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Aviation History
1958
1958 - 0253.PDF
FLIGHT, 28 February 1958 THOUGHTS ON THE D.H.121 267 The Evolution of a New Kind of Airliner THE political and industrial wrangling which followed B.E.A.'sselection of the D.H.121 has been discussed to the point oftedium. But now that B.E.A. are free to negotiate for the aeroplane of their choice, it is refreshing to look for a change atthe aeroplane itself as a particularly interesting exercise in aircraft design. The D.H.121 represents a new class of airliner. Short-rangeair transport has not previously yielded to economic exploitation by jet propulsion, and a jet airliner designed fpr such work is anexacting design-exercise that falls into a distinctive—some would say specialized—class of its own. B.E.A. have avoided using the word "specialized," in the sensethat it suggests a limited export market. But a jet airliner designed to make a profit over short-haul networks, which are traditionallyair transport's most unrewarding (from a financial viewpoint), must be specialized in the technical sense. Indeed, its market would belimited if it were otherwise—if, for example, it strove from the start to fulfil also the medium-haul requirement. Admittedly anyadvanced jet transport designed to meet the B.E.A. specification must tend to invade the medium-haul category to some extent;but this tendency is resultant rather than causative. The immediate past history of the D.H.121 goes back to theD.H.118. This project was conceived in the autumn of 1956, and was a short-lived, politically influenced attempt to fulfil B.O.A.C.'smedium/long-haul requirement. Already, in July 1956, B.E.A. had made known to the industry their own outline requirementsfor an optimum short-haul jet. A project numbered D.H.119, powered by four late-model Rolls-Royce Avon RA.29s, appearedin the last weeks of 1956, with B.E.A.'s requirements particularly in mind. But early in 1957, some months before the other Cor-poration's requirement was finally satisfied by the much larger Vickers VC.10, an attempt was made to arrive at a basic design tosatisfy the requirements of both Corporations. This was the D.H.I 20 project, for which four Rolls-Royce Avon RA.29s orRB.140s (scaled-down Conways) were considered. These were installed, as in the 119, behind the rear wing-spars, with theirintakes below the wing. As de Havilland had expected, this project, like the 119, satisfied neither Corporation. It was toobig for B.E.A., and B.O.A.C. were favouring the VC.10, and the long-haul potentialities which its uncompromised design offered.So D.H. concentrated on B.E.A.'s requirements; and thus it was that, a little less than a year ago—in May 1957—the D.H.121 wasborn. B.E.A.'s Requirements It is now generally known that B.E.A.'s basic requirementsspecified a 600 m.p.h. jet transport to carry about 70 first-class passengers over stages of 1,000 n.m., with a take-off field-lengthof 6,000ft. B.E.A.'s feelings on these points were not inflexible. Their specification constituted a basis for discussion, so that therecould follow a period for the cross-fertilization of ideas, in which the ability of the contending design teams could be tested. The first problem the designers had to tackle was the choiceand number of engines. Fortunately for de Havilland there was little restriction in choice: Rolls-Royce, with whom they workedfrom the start, announced their willingness to build an engine of whatever size was needed—so great has their experience nowbecome. Thus the eventual selection of the triple-RB.141 formula was not dictated by the non-availability of a smaller engine suitablefor a four-engine layout. The choice of the diree-engined scheme was clearly the resultof reasoning which ran along the following lines. The fewer the engines, the less the capital and maintenance costs—factors vitalto the economics of the aeroplane—and the lower the total installed weight. But why not two engines? Here the power-failure per-formance requirement would have called for engines so large and powerful that the economies of two engines would have been out-weighed by their very size and expense. The aggregate thrust of two engines would obviously have to be considerably more thanthe 36,000 lb of three RB.141s. The latter is a by-pass engine with a higher by-pass ratio than the present Conway. There were many other pro-three-engine considerations, amongthem the two-engine failure case (admittedly remote), and also the existence of a ready-made insurance engine in the RA.29. Allthese things tended to favour the contention that a right number of engines for a short-range jet is, in the words of a B.E.A.engineer, "less than four, but more than two." Next came the biggest problem—the method of installation.A wing-mounted arrangement was not possible: an underslung middle engine would have been a short-lived inhaler of runway-debris. Soon the choice narrowed to engines in, or on, the rear fuselage, particularly as this made possible a cleaner wing and aquieter cabin. Nevertheless, the positioning of the middle engine must havebeen a classic exercise in "optimizing" the factors of weight, intake- efficiency, tailpipe-losses, cooling, accessibility, and ease ofengine-changing. Several schemes were no doubt assessed before the final ope was chosen. It was obviously important that thecentre engine should not be buried within the fuselage and the fin structure, and the artist's impression (page 268) shows thatthis has been avoided. Cooling probably presented no great diffi- culty, and in any case the by-pass engine is not critical in thisrespect. Accessibility was not awkward, since the engine and its accessories could be readily exposed by unstressed cowlings. Theengine-removal aspect was perhaps less favourable, though the "up, out and down" slinging technique would present no particulardifficulties. The resulting compact engine arrangement appears to have leftplenty of space in die unpressurized rear fuselage for a well- planned grouping of engine-supplied power-system equipment,such as the main units of the electric, hydraulic and cabin-air systems. The Airframe It is clear that the tailplane ended up on top of the fin for fourmain reasons: maximum static stability by virtue of the greatest possible moment arm; lowest demand on elevator size for landing;maximum clearance from reverse thrust and jet blast; and lowest net weight. Fuselage size and capacity is probably not yet finally decided.It would seem sensible to adopt an internal diameter sufficient to accommodate an inevitable airline requirement for six-abreastseating—which means an internal diameter of at least lift. It is foreseeable that B.E.A. may require more than 70 (probably nearer80) first-class seats at 38in pitch five-abreast, which would suggest a maximum 34in six-abreast capacity of perhaps 100 or more.Windows would be numerous, to permit the maximum flexibility of seating. The fuselage cross-section would probably have twointersecting radii (almost a double-bubble), to procure in the most structurally economical way, the big freight capacity which B.E.A.need to ameliorate their off-peak passenger problems. The wing, being completely unobstructed by engines, is gener-ally a designer's dream. An exception is the problem of under- carriage attachment, which for balance reasons tends to be locatedbehind the main wing box. The main gear would be a four-wheel bogie. The possibility of employing high-lift devices which theclean wing affords ensures that there will be no worries about meeting the 6,000ft field-length requirement. The flying-control surfaces would almost certainly be capableof manual operation. Although Comet practice was full duplicated power-operation, advances in knowledge of high-speed controlmake manual controls an obvious choice today. Cruising Mach number would probably be of the order of 0.9—600 m.p.h. at30,000ft. Future Development One has to bear in mind that there are six years to go before the121 is required to enter service (B.E.A. are aiming for early 1964). Many points obviously must still be liable to change from week toweek during this early phase. But in general it may be said that it looks as though B.E.A.'s 1,000-mile full-payload stage length,and 6,000ft take-off requirements can be met by an aeroplane weighing nearer 100,000 lb than 120,000 lb, yet which offers auseful built-in stretch in payload/range capability for those customers who want it. D.H. have set their sights firmly on the short-range exportmarket, and they believe that the B.E.A. specification can produce an aeroplane capable of fulfilling the needs of that market moreefficiently than anything designed for medium-haul routes. The latter market is in any case well served by existing designs. Butthere seems to be no reason why the 121 should not operate at higher weights with extra fuel. The basic airframe appears tohave plenty of fuel capacity—at least enough for double the speci- fied stage-length—and with a little undercarriage beefingthe basic B.E.A. aeroplane could probably be offered with a higher gross weight to airlines who want the extra range and can acceptslight relaxation in the field-length required.
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