By William Pearce
In the 1930s, Japanese aviation began to make strides toward closing the technological gap with the Western World. In 1938, the Aeronautical Research Institute of the University of Tokyo, led by Shoroku Wada, began a high-speed aircraft research program. Gathering data on high-speed flight was the primary objective, but it was felt that an attempt on the 3 km absolute world speed record was an obtainable goal.
The aircraft project was known as KEN III (for Kensan III or Research III) and incorporated numerous advanced features new to Japanese aircraft. Approval was given for the aircraft’s development and a full-scale wooden mockup was finished in May 1941. Because of the outbreak of World War II, the project was taken over by the Imperial Japanese Army and designated Ki-78. A production contract for two prototypes was awarded to Kawasaki, under the direction of Isamu Imashi. Construction of the first prototype began in September 1941 at Kawasaki’s plant at Gifu Air Field.
The Ki-78 was an all-metal, low wing monoplane of conventional layout. The small streamlined fuselage was made as narrow as possible and was 26 ft 7 in (8.1 m) long. The wings possessed a laminar flow airfoil with a span of 26 ft 3 in (8 m) and an area of 118.403 sq ft (11 sq m). To reduce landing speed and improve low-speed handling, the wings incorporated drooping ailerons along with a combination of Fowler and split flaps, which was a first for a Japanese aircraft. When the Fowler flaps were deployed, the split flaps opened simultaneously to a similar extent. When the flaps were fully deployed, the ailerons automatically drooped down 10 degrees.
Power for the Ki-78 was provided by a Daimler-Benz DB 601A inverted V-12 engine driving a three-blade metal propeller. There is some question as to whether the engine was an imported German-built DB 601 or a license-built Kawasaki copy (Ha-40 Army Type 2), but it was most likely an imported German engine. Regardless, the DB-601 had a 5.91 in (150 mm) bore and 6.30 in stroke (160 mm), giving a total displacement of 2,070 cu in (33.9 L). It produced 1,175 hp (876 kW) at 2,500 rpm. The engine in the Ki-78 included a water-methanol injection system (another Japanese first) to boot the power output to 1,550 hp (1,156 kW) for short periods. The aircraft carried 66 gal (250 L) of fuel and 16 gal (60 L) of water-methanol.
Engine cooling was provided by two radiators: one mounted on each side of the rear fuselage. The radiators had a wide air inlet protruding slightly out from the fuselage. Airflow through each radiator was controlled by an actuated exit door. In addition, within the fuselage a small 60 hp (45 kW) turbine drove a fan to further assist cooling. The aircraft stood 10 ft 7/8 in (3.07 m) tall and weighed 4,255 lb (1,930 kg) empty.
The Ki-78 first flew on 26 December 1942 and was found to be extremely difficult to fly at low speeds and had poor stall characteristics. The aircraft was heavier than the design estimates, which increased the wing loading. Even with the special flaps and drooping ailerons, takeoff and landing speeds were both high at 127 mph (205 km/h) and 106 mph (170 km/h) respectively. In addition, elevator flutter was experienced at the relatively low speed of 395 mph (635 km/h) but was subsequently cured by fitting a horn-balance to the elevator.
High-speed flight tests were started in April 1943, and during the Ki-78’s 31st flight on 27 December, the aircraft achieved its maximum speed of 434.7 mph (699.6 km/h) at 11,572 ft (3,527 m). This was considerably less than the program’s speed goal of 528 mph (850 km/h). A study showed that extensive airframe modifications were needed to improve the Ki-78 flight performance. Consequently, the project was officially terminated after the aircraft’s 32nd flight on 11 January 1944. Only one prototype was built.
The unique Ki-78 survived the war but was crushed by American forces at Gifu Air Field in 1945.
World Speed Record Aircraft by Ferdinand Kasmann (1990)
Japanese Aircraft of the Pacific War by Rene Francillon (1970/2000)
General View of Japanese Military Aircraft in the Pacific War by Airview (1956)