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The Aichi Experimental Three-seat Reconnaissance Seaplane (HD-28) was the third of three Heinkel designs imported into Japan in 1926, and was tested for use as a long range reconnaissance aircraft.
Like many Japanese companies when Aichi decided to produce aircraft it began by creating a link with a European company, in this case Heinkel. The HD-25 and HD-26 were both designed as ship-borne reconnaissance aircraft. The HD-25 was produced in small numbers, but the HD-26 never progressed beyond testing.
The HD-28 was intended to serve as a long range reconnaissance aircraft. It was a single-span biplane, with a mixed wood and metal structure and a fabric covering. The fuselage was constructed around a frame of welded steel tubes, while the wings were wire-braced. The wings could be folded back for storage. The crew of three sat in tandem open cockpits.
The imported HD-28 was something of a disappointment. The Lorraine-Dietrich engine caused problems, and visibility was poor (a major failing in a reconnaissance aircraft). The Japanese Navy withdrew its interest in the design in 1928, but Aichi continued to modify the aircraft. First they installed a 450hp Nakajima Jupiter engine, compensating for the loss of power by making the aircraft smaller. The fuel tank was moved from the fuselage to the upper wing and the three seats were moved forward. These modifications didn't have the required effect, and a second set of changes were made. This time the wings were staggered and the lower wing was reduced in span, but again the changes were ineffective. Work on the HD-28 ended in 1929.
Although the HD-28 wasn't a success, Aichi learnt valuable lessons from its design, and the basic construction method was used on the AB-5 and AB-6.
Engine: Lorraine-Dietrich 18-III eighteen cylinder W-type water-cooled engine
Power: 650-710hp
Crew: 3
Span: 49ft 2.5in
Length: 35ft 11in
Height: 13ft 8.25in
Empty weight: 5,214lb
Loaded weight: 8,488lb
Max speed: 125mph at sea level
Cruising speed: 93mph
Climb Rate: 19min 24sec to 9,843ft
Service ceiling: 14,763ft
Armament: Two forward firing 7.7mm machine guns, one flexibly mounted rear firing 7.7mm machine gun
Bomb load: Two 242lb bombs if same weight of fuel was removed