The Kawasaki Ki-60 was a single-engine heavy interceptor powered by the German DB 601A inline engine that reached the prototype stage during 1941 but that was rejected in favour of the lighter Kawasaki Ki-61 Hien.
In 1938 Kawasaki gained the right to build and develop their own version of the Daimler Benz DB 601A, one of the best inline aircraft engines of the period. In April 1940 a Kawasaki team visited Daimler-Benz in Stuttgart, and returned to Japan with the blue-prints for the DB 601 and a number of completed engines. Work then began on Kawasaki's own version of the engine. The result was the Ha-40, with an improved take-off power of 1,175hp and a slight reduction in weight. The first Ha-40 was completed in July 1941 and it passed its ground tests by November, entering production as the 1,100hp Army Type 2 Engine. Kawasaki had a long record of co-operation with German engineers, and their design team had been led by Dr Richard Vogt for ten years, ending in 1933. He had used his influence to help Kawasaki in the negotiations with Daimler Benz, and the Kawasaki design team, headed by Takeo Doi, was still influenced by him. This may have explained why Kawasaki continued to work with inline engines at a time when most Japanese aircraft manufacturers moved onto radial engines.
While work on the new engine was going on the Air Headquarters of the Imperial Japanese Army was watching the progress of the air war over Europe, where the Spitfire, Hurricane and Bf 109 dominated the fighting. All three aircraft were high-performance aircraft with inline engines, some armour, and heavy armament, very different to the lightly armed, un-armoured, radial powered and slower but manoeuvrable fighters in Japanese service. In February 1940 the Japanese Army initiated the design of three aircraft - the Kawasaki Ki-61 light fighter, the Nakajima Ki-62 and the Kawasaki Ki-60 heavy interceptor, each to be powered by the Ha-40.
Previous Japanese fighter designs had focused on manoeuvrability rather than speed. This required a relatively low wing loading (weight divided by size of wing). The Ki-60 had a wing loading of 35.4lb/ square foot, nearly twice as high as on the Ki-43-Ia, which many Japanese pilots considered to be too heavily loaded. Generally the smaller the wing the higher the wing loading, the high the speed of the aircraft but the worse its manoeuvrability and the longer it's landing and take-off distances. On the Ki-60 speed, climb-rate and armament were seen as key, and the first aircraft were to be designed by two 20mm cannon and two 12.7mm machine guns. The pilot and fuel tanks received armour protection.
The first of the three prototypes, Ki-6001, made its maiden flight in March 1941. It had small wings, and was armed with two 20mm cannon and two 12.7mm machine guns. Speed was disappointing, at 342mph, down from the predicted speed of 373mph, and Japanese army pilots didn't like the high wing loading and resulting loss of manoeuvrability.
The second prototype, Ki-6002, had larger wings and a refined engine cowling. Its top speed increased to 348mph and manoeuvrability increased, but not by enough to impress the army pilots.
The third and final prototype, Ki-6003, retained the larger wings and had another new cowling. Weight was saved by replacing the 20mm cannon with two more machine guns, and speed increased to 354mph.
The Ki-60 was technically a success, but it didn't match the expectations of the Japanese pilots. In the skies over Europe high speed fighters were proving to be superior to slower but more manoeuvrable aircraft as long as suitable tactics were used. The faster aircraft were well suited to 'hit and run' tactics rather than to the more familiar dog-fighting techniques. Both the Japanese army and navy preferred the lighter, more manoeuvrable aircraft, and the early days of the Pacific war appeared to have proved their point, as the Mitsubishi Zero in particular gained a fearsome reputation. These early successes would turn out to have been misleading, having been gained against comparatively small numbers of second-string (at best) Allied aircraft. The lighter Japanese fighters would soon be outclassed by heavier, faster but less manoeuvrable Allied aircraft which had more in common with the Ki-60.
By the end of 1941 work on the Ki-60 had ended, and the Ki-61 was given high priority. The Ki-61 was not actually much lighter than the Ki-60. It was a rather more elegant design, with a 5ft wider wing span and 2ft longer fuselage than the Ki-60, and its wing loading was reduced at first, but soon increased against, and reached 30lb/ sq ft on the Ki-61-Ib, lower than on the Ki-60, but still higher than on earlier Japanese fighter aircraft.
Dimensions for second prototype
Engine: Daimler-Benz DB 601A twelve cylinder inverted-vee inline engine
Power: 1,150hp at take off, 1,100hp at 13,125ft
Wing span: 34ft 5in
Length: 26ft 9.5in
Height: 12ft 1 2/3in
Empty Weight: 4,740lb
Loaded Weight: 6,603lb
Wing loading: 34.8lb/ sq ft
Max Speed: 348mph at 14,765ft
Service Ceiling: 32,810ft
Armament: Two 12.7mm Ho-103 machine guns and two 20mm Mauser MG 151 cannon in first and second aircraft, cannon replaced with two 12.7mm Ho-103 machine gun in third