The Nakajima Ki-84 "Hayate" (キ84 疾風) was a single-seat fighter used by the Imperial Japanese Army Air Force in World War II. The Allied reporting name was "Frank"; the Japanese Army designation was Army Type 4 Fighter (四式戦闘機 yon-shiki-sentō-ki ). Featuring excellent performance and high maneuverability, the Ki-84 was considered to be the best Japanese fighter to see large scale operations during World War II. It was able to match any Allied fighter, and to intercept the high-flying B-29 Superfortresses. Its powerful armament (that could include two 30 mm and two 20 mm cannon) increased its lethality. Though hampered by poor production quality in later models, a high-maintenance engine, a landing gear prone to buckle,and lack of experienced pilots above all else, Hayates proved to be fearsome opponents; 3,514 aircraft were built.
Design of the Ki-84 commenced in early 1942 to meet an Imperial Japanese Army Air Service requirement for a replacement to Nakajima's Ki-43 fighter, just entering service. The specification recognized the need to combine the maneuverability of the Ki-43 with performance to match the best western fighters and heavy firepower. The Ki-84 first flew in March 1943 Although the design itself was solid, the shortage of fuel, construction materials, poor production quality, and lack of skilled pilots prevented the fighter from reaching its potential.
The Ki-84 addressed the most common complaints about the popular and highly maneuverable Ki-43: insufficient firepower, poor defensive armor, and lack of climbing power. The Ki-84 was a cantilever low-wing monoplane of all-metal construction, except for the fabric-covered control surfaces. It had retractable tailwheel landing gear. Armament comprised two fuselage-mounted, synchronized 12.7 mm (.50 in) machine guns and two wing-mounted 20 mm cannon, a considerable improvement over the two 12.7 mm (.50 in) machine guns used in the Hayabusa. Defensive armor offered Hayate pilots better protection than the unsealed wing tanks and light-alloy airframe of the Ki-43. In addition, the Ki-84 used a 65 mm (2.56 in) armor-glass canopy, 13 mm (.51 in) of head and back armor, and multiple bulkheads in the fuselage, which protected both the methanol-water tank (used to increase the effectiveness of the supercharger) and the centrally located fuel tank.
It was the Nakajima Ha-45 radial powerplant that gave the Hayate its high speed and prowess in combat. Derived from the Homare engine common to many Japanese aircraft, the Hayate used a direct-injection version of the engine, using water injection to aid the supercharger in giving the Ki-84 a rated 1,491 kW (2,000 hp) at takeoff. This combination—in theory, at least—gave it a climb rate and top speed roughly competitive with the top Allied fighters. Initial Hayate testing at Tachikawa in early summer 1943 saw test pilot Lieutenant Funabashi reach a maximum level airspeed of 624 km/h (387 mph) in the second prototype. After the war, a late-production, captured example was tested in the US with high octane fuel, and achieved a speed of 687 km/h (426 mph).
The complicated direct-injection engine required a great deal of care in construction and maintenance and, as the Allies advanced toward the Japanese homeland, it became increasingly difficult to support the type's designed performance. Compounding reliability problems were the Allied submarine blockade which prevented delivery of crucial components, such as the landing gear. Many further landing gear units were compromised by the poor-quality heat treatment of late-war Japanese steel. Many Hayates consequently suffered strut collapses on landing. Further damage was caused by inadequately trained late war pilots.