Tupolev ANT-21

Tupolev ANT-21

Company type	Prototype aircraft
Industry	Aerospace
Founded	1933
Founder	Andrei Tupolev, Alexander Arkhangelsky (chief designer)
Headquarters	Soviet Union
Products	Twin-engine heavy fighter aircraft

The Tupolev ANT-21 was a Soviet-designed, twin-engine heavy fighter accommodating four crew members, also known by the designation MI-3 (Mnogomestnyi Istrebitel, meaning multi-seat fighter). Only two prototypes were constructed, as it did not proceed to mass production.

Design and development

In early 1932, the Soviet Air Forces commissioned the TsAGI design bureau under the leadership of Andrei Tupolev to develop a twin-engine multi-seat fighter intended to succeed the KR-6 escort fighter variant of the Tupolev ANT-7, a versatile twin-engine aircraft. Tupolev delegated this project to a team headed by Alexander Arkhangelsky, marking his initial role as chief designer. The resulting ANT-21 resembled the ANT-7 in being a monoplane featuring corrugated all-metal wings derived from the ANT-7’s design but with a reduced wingspan and fabric covering over the corrugations to lower aerodynamic drag. The fuselage was newly crafted with an oval cross-section, utilizing a semi-monocoque structure partially flush-riveted and constructed from duralumin—a pioneering approach for Tupolev aircraft—and equipped with a twin-tail configuration. Additionally, it incorporated retractable tailwheel landing gear, an innovative feature for that period.

The pilot’s position was an open cockpit situated above the wing’s leading edge. The crew included a nose gunner operating two machine guns, a dorsal gunner manning another pair of machine guns, and a ventral gunner firing through a hatch below. The pilot controlled two fixed machine guns. Propulsion was provided by two Mikulin M-17 engines, identical to those powering the ANT-7.

The first prototype, designated MI-3 and nicknamed Mitrich, completed its maiden flight in August 1933. Initial flight trials were favorable, with test pilots expressing approval; however, during high-speed dives approaching 400 km/h (250 mph), significant tail flutter occurred, causing the starboard rudder to detach and forcing a heavy landing.

Following this incident, Arkhangelsky undertook a redesign that retained the original wings but introduced a new fuselage and tail assembly featuring a single vertical stabilizer. The crew compartments were enclosed; the nose gunner operated either a single 20mm cannon or a heavy machine gun, while the ventral gun position was eliminated. The dorsal gunner’s armament and pilot-operated fixed guns remained unchanged. The original M-17 engines were replaced by more powerful and contemporary M-34N engines. This revised version, known as ANT-21bis or MI-3D (Doubler, meaning understudy), was finalized in April 1934. Despite modifications to the tail design, vibrations persisted at specific power settings, necessitating the addition of bracing struts for stabilization. Official testing occurred between July and December 1934; however, the Soviet Air Force declined to adopt the aircraft as it shifted focus toward fighters equipped with heavy recoilless rifles, favoring instead the related Tupolev ANT-29 model.

References

• Duffy, Paul and Andrei Kandalov. Tupolev: The Man and His Aircraft. Shrewsbury, UK: Airlife Publishing, 1996. ISBN 1-85310-728-X.
• Green, William and Gordon Swanborough. The Complete Fighters Book. New York: Smithmark, 1994. ISBN 0-8317-3939-8.
• Gunston, Bill. The Osprey Encyclopedia of Russian Aircraft 1875–1995. London: Osprey, 1995. ISBN 1-85532-405-9.