The Ilyushin I-21, (Istrebitel-21, fighter-21), also known as TsKB-32, was a single-engined, single-seat fighter produced in the USSR in 1936-7 in response to a government specification.
Two specially modified Mikulin AM-34RNF (also written AM-34FRN) liquid-cooled V-12 engines provided the power via a non-reducing gearbox, which was used only to raise the thrust line, and enable the use of shorter undercarriage legs.
[1][2] Cooling for the first prototype's engine used an evaporative system which dispensed with radiators and used condensers built into the surface of the wing centre section.
The second prototype was fitted with a conventional cooling system using ethylene glycol coolant and a retractable radiator.
Flight trials started but soon showed that the evaporative cooling system was inadequate with both the engine and the wing centre section overheating.