Heinkel He 51 Evaluated by the Soviet Union
The Heinkel He 51 was one of the first modern fighters fielded by the newly formed Luftwaffe, but it also became an important subject of Soviet evaluation during the Spanish Civil War. Several German aircraft used by the Condor Legion were either captured or recovered by Republican forces, and at least one He 51 was eventually transported to the Soviet Union for detailed examination and testing.
In 1937–1938, Soviet specialists at the Air Force Scientific Research Institute (NII VVS) studied captured German aircraft obtained from Spain, including a He 51B fighter. The aircraft was repaired and test flown in the USSR, where it received the Soviet evaluation designation “I-25.” Soviet pilots assessed its handling, performance, and combat capabilities, while engineers examined its construction and equipment.
The tests confirmed combat experience already gained by Soviet volunteer pilots in Spain. Although the He 51 possessed good maneuverability and stable handling, its maximum speed and climb performance were inferior to contemporary Soviet fighters such as the Polikarpov I-15 and I-16. Soviet reports concluded that while the aircraft could still be dangerous in defensive combat or surprise attacks, it lacked the performance needed to maintain air superiority.
