Heinkel He 51

The Heinkel He 51 was a single-engine biplane fighter made in the 1930s. Operationally tested during the Spanish Civil War as equipment for the Condor Legion, it continued to be used, mainly by the Luftwaffe, before and during World War II. Equipped with a 690-horsepower BMW VI engine disguised as a civilian aircraft, like many other German aircraft made at the time, to circumvent the bans following Germany's defeat in World War I. Source wikipedia