Heinkel HD 21

The Heinkel HD 21 was a trainer aircraft built in Germany during the 1920s. It was a conventional single-bay biplane with staggered wings braced by N-type interplane struts and a fixed tailskid undercarriage whose main units were linked by a cross-axle. It had three open cockpits in tandem, although one of them was typically faired over, and the related HD 29 of 1925 was essentially similar but a pure two-seater. The Reichswehr flew examples at its clandestine aviation training facility at Lipetsk. One HD 21, registered SE-ACY, was flown as an air ambulance by Carl Gustaf von Rosen for the Red Cross during the Italian invasion of Abyssinia in 1935. A single Mercedes D.I engine of 75 kilowatts powered it, and the crew was a pilot and an instructor.