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The DH.4 was designed by Geoffrey de Havilland as a light two-seat combat aircraft, intended to perform both aerial reconnaissance and day bomber missions.
An early feature of the design was the intention for it to be powered by the newly-developed Beardmore Halford Pullinger (BHP) engine, capable of generating up to 160 hp.
According to aviation author J.M Bruce, the DH.4 was developed in parallel to the rival Bristol Fighter, developed and manufactured by the Bristol Aeroplane Company.
During August 1916, the prototype DH.4 conducted the type’s maiden flight, powered by a prototype BHP engine rated at 230 hp (170 kW).
Initial flight tests with the first prototype revealed it to have favorable handling and performance.
The Central Flying School (CFS) conducted early evaluation flights using the prototype, leading to it producing a favorable report on the aircraft, observing its high 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, unmatched by any of its predecessors.
While flying trials with the prototype had been producing promising results, it soon became recognized that the BHP engine would have required a major redesign prior to the unit entering production.
Even by the time of flying trials with the first prototype, there had been no finalized plans for quantity production of the BHP engine.
Coincidentally, another suitable and promising aero engine, 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 aiding 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 favorable 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.
During late 1916, the first order for 50 DH.4s, powered by 250 hp (186 kW) 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 synchronized 0.303 in (7.7 mm) Vickers machine gun for the pilot, while the observer was provided with a 0.303 in (7.7 mm) Lewis gun mounted upon a Scarff ring.
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 Autocar, Vulcan Motor and Engineering, and the Westland Aircraft Work.
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).
Overseas, SABCA of Belgium produced a further 15 DH.4s during 1926.
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, settling on the 375 hp (280 kW) 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 (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 of which were used to power, which encountered varying degrees of success, to production aircraft.
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.
(Eagle VIII engine)
Length: 30 ft 8 in (9.35 m)
Wingspan: 43 ft 4 in (13.21 m)
Height: 11 ft 0 in (3.35 m)
Wing area: 434 sq ft (40.3 m2)
Empty weight: 2,387 lb (1,083 kg)
Gross weight: 3,472 lb (1,575 kg)
Power plant: 1 × Rolls-Royce Eagle VIII water-cooled V12 engine, 375 hp (280 kW)
Maximum speed: 143 mph (230 km/h, 124 kn) at sea level
Endurance: 3 hr 45 min
Service ceiling: 22,000 ft (6,700 m)
Time to altitude: 9 min to 10,000 ft (3,000 m)
Guns: Forward-firing .303 in (7.7 mm) Vickers machine gun, Lewis gun on Scarff ring at rear
Bombs: 460 lb (210 kg) of bombs
Airco DH.1, Windsock Data File Special 1- J. M. Bruce.
Airco DH.2, Windsock Data File 48-B. J. Gray.
Airco DH.5, Windsock Data File 50- J. M. Bruce.
Airco DH.9, Windsock Data File 72- J. M. Bruce.
Airco DH.10, Windsock Data File 38- J. M. Bruce.
AIRCO: The Aircraft Manufacturing Company-Mick Davis.
The Complete book of fighters-William Green, Gordon Swanborough.