Stal-8, R.L.Bartini

68k drawing Stal-6/8 from 'Krylia Rodiny' magazine, courtesy of Sergey Andreev

The Stal-8 was a more practical development of the Stal-6. It was slightly larger, main construction material was an aluminum-magnesium alloy (altmag). All joints were performed by point wielding or wheel-wielding. Cooling system, landing gear was same as on Stal-6, but cockpit was conventional and two 7.62mm guns were installed in the engine cowling.

Not completed for several reasons:
• 'Aeroflot' (GVF) was not happy to be bothered by military aircraft, not serving its departmental needs.
• GUAP did not care much about the project because it was responsibility of the 'Aeroflot'.
• New fighters (I-15 and I-16) already were in production, and more vulnerable (despite superior in all other respects) Stal-8 was not a priority any more.

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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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References:
• "History of aircraft construction in the USSR", Vol.1 p.499-500;

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