The Kawasaki Ki-61 Hien ("Swallow") fighter represented a major departure for Japanese aircraft design in World War II. While other Japanese fighters were designed with air-cooled radials and were optimized for maneuverability, the Ki-61 used a liquid-cooled in-line engine and was designed for speed and power.
In fact, the Ki-61 was so different from other Japanese fighters that when the type was first encountered in combat over New Guinea in June 1943, the Allies thought it wasn't a Japanese design at all. At first they believed it was a copy of the German Messerschmitt Me-109, then suspected it was a copy of the Italian Macchi C.202 Foglore or similar Italian fighter. For this reason they gave it the code-name "Antonio", or "Tony", though by the summer of 1943 the Allies had realized the Ki-61 was in fact a Japanese design.
The Hien proved initially successful in combat against American fighters. As the war in the Pacific ground on, however, the Ki-61 found itself increasingly outclassed, but it soldiered on until the end of hostilities. This document describes the history of the Ki-61 and its radial-engine derivative, the Ki-100.
The confusion of the Ki-61 with German and Italian fighters had some basis in the aircraft's origins. Between 1923 and 1933, Kawasaki Aircraft Engineering Company's head designer was a German named Dr. Richard Vogt, who returned to Germany in 1933 to take a similar position at the firm of Blohm und Voss during the war. Not surprisingly, Kawasaki continued to be strongly influenced by Dr. Vogt's beliefs after he left, particularly a faith in the usefulness of liquid-cooled inline engines. This made Kawasaki something of a heretic among Japanese aircraft manufacturers, who preferred air-cooled radials.
In March 1938, Kawasaki signed an agreement with Daimler-Benz of Germany for manufacturing rights to the liquid-cooled inline engines then under development by the German firm. In April 1940, a Kawasaki engineering team visited Daimler-Benz in Stuttgart to obtain plans and samples of the DB-601A engine, then being used in the Me-109.
The Kawasaki engine team managed to increase the take-off power of their version of the engine to 875 kW (1,175 HP) and reduce its weight slightly. The engine was put into production in November 1941. It was designated the "Ha-40", or "Army Type 2", though it would be later redesignated the "Ha-60" in a combined Army/Navy nomenclature.
In the meantime, certain officers at the Air Headquarters of the Imperial Japanese Army were very interested in the new fighters powered by liquid-cooled being developed in Britain, the USA, the USSR, Germany, and France. The Japanese Army also had unpleasant experiences in air combat against the Soviet Polikarpov I-16 fighter during the beating the Imperial Army took in their Manchurian border clash with the USSR in the summer of 1939. This experience suggested the the single-minded focus on agility above all that characterized Japanese fighter design doctrine might need to yield to a focus on speed and improved armor protection and firepower.
* In February 1940, the Army initiated work with Kawasaki on two single-seat fighters based on the DB-601 derivative engine: a heavy interceptor, designated the "Ki-60", and a general-purpose fighter, designated the "Ki-61". Kawasaki decided to build the Ki-60 first, and the design team, under Kawasaki chief designer Takeo Doi and his deputy Shin Owada, constructed three prototypes of the interceptor in 1941.
Thanks to Aero_Shodanjo - For the Base Layer.