The Yokosuka K4Y1 Type 90 Seaplane Trainer was produced to replace the Yokosho K1Y Type 13 Seaplane Trainer, and was the first Japanese production aircraft to use a welded steel tube fuselage.
Work on the aircraft began in 1930, with Lt-Cdr Jiro Saha and Engineer Tamefumi Suzuki in charge. Saha was responsible for designing the welded steel tube fuselage, although the wings retained the usual wooden framework. Both fuselage and wings were fabric covered. The wings were staggered, with the upper wing just in front of the forward cockpit and the lower wing below the two cockpits. The aircraft had all-metal twin floats, and no rear float.
Originally the aircraft was powered by a 90hp Hatakaze inverted four-cylinder inline air-cooled engine, but the Japanese Navy tended to prefer radial engines and production aircraft used the 130hp Gasuden Jimpu seven-cylinder air-cooled radial engine.
Two prototypes were built in 1930. It was then accepted for service. The Jimpu powered version was accepted as the standard production version in May 1933 as the Type 90 Seaplane Trainer K4Y1. It replaced the Type 13 Seaplane Trainer, but not the Yokosho K2Y Type 3 Land-based trainer. The K4Y could be used as a land plane, but none of the production aircraft were used in that configuration.
As was often the case with Yokosuka aircraft, most were produced elsewhere. The first two prototypes were produced at Yokosuka. Production then began at Watanabe, where 156 were built in 1932-39. It then switched to Nippi, where the last 53 were built in 1939-40. The K4Y was the main primary floatplane trainer used by the Japanese Navy throughout the Pacific War.
Engine: Gasuden Jimpu seven-cylinder air-cooled radial engine
Span: 35ft 9in
Length: 29ft 8.5in
Height: 11ft 6in
Empty weight: 1,631lb
Loaded weight: 2,182lb
Max speed: 101mph at sea level
Climb Rate: 29min 20sec to 9,843ft
Service ceiling: 11,350ft
Endurance: 3.5 hours
Range: 196 miles