The Heinkel HD 21 was a trainer built in Germany during the 1920s. It was a conventional single-bay biplane with staggered wings braced with N-type interplane struts. The main units of the fixed tailskid undercarriage were linked by a cross-axle and the aircraft had three open cockpits in tandem although one of these was typically faired over. The HD 29 of 1925 was essentially similar but was purely a two-seater.
Made by Heinkel.