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Aviation History
1957
1957 - 1627.PDF
FLIGHT, 1 November 1957 715 AIRLINERS OF THE WORLD ACCOUNTA Aviation Traders Engineering), Ltd.,London W.I BETWEEN 1950 and 1952, Aviation Traders carried out acomprehensive survey of aircraft, in both civil and militaryservice, that were used to carry a combination of passengers and freight and could also be converted for ambulance, photo- graphic or similar irregular or semi-regular use. Analysis of the results showed that, while the demands for aircraft with 40 seats and more and for 16 seats and less were being adequately met by new equipment, the intervening categories were still being catered for by the ubiquitous DC-3. There were, at that time, an esti- mated 6,000 DC-3s still in service in various capacities throughout the world. Another category of aircraft for which a very large demand existed was also noted in the U.S. and Canada, where business flying was (and is) showing an upward trend that parallels the growth of the airlines. Here again, much of the equipment in use was converted war-surplus stock and there was apparently little being offered in the way of efficient and economical new executive aircraft to replace outdated equipment. Numerical analysis showed, that the majority of the world's aircraft could be classified in three ways: business aircraft, civil and military transports with a seating capacity of up to 28 seats, and those of up to 40 seats. The former two categories were greater numerically than the latter. Aviation Traders decided that their design would attempt to fulfil the requirements of the first two categories, but would be capable of extension to meet the requirements of die third if that were needed. The Accountant, then, can be considered to fall into the broad category of a "DC-3 replacement" although only certain features of the Douglas design required perpetuation. For an executive aircraft the overruling essentials are felt by Aviation Traders to be long range and speed, and for feeder line aircraft low initial cost; low operating costs; simplicity of maintenance and versatility in the carriage of passengers or freight. The fore- going presupposes comfort, good field performance and high tip to ground propeller clearances in each application. With this basis for their design philosophy, the company pro- ceeded with the fundamental decision to use Rolls-Royce Dart turboprops—at a time when world-wide operating experience had not been accumulated and assured acceptance was not so universal as is guaranteed today. Size and weight of the airframe would be similar to the DC-3, and it would be designed from the start with emphasis heavily placed on ruggedness and simplicity and built on fail-safe principles. Although a number of changes—some fairly major, involving a re-appreciation of the tension skin principle on which the fuselage and tailplane is constructed—have been put into effect in the design of aircraft subsequent to the prototype, the guiding principles remain as before. The prototype aircraft flew for the first time on July 9 from Southend, where two pre-production aircraft are now being built. Both machines incorporate the new design of fuselage which has a parallel centre section that can conveniently be extended to increase the seating capacity from a maximum of 28 to a maximum of 40. AIRFRAME The problem of fatigue has been given primary consideration in the design of the airframe. Production and pre- production aircraft have fuselages of 19 s.w.g. skin over conven- tional frames and stringers, and the general stress level arising from pressure differential is of the order of 7,000 lb/sq in, with a maximum level at points of stress concentration of 15,000 lb/sq in. There are a total of four emergency exits provided which adequately meet both B.C.A.R. and C.A.A. requirements. In areas such as the canopy, single load carrying members have been avoided and a design ultimate factor of 3 meets an American requirement for a factor of 1.15 on combined internal and external pressure loads with one element failed. Cabin windows abut on to a sealing ring on the skin and are clamped into position. Doors on the aircraft under construction are of the sideways opening type and will be locked by anti-jam throw-over latches. The windows are being increased in size to 14£in by 16in. In view of the 70,000 hr airframe life achieved by some DC-3s, Aviation Traders have set their sights at an "infinite life" in designing the wing, which is in five sections. The primary bending structure is a box formed by the front spar at 19 per cent chord, a rear web at 65 per cent, and the top and bottom surfaces of conventional skin-stringer construction. The bottom skin is single heat-treated L.64 or L.72 and, apart from multi-bolted joints outboard of the engines and inboard of the ailerons, is unbroken by cut-outs from the tip to the centre line of the fuselage, where another multi-bolted joint occurs. On the top skin, double-heat treated D.T.D. 687 and L.65 is used, with 12 s.w.g. skin over the centre section. The end load carried by the skin-stringer com- bination is diffused into two compression booms across the fuse- lage and into a keel member on the fuselage centre line. Inter- action between fuselage pressure differential and wing bending loads is thus minimized and cut-outs in the fuselage are reduced to simple reinforced slots. The lower section of the nose panels outboard of the powerplant hinges for access in daily servicing; flush-type press fasteners are used. Flaps stretch from aileron to aileron; they are in five sections connected by articulated joints. The cranked centre section flap is single slotted, but the remainder are double slotted with their geometry so arranged that at the take-off position the slat has not opened. Extensive use of glass-fibre mouldings is to be made for non-structural parts. A sturdy Dowty tricycle undercarriage is fitted. The main wheels retract into the nacelle "step" and the forward-retracting nosewheel protrudes slightly in the retracted position. Goodyear wheels, tyres, brakes and—on the mainwheels—anti-skid units are used. [Continued on page 716
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