|Function||Fighter prototype (3rd)|
|Weights and loads|
|Wing Load (kg/m2)||173|
|Power load (kg/hp)||2.5|
(later 1*23mm BT-23)
Concept of providing a fighter with maximum diving speed arose for a short time among Soviet aviation experts. It was caused by serious limitations of service fighters, caused by wing mechanical characteristics. In several occasions, fighter's wing simply collapsed during dive. M.M.Pashinin, at the time (ex-?)deputy of N.N.Polikarpov) was assigned to study the problem and design fighter with dive speed up to 950km/h.
M.M.Pashinin chose standard low-wing layout with special symmetric wing cross-section (NACA-0012-0009 on the outer sections). Engine exhaust reactive force also had to boost the speed. Project (factory designation IP-21) started in January 1940 and aircraft was built during Spring same year. It was a mixed-construction aircraft. Engine was mounted on the steel tube frame, while the fuselage was a plywood monocoque on pine frame. Wing had a duralumin ribs/spars and plywood skin. Control links - solid rods. It was planned to install M-107 engine, but this one was not ready on schedule.
On July 11 (May 18??), 1940 pilot P.F.Fokin took the I-21 into its first flight. On August 18 first I-21 was presented at Tushino Aviation Parade. Soon it was crashed during factory trials and not restored. The second prototype with tapered outer wing panels entered flight test program in October 1940. State trials finished in December, high landing speed and insufficient stability listed as a major disadvantages.
Suggested improvements were incorporated into third prototype, built in January 1941 and flown first time on April 5. Wing was redesigned: outer panels were clipped by 78cm each and swept back, tailplane also was modified. Armament was increased by replacing 20mm engine-mount cannon with 23mm one. This variant had better performance and good handling - except landing, which still was too complicated and demanded too long runways. I-21 was rejected mostly because it failed to demonstrate substantial advantages over competitors already in production: Yak-1, LaGG-3, MiG-3.
Pre-series assembly of five aircraft was discontinued.