Variants: DH9, DH9A, Dayton-Wright Cabin Cruiser
The Airco DH.4 was a British two-seat biplane day bomber of World War I. It was designed by Geoffrey de Havilland (hence "DH") for Airco, and was the first British two-seat light day-bomber to have an effective defensive armament. It first flew in August 1916 and entered service with the Royal Flying Corps (RFC) on 6 March 1917 with No. 55 Squadron in France. The majority of DH.4s were built as general purpose two-seaters in the United States, for service with the American forces in France.
The DH.4 was tried with several engines, of which the best was the 375 hp (280 kW) Rolls-Royce Eagle engine. Armament and ordnance for the aircraft consisted of one 0.303 in (7.7 mm) Vickers machine gun for the pilot and one 0.303 in (7.7 mm) Lewis gun on a Scarff ring mounting for the observer. Two 230 lb (100 kg) bombs or four 112 lb (51 kg) bombs could be carried.
The prototype that first flew in August 1916 was 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 engine. The first order for 50 DH.4s, powered by 250 hp (186 kW) Eagle III engines was placed at the end of 1916. In American production, the new Liberty engine proved suitable as a DH.4 engine, although the engine produced a slightly inferior performance to the Eagle. The Liberty was also to eventually power the British DH.9A.
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. There was some controversy (especially in American service) that this placement of the fuel tank was inherently unsafe. In fact, most contemporary aircraft were prone to catching fire in the air. The fire hazard was reduced, however, when the pressurised fuel system was replaced by one using wind-driven fuel pumps late in 1917, although this was not initially adopted by American-built aircraft. The otherwise inferior DH.9 brought the pilot and observer closer together by placing the fuel tank in the usual place, between the pilot and the engine.
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.
After the DH.4 entered service with No. 55 Squadron,RFC, more squadrons were equipped with DH.4s to increase the bombing capacity of the RFC. Two squadrons were re-equipped in May 1917 and a total of six squadrons by the end of the year. As well as the RFC, the RNAS also used the DH.4, both over France and over Italy and the Aegean front. The DH.4 was also used for coastal patrols by the RNAS. One, crewed by the pilot Major Egbert Cadbury and Captain Robert Leckie (later Air Vice-Marshal) as gunner, shot down Zeppelin L70 on 5 August 1918. Four RNAS DH.4s were credited with sinking the German U-boat UB 12 on 19 August 1918.
The DH.4 proved a huge success and was often considered the best single-engined bomber of World War I. Even when fully loaded with bombs, with its reliability and impressive performance, the air craft proved highly popular with its crews. The Airco DH.4 was easy to fly, and especially when fitted with the Rolls-Royce Eagle engine, its speed and altitude performance gave it a good deal of invulnerability to German fighter interception, so that the DH.4 often did not require a fighter escort on missions, a concept furthered by de Havilland in the later Mosquito in World War II.
Despite its success, numbers in service with the RFC actually started to decline from spring 1918, mainly due to a shortage of engines, and production switched to the DH.9, which turned out to be disappointing, being inferior to the DH.4 in most respects. It was left to the further developed DH.9A, with the American Liberty engine, to satisfactorily replace the DH.4.
When the IAF (Independent Air Force) was set up in June 1918 to carry out strategic bombing of targets in Germany, the DH.4s of 55 Squadron formed part of it, being used for daylight attacks. 55 Squadron developed tactics of flying in wedge formations, bombing on the leader's command and with the massed defensive fire of the formation deterring attacks by enemy fighters. Despite heavy losses, 55 Squadron continued in operation, the only one of the day bombing squadrons in the Independent Force which did not have to temporarily stand down owing to aircrew losses.
After the Armistice, the RAF formed No. 2 Communication Squadron, equipped with DH.4s to carry important passengers to and from the Paris Peace Conference. Several of the DH.4s used for this purpose were modified with an enclosed cabin for two passengers at the request of Andrew Bonar Law. These aircraft were designated DH.4A, with at least seven being converted for the RAF, and a further nine for civil use.
Adapted from various sources including Wikipeadia.