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Aviation History
1958
1958-1- - 0118.PDF
120 FLIGHT, 25 July Airco D.H.121 The Aircraft Manufacturing Co., Ltd., Hatfield, Herts. JUST as the Vickers VC.10 sprang from the requirements ofB.O.A.C., so the D.H.121 evolved from the needs of B.E.A.At one time, indeed, it was thought that one basic jet trans- port might fulfil the needs of the two British Corporations. But asthe British industry's project design-work progressed it became apparent that the two requirements were completely irreconcilable.Both Corporations required a British jet transport fully capable of meeting the high-subsonic air-transport requirements of the mid-sixties onwards. But there ended the broad similarity between the two Corporations' needs: a jet for B.E.A., which had to becapable of economic operation on stages of less than 1,000 miles was clearly a different design-exercise from the long-hauler. Itwas, indeed, more in the Viscount-replacement class. The D.H.121, which after a long political tussle was finallychosen in February 1958, represents a new class of airliner. Short- range air transport is difficult to exploit economically with jetpropulsion, because short stages allow less time for the jet's high speed to offset the inherently higher cost per hour (especially asmore time is spent low down). Thus the designers of the D.H.121 and its engines have had to "optimize," probably as no designershave been required to do before, all those factors which make for an economical jet airliner—very high payload in proportion tostructure and fuel weight, and very high cruising speed for the minimum fuel consumption. B.E.A. made known their requirements to the British industry inJuly 1956. The de Havilland Aircraft Company, in common with other manufacturers, had already done considerable project workto meet B.O.A.C.'s requirement (D.H.I 18); and they now evolved to B.E.A.'s specification the D.H.I 19, based upon four Rolls-RoyceAvon RA.29s—powerplant of the Comet 4. This developed, for political reasons, into the D.H.120 (four RA.29s or RB.140s),aimed at satisfying both Corporations. But this did not suit either B.O.A.C. or B.E.A.; at length B.O.A.C. ordered VC.lOs, andde Havilland concentrated on B.E.A.'s requirements. Thus was born, in May 1957, the D.H.121, which was to com-pete for B.E.A. and Government favours with the Bristol 200 (page 148) and the Avro 740 project (later dropped when theHawker Siddeley Group decided to co-operate with Bristol on the 200). The basic design decision to be taken was the choice and numberof engines; B.E.A.'s and de Havilland's preference was for a An unofficial general arrange- ment drawing of the D.H.121. Rolls-Royce powerplant, and Rolls-Royce were prepared to designand build an engine of whatever size was required. Eventually a triple-engined layout was evolved, with RB.141s—scaled-downby-pass Conway derivatives developing 12,000 lb thrust each. The choice of three engines was the result of reasoning which probablyran as follows: the fewer the engines, the lower the capital and maintenance costs and installed weight—all factors vital to jeteconomics. Two engines would have been so large—because of engine-out performance requirements—as to outweigh the twin-engine economies of cost and weight. Four engines would have been excessive in an aircraft of this class. The method of installation was no easy problem: a rear-mounted Caravelle-type arrangement was preferred because of the resultant advantages of a clean wing and cabin quietness; it wasalso virtually obligatory, since no other practical position exists for a triple-engine scheme. Many layouts were studied; the finalarrangement, as shown in the latest artist's impression above, is evidently different from that illustrated at the time of the B.E.A.order last February, when the middle engine was well forward oi the fin, instead of beneath, as it is now. This suggests that theengine is now removed by dropping it downwards instead of lifting it up and sideways first, as before; the power-systems bay,previously aft, has presumably been shifted forward. (It is note- worthy that the length of the fuselage forward of the wings hasincreased considerably since publication of the original drawing— no doubt to improve balance characteristics, which would befurther improved by a forward-positioned equipment bay.) Position of the tailplane, right on top of the swept-back fin, wasdictated by the requirements of maximum elevator moment-arm to give the smallest elevator size for the most critical case, i.e.,landing; by the need for maximum clearance from reverse-thrust jet blast; and to attain the lowest net structure weight. It isevident from the latest illustration that the middle engine will not, contrary to earlier sketches, be fitted with reversers. As already noted, the fuselage appears to have increased inlength since the early project stages. There are more windows (32 per side) than is demanded by the seating-capacity requirement(up to 100 five-abreast), which suggests that there will be more than one window per seat row. A departure from previous D.H.(Comet) practice, and in conformity with Vickers and Bristol design, is the shape of the windows—vertical, rather than hori-zontal, ellipses. Fuselage width is likely to be at least lift, in order to meet the inevitable airline requirement for six-ubreastseating. Cross-section is neither circular nor double-bubble, suggesting two intersecting radii. Nose-shape has changed con-siderably since the early drawings. The wing, entirely unobstructed by engines, will have large,highly efficient flaps; no suggestion that slats will be fitted—a la VC.10—is apparent. The main undercarriage, set well back on thewing, has inwards-retracting four-wheel bogies. Airco, the de Havilland-Hunting-Fairey consortium which isdesigning, developing and building the D.H.121, have issued no data about the aircraft, which is still in the formative stages (entryinto service with B.E.A., who have ordered 24 plus an option on 12, will not be until 1963/1964). In the meantime it is possible topredict the following approximate basic data: Gross weight, about 120,000 lb; payload, 20,000 1b plus; max-payload range withreserves, eventually up to 2,000 st. m. (without compromising B.E.A.'s requirement for the optimum 1,000-mile aircraft); cruis-ing speed 600 m.p.h. at 30,000ft (Mach 0.9); take-off thrust, 36,000 lb total from three Rolls-Royce RB.141s. Comet experiencesuggests a first flight date for the D.H.121 of spring 1961.
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