Kalinin K-7

The Kalinin K-7 was a heavy experimental aircraft designed and tested in the Soviet Union in the early 1930s. It was of unusual configuration, with twin booms and large underwing pods housing fixed landing gear and machine gun turrets. It was proposed as a heavy bomber/civilian transport. In the passenger version, seats were arranged inside the 2.3-meter thick (7 ft 7 in) wings. The original design called for six engines in the wing leading edge, but when the projected loaded weight was exceeded, two more engines were added to the trailing edges of the wing, one right and one left of the central passenger pod.

The K-7 first flew on 11 August 1933. The very brief first flight showed instability and serious vibration caused by the airframe resonating with the engine frequency. The solution to this was thought to be to shorten and strengthen the tail booms, little being known then about the natural frequencies of structures and their response to vibration. The aircraft completed seven test flights before a crash due to structural failure of one of the tail booms on 21 November 1933.

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