Design and Development
The DH.4 was designed by Geoffrey de Havilland as a light two-seat day bomber powered by the new BHP engine. The prototype first flew in August 1916, powered by a prototype BHP engine rated at 230 hp (170 kW). While the DH.4 trials were promising, the BHP engine required major redesign before entering production, and the Rolls-Royce Eagle engine was selected as the DH.4's powerplant. The first order for 50 DH.4s, powered by 250 hp (186 kW) Eagle III engines was placed at the end of 1916.
The aircraft was a conventional tractor two bay biplane of all wooden construction. The crew of two were accommodated in widely spaced cockpits, separated by the fuel tank. It was armed with a single forward-firing synchronised Vickers machine gun and one or two .303 in (7.7 mm) Lewis Guns fitted on a Scarff ring fired by the observer. A bomb load of 460 lb (210 kg) could be fitted to external racks. While the crew arrangement gave good fields of view for the pilot and observer, it caused communication problems between the two crew members, particularly in combat, where the speaking tube linking the cockpits was of limited use.
As production continued, DH.4s were fitted with Eagle engines of increasing power, settling on the 375 hp (280 kW) Eagle VIII, which powered the majority of front line DH.4s by the end of 1917. Because of the chronic shortage of Rolls-Royce aero engines in general, and Eagles in particular, alternative engines were also investigated, with the BHP (230 hp/170 kW), the Royal Aircraft Factory RAF3A (200 hp/150 kW), the Siddeley Puma (230 hp/170 kW) and the 260 hp (190 kW) Fiat, all being used in production aircraft. None of these engines could match the Rolls-Royce Eagle, however there were simply not enough Eagles available.
In American production, the new Liberty engine proved suitable as a DH.4 powerplant although the engine produced a slightly inferior performance to the Eagle. The Liberty was also to eventually power the British DH.9A.
Read more about this topic: Airco DH.4
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