The Kawasaki Ki-61 Hien or Type 3 Fighter remains to this day one of the most recognizable Japanese fighters of the World War II era. What makes Hien unique is the powerplant – it was the only mass-produced Japanese fighter powered by an inline, liquid cooled engine.
The Ki-61 began to arrive at the frontlines in large numbers in the summer of 1943 and took part in battles over New Guinea and later over the Philippines and Okinawa, as well as in the defense of the Japanese Home Islands. In total over 3,000 examples of various Ki-61 variants and derivatives were built. The Ki-100, a Ki-61-II Kai airframe mated to the Ha-112-II radial engine, entered service towards the end of the war.
Origins and development of the design
On July 1, 1938 the Rikugunsho (Japanese Ministry of the Army) signed off on the expansion and fleet modernization program of the Dai Nippon Teikoku Rikugun Kokutai (Imperial Japanese Army Air Force, IJAAF), known as Koku Heiki Kenkyu Hoshin (Air Weapons Research Policy). The program, prepared by Rikugun Koku Honbu (Army Aeronautical Department), included the development of two single-seat fighter types by Nakajima – light Ki-43 and the Ki-44 heavy fighter. “Light” and “heavy” designations did not reflect the weight or size of the aircraft, but rather the caliber of offensive armament carried by the fighters. According to the program’s requirements, the light single-seat fighter (kei tanza sentoki) was to be armed with a pair of 7.7 mm machine guns, i.e. standard weapons carried by the Army Air Force fighters since its inception. The aircraft, designed as a weapon against enemy fighters, was supposed to be very maneuverable and fast. On the other hand, the heavy single-seat fighter (ju tanza sentoki) was to be used against enemy bombers. That type of mission required a machine with a high level flight speed, a good rate of climb and a heavy offensive punch. The proposed heavy single-seat fighter was therefore required to be armed with two 7.7 mm machine guns and one or two “cannons”, which in reality meant large caliber machine guns
In June 1939, less than a year after the modernization program had been approved, the officials of Rikugun Kokugijutsu Kenkyusho (Army Air Technical Research Institute, often known under its abbreviated name Kogiken or Giken) began a series of consultations with the representatives of aeronautical companies in order to work out technical requirements for a new generation of combat aircraft, whose development would be included in the 1940 Koku Heiki Kenkyu Hoshin program. During the consultations the Kogiken officials met twice (in June and in August) with the Kawasaki engineers. In addition to talks and consultations with the local aeronautical industry leaders, the Kogiken team studied lessons learned from the battles against the Soviet air force over Khalkhin-gol (Nomonhan) and reports of the Japanese observers covering operations of the Luftwaffe against Poland. The newest trends and developments in aviation technology in nations considered global aviation powers (especially Germany, Britain and the U.S.) were also carefully studied and scrutinized.
In February 1940 Rikugun Koku Honbu Gijutsubu (Army Aeronautical Department, Engineering Division) used the results of the studies to commission several Japanese aircraft manufacturers to develop new combat aircraft designs, with considerably better performance, stronger construction and heavier armament than the types in active service or in development at that time. In the single-engine, single-seat fighter category the division into light and heavy types was maintained. Kawasaki received orders to develop two fighter designs powered by inline, liquid cooled engines – the heavy Ki-60 and the light Ki-61 fighter. Orders for similar types, but powered by radial, air cooled engines, were placed with Nakajima (the light Ki-62 fighter and the heavy Ki-63). In addition, Kawasaki designers were tasked with the development of the ground-breaking Ki-64 fighter, while Mitsubishi was to produce the Ki-65 heavy fighter. The winning designs in each category were to be officially selected in March 1942.
Takeo Doi, chief designer at Kawasaki Kokuki Kogyo Kabushiki Kaisha (Kawasaki Aircraft Industries Co., Inc.) considered the division into heavy and light fighters completely artificial. He argued that in real combat conditions it was unrealistic to expect enemy fighters to be engaged exclusively by light fighter types, or bombers to be attacked by heavy machines only. In Doi’s opinion the best way forward was to develop a universal or “medium” fighter type (chukan sentoki), capable of dealing with any type of air threat, be it bombers or fighters. Such a machine would have to combine the best features of light and heavy fighters, i.e. excellent performance, agility and heavy armament. Other desirable features included high cruising speed, respectable range and robust design. Despite Doi’s objections, Kawasaki design team went to work to design both fighter types, as required by Gijutsubu. Doi’s deputy in charge of the development of the Ki-60 heavy fighter (factory designation KDA-202) was Kenkichi Kiyota, while the team developing the Ki-61 light fighter (KDA-21) was led by Shin (Shinji?) Owada.