I-21 (TsKB-32) fighter by S.V.Ilyushin

Experimental single-seat compact all-metal fighter with evaporative engine cooling. The wing surface (riveted and soldered stainless steel) was used for water vapor cooling and condensation. The M-34RNF engine was specially modified for this aircraft. Gearbox was installed to boost the rate from 1800rpm to 2400rpm and carter was cast with rectangular "hole" for the wing spar.

The main landing gear with very short legs was retractable into the centroplane. Fuselage and tail were made of duralumin. Fuselage cross-section was very small, almost equal to one of the engine.Despite small size, I-21 had to carry unusually heavy armament for its time - two 20mm cannons.

Cockpit was placed well aft, behind the wing trailing edge. The wing had a very broad chord, with a strongly swept leading edge (looks like S.V.Ilyushin was on the right path towards the swept-wing high-speed aircraft!). RAF-38 airfoil (not perfect selection for a fighter) was chosen for the tapered wing, but it was not the cause of I-21 failure.

Two experimental I-21s were built in the end of 1936. Even before the factory trials, assembly of 20 pre-series machines was launched.

But trials (test-pilot V.K.Kokkinaki) revealed number of problems. The engine and wing were overheating, as well as engine oil. V.K.Kokkinaki complained that he can not fly a SAMOVAR (Russian authentic water-boiler for tea party). Aircraft weight exceeded the design limits due to poor assembly quality. Takeoff and landing runs were too long. Gliding was found way too steep.

S.V.Ilyushin decided to put factory tests on hold after only three flights. Efforts were concentrated on three major "fronts":

• improvement of a cooling system
• engine development (resulted in the birth of AM-35)
• gun mounts - later used on TsKB-55 and Il-2

In May 1939 was dropped from the experimental development plans. All works were cancelled.

There was a widely used argument against evaporative cooling systems - vulnerability. But... this is a common feature of water-cooled engines (except special cases with armor-shielded cooler like Il-2). Large area of cooling surface is not a weakness - higher probability of combat damage is compensated by relatively lower impact of a hole on the system.

During trials of Stal-6 A.B.Yumashev performed few flights with imitation of a cooler combat damage. Each flight lasted more than 30min! Taking into account that flight endurance of (most) Soviet WW-II single-engine fighters was close to 1hour, damaged fighter could not only return home safely, but do it after the fight is over.

There also was a room for improvement - Stal-6 had single-unit cooler, while segmented system could be even less sensitive to damage.

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Created January 25, 1996 Modified November 25, 2000

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