The Imperial Japanese Army became interested during the late 1935 in autogyro developments taking place in the USA and, believing that such an aircraft might be developed for use as an artillery spotter, imported from America in 1939 a Kellett KD-1A. However, soon after this aircraft had arrived in Japan it was irreparably damaged during a flight test and the army arranged for the wreck to be transferred to the Kayaba Industrial Company, which had been carrying out research into the autogyro configuration. Shortly afterwards Kayaba was requested to proceed with design and development of a two-seat autogyro based on the Kellett. The resulting Kayaba Ka-1 prototype comprised a fuselage with two separate open cockpits in tandem, a tail unit incorporating a tailplane with twin inverted fins, fixed tailwheel landing gear, and an Argus As 10c engine mounted conventionally in the nose of the fuselage to drive a two-blade tractor propeller; the pylon for the three-blade unpowered rotor was incorporated in the fuselage structure, mounted just forward of the front cockpit.
First flown on 26 May 1941, the Ka-1 proved successful in early flight testing, and the type was ordered into production to serve in the originally intended role as a spotter-plane for artillery units. At the same time Japanese shipping losses were beginning to rise and it was suggested that the very short takeoff required by such aircraft would make them suitable for operation from the light escort carrier Akitsu Maru. A few production Ka-1s were modified for this role and equipped to carry two 132-lb (60-kg) depth charges, but because of their limited payload capability these had to be flown as single-seaters. Operating for some time off Japanese coastal waters they were the world's first operational armed rotary-wing aircraft, with production of both versions totalling approximately 240. This number included one Ka-1 KAI which was tested with rockets attached to the rotor tips in an attempt to improve payload capability, and a single aircraft that was evaluated with a 240-hp (179-kW) Jacobs L-4MA-7 7-cylinder radial engine, allocated the designation Ka-2.
Technical data for Kayaba Ka-1
Length: 9.2 m (30 ft 2⅛ in)
Rotor diameter: 12.2 m (40 ft 0¼in)
Disc area: 117 m² (10.9 ft²)
Empty weight: 775 kg (1,709 lb)
Max takeoff weight: 1,170 kg (2,574 lb)
Powerplant: 1× Argus As 10c air-cooled inverted V8 engine, 180 kW (241 hp)
Maximum speed: 165 km/h (89 knots, 102 mph)
Cruise speed: 115 km/h (62 knots,71 mph)
Range: 280 km (151 nm, 174 mi)
Service ceiling: 3,500 m (11,500 ft)
Rate of climb: 5 m/s (980 ft/min)
1x 60 kg (132 lb) depth charges