Ilyushin I-21

Ilyushin I-21

Company type	Fighter aircraft prototype
Industry	Aerospace
Founded	1936
Headquarters	Soviet Union

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The Ilyushin I-21, also referred to as Istrebitel-21 (fighter-21) and designated TsKB-32, was a single-seat, single-engine fighter developed in the Soviet Union during 1936–1937 in response to a governmental requirement.

Design and development

Constructed entirely from metal, the I-21 featured a low-wing cantilever monoplane design equipped with retractable landing gear and an enclosed cockpit. Its wings exhibited a pronounced taper with straight trailing edges, resulting in a swept leading edge. Propulsion was provided by two specially adapted Mikulin AM-34RNF (also known as AM-34FRN) liquid-cooled V-12 engines, which were connected through a non-reducing gearbox. This gearbox served solely to elevate the thrust line, allowing for shorter landing gear legs.

The initial prototype employed an evaporative cooling system that eliminated traditional radiators, instead integrating condensers into the wing center section's surface. In contrast, the second prototype utilized a conventional cooling arrangement with ethylene glycol coolant and a retractable radiator. Flight testing commenced but quickly revealed that the evaporative system was insufficient, causing overheating issues in both the engine and wing center section. Although Ilyushin recognized the system’s susceptibility to combat damage, he was compelled by GUAP directives to proceed with this cooling method. Development ceased in 1939, and the I-21 designation was later reassigned to the Pashinin I-21.

Variants

Information sourced from: OKB Ilyushin

TsKB 32

(I-21 initial prototype) featuring an evaporative cooling system

TsKB 52

(I-21 second prototype) equipped with a standard ethylene glycol cooling mechanism, project halted due to engine supply issues.