Armstrong Whitworth F.K.6

The Armstrong Whitworth F.K.5 and F.K.6 were experimental intended to serve as escort fighters built by Armstrong Whitworth during the First World War.

Early in 1916 the British war office drew up an specification for a multi-seat escort fighter to be powered by the new Rolls-Royce Eagle engine. It had to have at least 7 endurance hours and it had to protect the bomber formations from German fighters such as the Fokker E.I and it had to be capable also of destroying enemy airships. The orders for prototypes were placed from Armstrong Whitworth, Sopwith and Vickers, and all of them presented unorthodox designs given the need to give the gunners a good field of fire, specially considering the lack of syncronisation gear.

Frederick Koolhoven originally designed the F.K.5 as a tractor triplane with the middle wing having much greater span than the upper and lower ones. Gunners were mounted in nacelles placed on top of the middle wing making them to sit ahead of the propeller blade and the pilot cockpit placed behind the wings having that way a poor view. Undercarriage consisted in a sprung stut with two main wheels under the engine with two stabilishing ones at the wingtips of the lower wings with a tail slid aft of the trailing edge of the lower wing. This design never flew because the head of Armstrong Whitworth aircraft department, had forbidden the test flights.

However, Koolhoven radically redesigned the F.K.5 in order to design the F.K.6. It still was a triplane with the middle wing having much greater span than the upper and lower ones. Gunner nacelles were slung under the middle wing and were significally shorter so the gunners sat behind and outboard the propeller. Fuselage was also deeper than the F.K.5 one, giving that way an slighlty better view from the cockpit and the undercarriage had two pair of wheels with a narrow track under the fuselage and a more conventional tail skid.
It was powered by a 250hp Rolls-Royce 12-cylinder water-cooled engine and would have carried two 0.303in Vickers machine guns, one in each nacelle.

Four airplanes were ordered in April 1916, two of which were to enter service with the Royal Naval Air Service, but only one was built as it demonstrated poor performance after flight tests. As effective synchronisation gears were already available by then, this type of airplane was abandoned and none of them were brought into production.

Sources:
1. https://en.wikipedia.org/wiki/Armstrong_Whitworth_F.K.6
2. Salamander Books - The Complete Book of Fighters.