The Aichi Experimental Type-H Carrier Fighter (HD-23) was the first single-seat fighter designed by Heinkel, and was a generally unsuccessful design that failed to win a production contract in Japan.
In April 1926 the Japanese navy issued a specification for a new fighter to replace the Mitsubishi Type 10 Carrier Fighter (the first purpose-built carrier fighter in the world). Aicho approached their German partner Heinkel and asked them to design and built a single-seat fighter. This was the first aircraft of that type to be built by Heinkel and the new design wasn't a great success.
The HD-23 was a single bay biplane, with a wooden structure, mixed fabric and plywood covering and a metal frame for the tail. Heinkel tested it as both a landplane and a float plane, but the landplane was the version of interest in Japan.
One of the requirements of the Navy specification was that the new fighter should be easy to ditch. Mitsubishi and Aichi both made quite elaborate arrangements to satisfy this requirement, but the victorious Nakajima design ignored it. The HD-23 had a jettisonable undercarriage, and the propeller could be stopped in the horizontal position. The underside of the fuselage was shaped like a boat hull, while the fuselage and wing leaded edges were made watertight.
The two prototypes reached Japan in the summer of 1927. One was powered by a BMW VIA engine and the other by a Hispano-Suiza engine. Tests on both aircraft began in December 1927 and the aircraft proved to be disappointing. It was considered to be too heavy, had limited manoeuvrability and was unstable when landings, a critical flaw in a carrier fighter. The fighter contest was won by the Nakajima entry, a modified Gloster Gambet, which entered service as the Type 3 Carrier Fighter.

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