Development of the I-6 (
Istrebitel—fighter) began in September 1928 with a deadline for delivery for the first prototype of 1 August 1929 after the first prototypes of the
Polikarpov I-3 were completed. Although the new fighter shared many of the characteristics of the earlier design, including the staggered sesquiplane, single-bay, layout of the wings, it was a new design which used a nine-cylinder, single-row, air-cooled
Bristol Jupiter radial engine rather the water-cooled
inline engine of its predecessor. It was designed by the OSS (—Landplane Department), later redesignated as OPO-1 (—Experimental Department) of
Aviatrest ("Aviation Trust") under the supervision of Nikolai Polikarpov, head designer of the department. It was originally intended to be compared to the I-3, but this was changed to an evaluation of construction methods with the wooden construction I-6 compared to the mixed construction Polikarpov I-5. Both aircraft used the Jupiter VI engine for which a license had recently been negotiated. The I-6 had an oval-section semi-
monocoque fuselage covered with 'shpon', molded birch plywood, with a small headrest faired into the fuselage, although the engine was enclosed in a metal cowling that left the cylinder heads exposed for better cooling. The two-
spar wings were covered in plywood and fabric and had a
Clark Y profile. Internal bracing wires were fitted to reinforce the wings. The control surfaces were framed in
duralumin, but covered in fabric. The duralumin
N-type struts that separated the wings, and attached the upper wing to the fuselage, had a teardrop profile. They were reinforced with steel bracing wires. The
conventional undercarriage was fixed with rubber shock absorbers. The wooden
propeller was given a spinner. The lighter weight of the air-cooled Jupiter engine, which required neither a heavy radiator nor coolant, meant that the I-6 had an empty weight only 62% of that of the I-3. and this delayed the first flight of the I-6 until 30 March 1930. The second prototype was completed shortly thereafter and both aircraft appeared in that year's
May Day fly-past over
Moscow. Both aircraft likely used imported engines before they were replaced by the Soviet-built copy of the Jupiter, the Shvetsov M-22. One I-6 crashed on 13 June 1930 after the test pilot bailed out, without justification, in the opinion of the Soviet aviation historian Vadim B. Shavrov. ==Specifications==