. "The Heinkel HD 36 was a trainer developed in Germany in the 1920s at the request of the Swedish Air Force, which was in search of a new trainer aircraft. The newly formed air force had previously evaluated the HD 35, found it to be underpowered, and asked Heinkel to address this problem. Heinkel's response was a development of the HD 35 modified to use the Mercedes D.III engine instead."@en . . . .