplant weightshould be less, and so would maintenance and hence direct operating cost. Furthermore, it was conceivable that engines (and pod units) couldbe used which would be interchangeable with those of the larger four- engined jet transports. Someone even argued that with a smaller numberof engines the incidence of engine failures would statistically be lower; but this was probably stretching the case for three engines a bit far. There were two chief disadvantages to the three-engine proposal. Onewas technical, the other commercial. The technical problem of where to put the third engine was not aneasy one. The only position worth considering was in the tail, and con- siderable engineering ingenuity was brought to bear on making the mostefficient installation in an inherently awkward place. Awkward, that is, from the point of view of accessibility, intake and jet-pipe ducting andcrash-landing safety. The first items were matters of engineering skill, the last a matter of opinion. Was it wise practice to have a transportaeroplane's engine in the fuselage, albeit aft of the pressure dome, and within the vital tailplane structure? Wherever a powerplant is installedan engine irruption might cause structural damage to the aeroplane: installed within the tail the chances of mortal damage by a burstingengine would be high. And with a centrally mounted aft engine there was the possibility that in a heavy crash-landing it might rip forward intothe passenger cabin. The commercial objection to three engines was more open to argument,particularly by the technicians, and less easy to express. It was the feeling that four engines sell more tickets—or rather that three engines are moreprone to be "unsold" by four-engined competitors. The commercial department of an airline has to see a sales-value in being technicallydifferent; and there was no doubt that three-engined jet travel, notwith- standing all its claimed technical advantages, would be different. Theaccepted norms of trunk-line travel, short-haul, medium-haul and long- haul, would, the commercial people felt, be in four-engined aeroplanes.Even among the designers, especially those (still too few in this country?) with a flair for sales, there was a feeling that a four-engined aeroplanewould be more readily acceptable in the export market—a market domi- nated by an American industry which appeared to be favouring four-engined air transport. During the latter half of 1956, the U.S. airlines, for the first time in their history, were operating more four-engined thantwin-engined transports; and it could be estimated from the numbers of Possible "extreme" three-engine solution, attrac- tive from balance and installation viewpoints. Brings the centre of pressure near to e.g.; treats the cluster as a neat power-package, well clear of airframe (the future nuclear powerplant solu- tion?); and utilizes the maximum of fuselage. FLIGHT, 6 September 195: new types on order that by 1956 perhaps 80 per cent of the total U.S.civil fleet would be four-engined. Should a relatively clear-cut technical issue by swayed by doubtfulcommercial arguments of this kind? The instinct of the industry, sharpened by ten years of competing with the Americans, was to accedeto the commercial point of view. There was already another aspect of the design which, though far from being unconventional, was counterto the American trend. This was the mounting of the engines in a tail-cluster (Caravelle-style)instead of in wing-mounted pods. Wing pods were the classic American way—the Boeing, Douglas and Convair way—of installing jets in trans-ports. Should British jets have wing pods just to accord with the American style? Wing Pods or Tail-Cluster? The answer was not an immediate, chauvinistic negative. If Britishjets were to be accepted by a U.S.-dominated market, and certificated by the U.S. airworthiness authorities, an established technical practiceshould not be lightly rejected. But the more that most British designers looked at wing pods, the more they favoured the tail-cluster. Wing pods,after all, only became a virtue out of the necessity of the wing bending- moment relief and mass-balancing of the thin, flexible, high-aspect-ratiowings of the Boeing long-range bombers. The short-range jet did not need that kind of wing: on the contrary, the cruising L/D requirementwould be less, short-spanned stiffness for high-speed medium-altitude cruising would be necessary anyway, and furthermore wing pods wouldbe uncomfortably close to the ground (debris) in the smaller aeroplane being considered. There were doubts, too, whether the lack of Britishtunnel and flight-test experience of the structural and aeroelastic problems of wing pods might not run away with a lot of development time. Therewere many other pros and cons of wing pods and tail clusters to be searchingly examined. Some of these were listed in Flight of May 31.Two items in particular made the tail-cluster an attractive proposition: cleaner wing and flaps, and a quieter cabin. The possibility of a com-pletely unobstructed wing and flaps was, of course, an aerodynamicist's dream, especially as they alleviated considerably the difficult take-offrequirement. With a flap-blowing system, which we can assume showed promising theoretical possibilities for this class of aircraft, the take-offand low-speed handling performance looked especially good. So much for the design. What about its market prospects? Would ashort-haul jet, custom-tailored for B.E.A., be acceptable off-the-peg to foreign airlines, and if so in what quantities? Flight has already broadly assessed (May 3, 1957) the potential worldmarket for jet transports of the three main traffic categories—long-, medium-, and short-haul. The estimate for the last-named category wasthat a market for 1,200 aircraft can be foreseen between 1960 and 1980. The assumptions made as to (1) rate of traffic-growth and (2) the relativeproportions of traffic shared between jets and turboprops were stated, and—like all long-range speculations—may be challenged. It might (forexample) be argued that the assumed annual rate of growth was too conservative, or that the allotted proportions of traffic showed too great abias in favour of the jet. But one such argument usually tends to balance the other; and the figure of 1,200 aircraft—worth about £1,700 millionto the manufacturers concerned—gives a rough measure of the amount of short-haul jet business likely to be transacted during the next twodecades. More than half of the world's air traffic is carried on stages of up to 1,000 miles—the realm of B.E.A.'s new jet. The Trend of The Market For however great the temptations may be to indulge in technicalleapfrogging, and notwithstanding the oft-heard arguments against "doing aeroplanes that are only the same as the Americans'," the fact is that amarket is not captured by running counter to its trend. And the trend is that jet transports in the image of the DC-8 and 707 will be sought indifferent sizes by the airlines during the next twenty-odd years. Now the nation is to be off-the-mark with two contenders—the VC-10in the medium-size, and the small B.E.A. jet for the short ranges. Twenty years may seem a long period for such aircraft to be in demand, so fastis the rate of technical progress; but the new B.E.A. jet, and the VC-10, Convair 880, DC-8 and Boeing 707 may, in the seventies, resemble theiroriginal models as much as the DC-7C today resembles the DC-4 of 1939. Who is to say for certain now that in the seventies the B.E.A. jetwill not have full laminar flow, area rule, transonic cruising speeds, twice its original payload, half its original airfield needs, or that it willbe as quiet as the Britannia? Such imaginings may seem wishful fantasy now: but these are the lines along which sound business is done, andwhich will lead the nation—quite incidentally—towards the prestige and air-power it seeks.
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