The K-7 was an exceptionally large experimental bomber of 1933, having two faired underslung tandem-wheel landing-gear units and six engines.
Designed by World War I aviator Konstantin Kalinin, the K-7 was one of the biggest aircraft built before the jet age. As originally designed the K-7 was to have engines in the undercarriage sponsons. As completed the bomber had gun positions, the bomb load and an internal staircase as well as two large wheels in each massive sponson.
It was powered by seven engines, six pulling on the wing leading edge and one pushing at the rear. The K-7 was one of the first metal aircraft with a twin-boom layout. The K-7's control surfaces were all deflected by the use of large trim tab surfaces mounted on struts.
The K-7's very brief first flight showed up instability and serious vibration caused by the airframe resonating with the engine frequency. The solution to this 'flutter' 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. On the 11th flight, during a speed test, the port tailboom vibrated, fractured, jammed the elevator and caused the giant aircraft to plough into the ground. The K-7 was said to have had a pilot, 18 crew members and one passenger when it crashed, killing all but five crew.
Undaunted by this disaster, Kalinin's team began construction of two further K-7s in a new factory, but the project was abandoned.
Engine: 7 x M34F, 550kW
Max take-off weight: 38000 kg / 83776 lb
Empty weight: 24400 kg / 53793 lb
Wingspan: 53.0 m / 173 ft 11 in
Length: 28.0 m / 91 ft 10 in
Wing area: 454.0 sq.m / 4886.81 sq ft
Max. speed: 234 km/h / 145 mph
Cruise speed: 180 km/h / 112 mph
Ceiling: 4000 m / 13100 ft
Range: 3030 km / 1883 miles