The Polikarpov I-6 was a biplane of wooden construction designed to compete against the metal framed I-5, but that never progressed beyond the prototype stage. Both aircraft were ordered as part of the experimental aircraft section of the 1928 Five Year Plan. Both aircraft were to use the same imported Bristol Jupiter radial engine.
Work on the I-6 began in September 1928, with a deadline of 1 August 1929. The fuselage was constructed by gluing thin layers of 'shpon' (veneered wooden sheets) as were the wings and struts. The empty weight of the I-6 was 38% lower than that of Polikarpov's earlier I-3, partly because the 480hp Bristol Jupiter VI engine didn’t need the coolant or radiators of the liquid cooled BMW VI engine used in the earlier aircraft.
The original timetable for both the I-5 and I-6 proved to be over-ambitious, but rather that admit this the Soviet authorities responded by arresting large numbers of aircraft designers and engineers. Polikarpov was arrested in September 1929, charged with industrial sabotage and sentenced to death. This was soon commuted to ten years imprisonment, and he formed part of the Vnutrennaya Tyurma, or House Arrest design team, where he eventually took over development of the I-5.
Much of Polikarpov's team shared his fate, leaving the survivors to continue work on the I-6. The first prototype finally made its maiden flight on 30 March 1930 (two days after the wooden mock-up of the I-5 had received official approval). A second prototype soon followed. Both aircraft were originally powered by imported Jupiter VI engines, but they were soon given the Soviet licence-built version, the M-22.
The I-6 had a similar top speed, climb rate and general manoeuvrability as the I-5, but its rate of turn was slower and its structure was seen as somewhat weaker. This probably played a part in the decision to put the I-5 into production, as did the crash of one prototype on 13 June 1930.
Engine: Bristol Jupiter VI then Soviet M-22
Power: 480hp (both)
Wing span: 31.8ft (upper), 24.6ft (lower)
Empty Weight: 1,914lb
Loaded Weight: 2,822lb
Max Speed: 174mph
Service Ceiling: 24,600ft
Range: 435 miles
Armament: Two 7.62mm machine guns (planned)