I-15, TsKB-3 by N.N.Polikarpov

I-15 with M-22 engine, 8k drawing from "History of aircraft construction in the USSR", Vol.1 p.486

In May 1932 S.V.Ilyushin (at the time - head of the TsKB) made N.N.Polikarpov as a deputy of P.O.Sukhoj - head of brigade 3, working on advanced I-14 monoplane fighter. S.V.Ilyushin suggested more conservative 'backup' sesquiplane I-14B (with retractable gear) and I-14A (with fixed landing gear). N.N.Polikarpov accepted responsibility for the second one, which gradually evolved into the I-15 design.

Mock-ups of the I-14 (P.O.Sukhoj) and I-14A (N.N.Polikarpov) were presented in November 1932. On December 8 both projects were approved, I-14 as a high speed interceptor and I-15 as a highly maneuverable dogfighter. At the time N.N.Polikarpov got his separate design brigade.

I-15 was a small biplane fighter, development of the I-5 and I-6 with improved airdynamic. The most distinctive features were gulled upper wing and very clean fixed landing gear, often covered with fairing. Roots of the upper wing were included into fuselage structure. Upper wing shape improved pilot view and reduced drag.

Front section of the fuselage from engine to cockpit was covered with duralumin sheets, the rest of aircraft - with fabric. Wheels were equipped with disk brakes. First aircraft was equipped by imported 630/715hp high altitude Wright-Cyclone SGR-1820 F-3 engine. Armament included pair of synchronized PV-1 machineguns and 40kg of bombs (overload). Interestingly, the tail of the first TsKB-3 carried same letters VT as the VT-11 prototype of the I-5.

Flight tests were performed in October-November 1933 by V.P.Chkalov. The second prototype - on skis - was flown in December. During continuous dive one of skis was lost, and aircraft turned over during landing.

Series production of I-15 started.

Early series were powered by Wright-Cyclone engine. Planned production of its license version M-25 was delayed, and during 1934-36 few hundred of I-15 were equipped with M-22. Interestingly, low altitude performance did not suffer despite less powerful engine was used. Since 1936 all production aircraft had M-25 engine, demonstrating improved performance at high altitudes. Production aircraft carried four PV-1 machineguns or (in 1938) pair of 12.7mm BS heavy machineguns.

Stripped version of the I-15 was used in 1935 by V.K.Kokkinaki to set the World altitude record (14,575m). Series of trials were performed in 1937 with pressurized cockpit. They had no value for series I-15 (due to ceiling), but those were first successful tests of pressurized cockpits on Soviet fighters.

Gull-wing caused heated debates among specialists and pilots, despite the I-15 was superior compared with other contemporary fighters. As a result N.N.Polikarpov had to convert design back to conventional wing. This conversion forced production delay in 1936 (only 12 I-15 rolled out). But while large series with straight wing started in 1937 (picture below), extensive airdynamic tests at TsAGI proved advantage of the gull wing at high speeds, and in 1938 I-15 production continued with (modified) gull wing.

Straight-winged I-15 of 1937 series (104k!) from 'Aviatsia i Vremia' ('Aviation and Time') magazine, courtesy of Sergey Andreev

The I-15 was used in combat in Spain and proved to be one of the best fighter biplanes of its time. Performance was high enough to force pilots of early Bf-109 to avoid close combat with more agile and well-armed 'Chato'. But biplane era was already fading, and in 1939 during clashes with Japanese I-15 was already obsolete.

I-15 deserves special honor from American military pilots. It was flown by ace of Spanish War Frank G. Tinker, who became the first American to shot down Messerschmitt Bf-109...

674 (?) built.

Predecessors		Modifications
I-5	I-6	I-152 I-15bis	I-153 I-15ter