Design and Development
The DH.9 was designed by de Havilland for the Aircraft Manufacturing Company in 1916 as a successor to the DH.4. It used the wings and tail unit of the DH.4 but had a new fuselage. This enabled the pilot to sit closer to the gunner/observer and away from the engine and fuel tank. The other major change from the DH.4 was the choice of the promising new BHP/Galloway Adriatic engine, which was predicted to produce 300 hp (224 kW) and so give the new aircraft an adequate performance to match enemy fighters.
By this time, as a result of attacks by German bombers on London, the decision was made to almost double the size of the Royal Flying Corps, with most of the new squadrons planned to be equipped with bombers. Based on the performance estimates for the DH.9 (which were expected to surpass those of the DH.4), and the similarity to the DH.4, which meant that it would be easy to convert production over to the new aircraft, massive orders (4,630 aircraft) were placed.
The prototype (a converted DH.4) first flew at Hendon in July 1917. Unfortunately, the BHP engine proved unable to reliably deliver its expected power, with the engine being de-rated to 230 hp (186 kW) in order to improve reliability. This had a drastic effect on the aircraft's performance, especially at high altitude, with it being inferior to that of the DH.4 it was supposed to replace. This meant that the DH.9 would have to fight its way through enemy fighters, which could easily catch the DH.9 where the DH.4 could avoid many of these attacks.
While attempts were made to provide the DH.9 with an adequate engine, with aircraft being fitted with the Siddeley Puma, a lightened and supposedly more powerful version of the BHP, with the Fiat A12 engine and with a 430 hp (321 kW) Napier Lion engine, these were generally unsuccessful (although the Lion engined aircraft did set a World Altitude Record of 30,500 ft (13,900 m) on 2 January 1919) and it required redesign into the DH.9A to transform the aircraft.
Read more about this topic: Airco DH.9
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