The Airco DH.4 is a British two-seat biplane day bomber of the First World War. It was designed by Geoffrey de Havilland (hence "DH") for Airco, and was the first British two-seat light day-bomber capable of defending itself.
It was designed and developed specifically as a bomber, as well as aerial reconnaissance missions. The DH.4 was to have been powered by the new Beardmore Halford Pullinger (BHP) engine, but problems with that resulted in numerous other engines being used, perhaps the best of which was the Rolls-Royce Eagle engine. The DH.4 first flew in August 1916 and it entered operational service in France on 6 March 1917 less than a year later. The majority were manufactured as general purpose two-seaters in the United States for the American expeditionary forces in France, becoming the only American made plane to see combat in World War I.
Following the Armistice of 11 November 1918, many DH.4s were sold to civil operators where it was found to be particularly useful as a mailplane. Early commercial passenger airplane service in Europe was initiated with modified variants of the DH-4. War-surplus DH-4s became key aircraft in newly emerging air forces throughout the world. The U.S. Army later had several companies re-manufacture its remaining DH.4s to DH.4B standard and they operated the type into the early 1930s.
Development Origins The DH.4 was designed by Geoffrey de Havilland as a light two-seat combat aircraft, and was intended to perform both day bomber and aerial reconnaissance missions. The intention was for it to be powered by the newly developed Beardmore Halford Pullinger (BHP) engine. The DH.4 was developed in parallel to the rival Bristol Fighter.Bruce, 1966, p.3 During August 1916, the prototype DH.4 made its first flight, powered by a prototype BHP engine.Jackson, 1987, p.53
Initial flight tests revealed it to have favourable handling and performance. The Central Flying School (CFS) conducted early evaluation flights using the prototype, leading to it producing a favourable report on the aircraft, observing good stability in flight, light flying controls and its relatively comfortable crew positions. During its flights with the CFS, it was able to attain previously unheard-of time-to-altitude figures which were unmatched by any of its predecessors. While flying trials with the prototype had been producing promising results, it soon became clear that the BHP engine would require a major redesign prior to entering production.Bruce, 1966, p.4
Even by the time of flying trials with the first prototype, there had been no finalised plans for quantity production of the BHP engine. Coincidentally, another suitable and promising aeroengine, the water-cooled Rolls-Royce Eagle in-line engine, was approaching the end of its development process. According to Bruce, the Eagle shared the same basic configuration as the BHP engine, which greatly aided in its adoption by de Havilland, as did the engine's endorsement by William Beardmore. During the summer of 1916, a second prototype, equipped with the Rolls-Royce engine, conducted its first flight.
In response to its favourable performance, the Royal Flying Corps (RFC) decided to place an initial order for the type during late 1916. Separately to the RFC's interactions with the DH.4, it had received substantial interest from the Royal Navy as well. The Admiralty decided to order a further pair of prototypes, configured to suit the service's own requirements, for evaluation purposes; however, according to Bruce, it is unlikely that the second of these was ever constructed. Following trials with the first of these prototypes, orders were placed for the production of DH.4s to equip the Royal Naval Air Service.Bruce, 1966, p.5
Production
During late 1916, the first order for 50 DH.4s, powered by Eagle III engines, was received from the RFC. According to Bruce, it was not a surprise to most observers that the Eagle had been selected to power the first batch of production DH.4s. The initial production aircraft were largely identical to the second prototype, the main difference being the adoption of armament, which included a single synchronised Vickers machine gun for the pilot, while the observer was provided with a Lewis gun mounted on a Scarff ring.Bruce, 1966, pp.4–5
Production of the DH.4 was performed by a variety of companies beyond Airco themselves; these included F.W. Berwick and Co, Glendower Aircraft Company, Palladium Autocars, Vulcan Motor and Engineering, and the Westland Aircraft Works. By the end of production, a total of 1,449 aircraft (from orders for 1,700 aircraft) were constructed in Britain for the Royal Flying Corps (RFC) and the Royal Naval Air Service (RNAS).Jackson, 1987, p.54 Overseas, SABCA of Belgium produced a further 15 DH.4s during 1926.Jackson, 1987, p.60
As production progressed, various changes and improvements to the design were introduced upon the DH.4. As time went on, production DH.4s were fitted with Eagle engines of increasing power, ending with the Eagle VIII, which powered the majority of frontline DH.4s by the end of 1917. However, this transition was greatly hindered as by January 1917, it had become clear that there was a chronic shortage of Rolls-Royce aero engines, and of the Eagle in particular; it has been claimed by Bruce that this shortfall was partially the result of protracted decision-making on the part of the Air Board.
In response to the limited availability of the Eagle, extensive investigations into the use of alternative engines for the DH.4 were conducted. This resulted in aircraft being outfitted with a diverse range of engines; these included the BHP (, the Royal Aircraft Factory RAF3A (), the Siddeley Puma () and the Fiat, all of which were used to power production aircraft, with varying degrees of success. None of these engines proved to be capable of matching the performance of the Eagle engine, which remained the preferred options despite the persistent supply constraints.Bruce, 1966, pp.5–8
American versions
At the time of entry of the United States into the First World War on 6 April 1917, the aviation section of the U.S. Signal Corps was unprepared, not being equipped with any aircraft suitable for combat. However, considerable optimism and energy was put into addressing this identified need, leading to the mobilization of American industry to set about the production of contemporary combat aircraft. As there were no suitable aircraft domestically, a technical commission, known as the Bolling Commission, was dispatched to Europe to seek out the best available combat aircraft and to make arrangements to enable their production to be established in the United States.Bowers, 1966, p.3 As a result of the efforts of the Bolling Commission, the DH.4, along with the Bristol F.2 Fighter, the Royal Aircraft Factory S.E.5, and French SPAD S.XIII were selected. On 27 July 1917, a single DH.4 was sent to the United States as a pattern aircraft. It was not until 1918 that the first American-built DH.4s came off the production line.Bowers, 1966, pp.3, 10 Several different manufacturers, including the Boeing Airplane Corporation, Dayton-Wright Company, the Fisher Body Corporation, and the Standard Aircraft Corporation produced this Americanized variant of the DH.4, featuring over 1,000 modifications from the original British design, to equip the American air services.Bowers, 1966, pp.3–4 A total of 9,500 DH.4s were ordered from American manufacturers, of which 1,885 actually reached France during the war. In American production, the new Liberty engine, which had proved suitable as a DH.4 power plant, was adopted. The Liberty was also eventually adopted by the British to power the DH.9A variant of the type.Jackson, 1987, p.58Bowers, 1966, pp.6–7
After the war, a number of firms, most significant of these being Boeing, were contracted by the U.S. Army to remanufacture surplus DH.4s to the improved DH.4B standard. Internally referred to by Boeing as the Model 16, deliveries of 111 aircraft from this manufacturer took place between March and July 1920; reportedly, roughly 50 of these were returned for further refurbishments three years later.Bowers, 1989, p.67Bowers, 1966, pp.7–8
During 1923, the Army placed an order for a new DH.4 variant from Boeing, distinguished by a fuselage of fabric-covered steel tube in place of the original plywood structure.Bowers, 1966, p.9 These three prototypes were designated DH.4M-1 (M for modernized) and were ordered into production alongside the generally similar DH.4M-2 developed by Atlantic Aircraft. A total of 22 of the 163 DH.4M-1s were converted by the Army into dual-control trainers (DH.4M-1T) and a few more into target tugs (DH.4M-1K). Thirty of the aircraft ordered by the Army were diverted to the Navy for Marine Corps use, these designated O2B-1 for the base model, and O2B-2 for aircraft equipped for night and cross-country flying.Bowers, 1989, p.70
Design
The Airco DH.4 was a conventional tractor two bay biplane of all-wooden construction. It was entirely built of traditional materials. The forward fuselage section and the underside of the tail area was covered by a 3mm plywood skin; this construction led to the fuselage being both strong and lightweight, heavily contributing to cross-bracing only being used for the four bays directly behind the rear cockpit. The nose of the aircraft was considerably longer than necessary, the cowling having been originally designed to accommodate the Beardmore Halford Pullinger (BHP) engine, rather than the Rolls-Royce Eagle that was adopted for production instead.
The DH.4 was powered by a variety of engines, including the Eagle, the BHP, the American Liberty, Royal Aircraft Factory RAF3A, the Siddeley Puma and the Fiat. Regardless of the engine used, it drove a four-bladed propeller mounted upon the nose. Cooling for the engine was provided via an oval-shaped radiator, while a port-mounted exhaust manifold discarded waste emissions above the upper wing. An unusual modification featuring on a small proportion of production DH.4s was the inversion of the engine, a design change that had been implemented in order to better accommodate the relatively-tall Ricardo-Halford-Armstrong (RHA) supercharged engine, which would otherwise unduly obstruct the pilot's forward field of view.
The DH.4 was operated by a crew of two, who were accommodated in widely spaced cockpits, between which the fuel tank was positioned. While the crew arrangement provided good fields of view for both the pilot and observer; however, it had the noticeable downside of causing communication problems between the two crew members, particularly during combat situations, where the speaking tube that linked the two cockpits was of only limited use.Bruce, 1952, p.507Bruce, 1966, pp.3–4 On the majority of American-built aircraft, the pilot's seating and fuel tank arrangement were switched around; aviation author Peter M Bowers credits this change with improving the pilot's safety in the event of a crash, as well as allowing for better communication with the observer.Bowers, 1966, p.7
The DH.4 was armed with a single forward-firing synchronised Vickers machine gun along with either one or two Lewis guns fitted on a Scarff ring fired by the observer. It could carry of bombs, which could be mounted upon external racks. There were several alterations to the aircraft's armament throughout its production life, such as improvement in the ergonomics of the observer's Lewis gun and the installation of an additional Vickers gun. A pair of DH.4s were outfitted with COW 37 mm guns for experimental purposes, but the war came to a close prior to firing trials being conducted. All armaments would typically be removed from those DH.4s that were used by civil operators, including ex-military aircraft that were sold on in great numbers following the end of the Great War.