The Sopwith 3F.2 Hippo was an unsuccessful design for a two seat fighter that only reached the prototype stage.
The Hippo was a two bay biplane, with the upper wing level with the top of the fuselage and backwards stagger, with the lower wing in front of the top wing. The pilot was seated just in front of the upper wing, and the observer within the wing, just behind the rear wing spar. There were large cut-outs in the upper wing to improve the observer’s view, and that was also the reason for the unusual backwards stagger, as it gave the observer a better view down. However this arrangement did result in an unusually large gap between the two crewmen, which made communications between them difficult. The pilot’s view was somewhat obscured by the large cowling for the engine and the twin forward firing Vickers guns.
The first company drawings of the Hippo were approved on 30 April 1917. This version had plain ailerons and more rounded vertical tail surfaces than seen on the prototype. The type was to be powered by a new 200hp Clerget 11Eb eleven cylinder rotary engine, and the first example reached Sopwith in September 1917. The prototype was built as a private venture, and the Hippo was probably meant to replace the Sopwith 1 ½ Strutter in French production. As originally built the prototype had a curved rudder, but a rectangular vertical fin.
This first engine was initially installed in the similar Sopwith 2FR.2 Bulldog, but in November it was moved to the Hippo and the prototype went through builder’s trials at Brookland. This single engine appears to have been moved between the two aircraft several times, as the new Clerget 11Eb was scarce and demand was high.
In December 1917 a new set of wings with balanced ailerons was installed.
In January 1918 the Hippo underwent trials at Martlesham Heath, with the number X11. The aircraft was judged to be very slow and heavy on lateral control, the pilot’s view poor and the pilot and observer too far apart. However the rudder and elevator controls were described as being fairly light. This ended any official British interest in the type.
The Hippo was returned to Sopwith, where more work was carried out. It was given a Scarff ring carrying a single Lewis gun in the observer’s position, and a new landing gear with streamlined steel tubes replaced the original wooden version. The backstagger between the wings was reduced to 1ft 9 3/8in dihedral was increased to 5 degrees. At the same time a new vertical fin with a curved top was added, which merged smoothly into the rudder
In June 1918 a Hippo numbered X18 was flying, although it isn’t clear if this was the modified prototype or a second aircraft. Late in 1918 Sopwith produced performance figures for a Hippo with reduced wingspan and a top speed of 119mph at sea level, but again it isn’t clear which aircraft this refers to.
Stats for tested version
Engine: Clerget 11Eb
Span: 38ft 9in
Empty Weight: 1,481lb
Maximum Weight: 2,590lb
Maximum Speed: 115.5mph at 10,000ft, 101mph at 15,000ft
Climb rate: 13m 25s to 10,000ft
Guns: Two fixed forward firing Vickers guns, two flexibly mounted machine guns for observer