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Aviation History
1952
1952 - 3038.PDF
FLIGHT, 17 October 1952 An early Eagle-engined specimen of the D.H.4 with the short undercarriage and with the Scarff gun ring mounted directly on the top longerons. THE DE HAVILLAND D.H.4 No. 1 of the Series By J. M. BRUCE, M.A. IT is not surprising that, in the first major war in which aircraft were used, it was some little time before the strategy of aerial warfare was evolved, and with it the conception of specialized types of aircraft, each with a specific function to fulfil. Until the end of 1916, bombing was done by aeroplanes which were in reality general-purpose types— converted fighters such as the Martinsyde Elephant, Sopwith i^-Strutter, and F.E.2b, or corps reconnaissance machines like the B.E.2C and 2d—all of which were quite capable of carrying the small bombs of the period. When it was realized that an aeroplane of a special type was required for day-bombing purposes, the task of designing such a machine was entrusted to the Aircraft Manufacturing Company, the ultimate responsibility for the design falling upon Geoffrey de Havilland. The aeroplane, designated the D.H.4, was actually designed around the then-new 200 h.p. six-in-line B.H.P. engine, and the prototype first appeared in August, 1916. By a happy coincidence, at the time of the D.H.4's appearance, the great Rolls-Royce concern was developing the liquid-cooled 12-cyiinder vee engine which was later known as the Eagle and which, in its Mk VIII form, delivered no less than 375 h.p. Prototype tests of the D.H.4 with both the B.H.P. and Rolls- Royce were highly successful and production was initiated with such efficiency that No. 55 Squadron, R.F.C., was able to go to France fully equipped with Eagle-powered D.H.4S on March 6th, 1917. Various marks of Eagle were fitted to the D.H.4 m service, and could be distinguished from the similarly cowled Rolls-Royce D.H.4 day bombers (B.H.P. engine) of No. 27 Squadron at Serny aero drome. Pas de Calais, France, on February 17th, 1918. For this initial study in a special "Flight" series to be devoted to famous British military aircraft, the subject is one of the most significant bombing and general-purpose aircraft of all time— the Mosquito of its day—the D.H.4. Falcon (also fitted to the D.H.4) by the left-hand airscrew. The Falcon, of course, drove a right-hand airscrew. The prototype D.H.4 which had the B.H.P. power-unit differed from standard in having the rear centre-section struts meeting the upper longerons at a point farther aft than on production machines, with the result that these struts sloped forward in side elevation. A four-blade right-hand airscrew was fitted, but most production D.H.4S with the B.H.P. engine had two-blade airscrews. The exhaust-pipe of the prototype led upwards to a point above the leading edge of the centre-section on the port side, a feature repeated on some of the production B.H.P.-powered machines, though others had a simple horizontal pipe on the port side only. Several other types of engine were fitted to the D.H.4, and performance varied with the power of the engine installed. The D.H.4S which had the R.A.F. 3A* power unit could be distin guished by the single central exhaust stack, four-blade left-hand airscrew, and radiator wider at the top than at the bottom. The Fiat installation bore a resemblance to the later D.H.9's nose configuration, but the lines of the cowling around the lower part of the Fiat were more bulbous than those of the cowling of the D.H.9's B.H.P. The Fiat's exhausts were on the starboard side and consisted of two short manifolds, each of which carried away gases from three cylinders. Structually, the D.H.4 was typical of the 1914-1918 period. The fuselage was the usual box girder, but in it a slight departure was made from the convention of the time, for the front portion was covered with plywood as far aft as the gunner's cockpit, and there was no internal wire bracing. The rear section of the fuselage was constructed as a separate unit and was attached to the forward portion by means of fishplates. Normal wire cross-bracing was used in the rear portion and fabric covering was applied. There were four wire-braced bays in the rear fuselage, the fifth and rearmost being sub-divided by two vertical spacers and covered with plywood. The longerons were of spruce, and ash engine- bearers were fitted. The wings and tail unit were conventional in construction, and were braced with "Raf-wires." The mainplanes had two spars of spruce, spindled out between the compression ribs. The ailerons were attached to the rear spars, and their balance-cables ran exter nally along the leading edges of the wings. The incidence of the tailplane was controllable and could be varied from + 2 deg to + 5 deg. The undercarriage was a simple, sturdy vee structure, with rubber cord shock-absorbers at the apices of the vees. Later in its career the D.H.4 was provided with a taller undercarriage. * These initials stood for Royal Aircraft Factory, the former name of the Royal Aircraft Establishment, Farnborough.
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