10/31/2013. Remarks by Johan Visschedijk: "Built as a private venture, the Hippo two-seat fighter featured negative wing stagger, the gap between the wings being completely filled by the deep fuselage, and serialed in the Licensed Series as X11, the first of two prototypes was flown on September 13, 1917. A two-bay biplane powered by a 200 hp Clerget 11Eb eleven-cylinder rotary, the Hippo had an armament of two fixed synchronized 0.303 in (7.7 mm) machine guns and (initially) two free-mounted guns of similar caliber, or (later) one 0.303 -in (7.7 mm) gun on a Scarff mount in the rear cockpit.

Official trials were performed at Martlesham Heath in January 1918, these having been delayed by engine problems. The performance of the Hippo was considered inferior to that of the Bristol F.2B and lateral control was criticized, and, on February 2, 1918, the aircraft was returned to Sopwith. Despite official rejection, the manufacturer fitted new wings, plain ailerons and an enlarged fin. Wing dihedral was increased and stagger was reduced, and with these modifications the Hippo re-emerged in April 1918, with a second prototype (serialed X18) following in June.

Subsequently both aircraft were reserialed as H4420 and H4421 respectively. By that time, the Bristol F.2B was giving satisfaction in service and it became apparent to Sopwith that the Hippo was too late, further development being discontinued. The following data relate to the Hippo in its original form."

Span: 38 ft 9 in (11,81 m)
Length: 24 ft 6 in (7.47 m)
Height: 9 ft 4 in (2.84 m)
Wing area: 340 sq.ft (31.59 sq.m)
Empty weight: 1,481 lb (672 kg)
Loaded weight: 2,590 lb (1,175 kg)
Max speed: 115 mph (185 kmh) at 10,000 ft (3,050 m)
Climb: to 10,000 ft (3,050 m) 13 min 25 sec
Service ceiling: 18,000 ft (5,480 m)