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Aviation History
1958
1958 - 0696.PDF
712 FLIGHT23 Afay 1958 "The one and onfy DC-4E prototype was . . . flown on June 7, 1938." Origins of the Modern Airliner PART4: THE DC-4 GENERATION By PETER W. BROOKS, B.Sc, A.C.G.I., A.F.HAeJ. THIS i« the concluding part of Air. Brooki's article, die previous instal-ments of which appeared in our issues of April 11 and 25 and May 9. The author wishes to express his indebtedness for assistance given byMessrs. B. S. Shenstone, S. F. Wheatcroft, D. Sawers and J. M. Stroud in its preparation. RETURNING to the American scene, in mid-1935 discus-sions started between Douglas and the five largest Ameri-can airlines—United, American, T.W.A., Eastern and Pan American—about a new four-engined trunk-line aeroplane. InMarch 1936 each of these airlines put up $100,000 (£20,000) for the design and construction of a prototype, which was put inhand. The one and only DC-4E prototype was completed and firs; flown on June 7, 1938. It was a very large aeroplane for itstime, with a wing area of 2,155 sq ft—not quite as big as the Ensign, but bigger than the Ju 90 and more than double the sizeof the DC-3—and was the first American airliner to adopt the four-engined low-wing monoplane configuration already pioneeredin Europe. Construction was of the now standard 24S alloy and the wing was also of typical Douglas design. It had a split flap underthe centre-section and plain flaps outboard. The loaded weight of the DC-4E was initially 61,000 lb but this was later increasedto 66,500 lb. There were passenger seats for 42 people, accom- modated in abreast pairs each side of a central aisle. The aero-plane at first had triple fins and rudders, but these were later changed for a more convential single unit. In spite of many impressive features the DC-4E was a failureand was not proceeded with. It was, however, important as the first large aeroplane to have the tricycle-type undercarriage thatwas to become a standard feature of the "DC-4 generation." It was also the first transport to have power-boosted controls, alter-nating-current electrical system, an auxiliary power unit and new specially-developed Pratt and Whitney R-2180 engines. In addi-tion, it was the first American airliner with flush-riveted skin, a practice which had, however, been adopted earlier in Europe;U.S. manufacturers had previously used mushroom-headed rivets. More significant was the fact that the DC-4E was also the firsttransport aeroplane designed to performance standards which, since the war, have become the basis of modern airline operation.The conception of catering for the "worse case" failure of an engine became feasible for the first time with four engines.N.A.C.A. data on basic control and stability requirements were included, also for the first time, in the airlines' specification forthe DC-4E. The airlines which had sponsored the DC-4E were not allentirely happy with what they were getting, but it was a con- siderable time before the project was finally dropped. In mid-1936, when construction of the prototype was still at an early stage, two of the sponsor airlines (Pan-American and T.W.A.)started discussions with Boeing about a possible civil transport development of the B-17 bomber, which had appeared in proto-type form the previous year. The Boeing 307 Stratoliner was con- siderably smaller than the DC-4E—it had 30 per cent less wingarea—and employed the wings, tail unit and tailwheel under- carriage of the B-17 but, like the Douglas, incorporated a large-diameter circular-section fuselage which was intended to be pres- surized. The wing was a two-spar structure mainly of conventionalaluminium alloy, as was the remainder of the airframe. Some use, however, was also made of steel. The 307 prototype made its first flight on the last day of 1938.Well before this, in the spring of 1937, T.W.A. had ordered six 307s and Pan-American three and had thus helped Boeing tofinance the project. Pan-American were particularly keen on the pressurization feature and specifically helped to finance its develop-ment. An accident to the first aeroplane in March 1939, when it suffered structural failure while coming out of an accidentalspin during a demonstration flight for K.L.M., caused delays, but development went forward and the production aircraft weregiven larger vertical tail surfaces. The type was also developed to take more powerful Wright Cyclone engines and—in the Model307B—the flaps were changed from the split to the slotted type. The first 307 was delivered to Pan-American early in 1940 butthe first to go into service were five 3O7Bs of T.W.A. which were used on the transcontinental and New York - Chicago routesfrom April 1940. Later the 307s were used by T.W.A. for trans- atlantic services on behalf of the United States military authorities.The 307, like the Lockheed 14 before it, had full-feathering con- stant-speed propellers. It was also the first transport aeroplaneto be put into service with cabin pressurization. The pressure differential was 2 Ib/sq ft, equivalent to a cabin attitude of8,000ft when the aircraft was at 14,700ft. Although technically a success, the Boeing 307 was not produced in numbers. The DC-4E prototype was handed over to United Airlines in1939 and was evaluated for a time on their routes. It received C.A.A. certification on May 5, 1939. United contemplated order-ing a 52-passenger version but, in the end, the sponsor airlines agreed that the DC-4E was not what they wanted. There was"too much aeroplane" for the job to be done and the feeling was that an aircraft more of the size of the Boeing 307 was whatwas required. The DC-4E prototype was sold to Japan towards the end of 1939. In mid-1939, as a result of airline reaction to the DC-4E,Douglas started work on an entirely new design. Construction of the first aircraft started early in the following year, by which timewar had broken out in Europe. The new aeroplane, also known— confusingly—as the DC-4, was considerably smaller, structurallymore efficient and aerodynamically cleaner than the DC-4E. Its size was, in fact, similar to that of the Boeing 307 with a wingspan of 117ft 6in and an area of 1,457 sq ft (Boeing 307: 107ft 3in and 1,486 sq ft) The power units were four of the new Pratt andWhitney R-2000 radials of 1,350 h.p. and the layout was similar to that of the DC-4E, although the wing was of higher aspect-ratio. The wing was of the same N.A.C.A. 23-series section—16 per cent thick at the root and 12 per cent at the tip—but had slottedflaps. The design wing-loading was increased from 28 to 34 lb/sq ft. The inner wings were of typical Douglas "multi-cellular" three-spar construction but the outer panels had single webs and heavy booms—which suggests that the structural attrac-tions of the technique used in the DC-2 and DC-3 had still not been fully appreciated. The initial design gross weight of theDC-4 was 50,000 lb but this was soon increased and the produc- tion aircraft were operating at 73,000 lb before the end of the war.The tail was a simple single fin and rudder unit of clean design. The undercarriage retracted vertically into the inner enginenacelles instead of laterally into the wings as it had done on the DC-4E. Structure was again mainly of 24S aluminium alloy. Thefuselage was a long "parallel-section" tube of slightly egg-shaped section. An internal cabin-width of 118in provided space for two-and-two seating in seats 24in wide on each side of a 22-inch aisle. These figures represented a substantial advance even on the two-and-one layouts of the DC-3. Later, this same DC-4 fuselage section—the fuselage section seems to be one of the few immutablethings about a transport aeroplane—was to take five abreast (three- and-two) high-density seating in versions of the whole DC-4 toDC-7 series. Sixty-one of the new DC-4s were ordered "off the drawing
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