page_title: Nakajima Ki-84
section_title: Design and development
hierarchical_section_title: Nakajima Ki-84 / Design and development
caption_attribution_description: English: Nakajima Ki-84 Type 4 Hayate (1st additional prototype #124) of the Army Air Inspection Section, Imperial Japanese Army Air Force. 日本語: 陸軍航空審査部の中島 キ84 四式戦闘機 疾風（第一次増加試作機 124号機）。
context_page_description: The Nakajima Ki-84 Hayate is a single-seat fighter flown by the Imperial Japanese Army Air Service in the last two years of World War II. The Allied reporting name was "Frank"; the Japanese Army designation was Army Type 4 Fighter. The Ki-84 is generally considered the best Japanese fighter to operate in large numbers during the conflict. The aircraft boasted high speeds and excellent maneuverability with an armament that gave it formidable firepower. The Ki-84's performance matched that of any single-engine Allied fighter it faced, and its operational ceiling enabled it to intercept high-flying B-29 Superfortress bombers. Pilots and crews in the field learned to take care with the plane's high-maintenance Nakajima Homare engine and a landing gear prone to buckling. The difficulties of Japan's situation late in the war took a toll on the aircraft's field performance as manufacturing defects multiplied, quality fuel proved difficult to procure and experienced pilots grew scarce. Nevertheless, a well-maintained Ki-84 was Japan's fastest fighter. A total of 3,514 aircraft were built.
context_section_description: Design of the Ki-84 commenced in early 1942 to meet an Imperial Japanese Army Air Service requirement for a replacement to Nakajima's own, earlier Ki-43  fighter, then 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  and deliveries from Nakajima's Ota factory commenced in April 1943. Although the design was itself solid, growing difficulties in securing skilled pilots, proper fuel and construction materials, and adequate manufacture often prevented the aircraft from reaching its full potential in the field.
  The design of 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 speed. The Ki-84 was a cantilever low-wing monoplane of all-metal construction, except for the fabric-covered control surfaces. It had a retractable tailwheel landing gear.
  Armament comprised two fuselage-mounted, synchronized 12.7 mm (.50 in) machine guns — these proved challenging to synchronize properly with the Hayate's four-blade propeller — and two wing-mounted 20 mm cannon, a considerable improvement over the two 12.7 mm (.50 in) machine guns used in the Ki-43 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 firm's own-designed 35.8 litre displacement, Ha-45 Homare ("Praise or Honor") air-cooled eighteen-cylinder radial engine, first accepted for military use in 1941, that gave the Hayate its high speed and prowess in combat. Derived from the Nakajima Homare engines common to many Japanese aircraft, the Hayate used the Homare 21 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 theoretically 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 captured late-production example was tested in the US and achieved a speed of 680 km/h (422 mph) using 92 octane AvGas, plus methanol injection.
  The complicated direct-injection engine was a compact design (only 3 cm/1¼ inches larger in diameter than the  Ki-43's 27.9 L 14-cylinder Nakajima Sakae radial) that required a great deal of care in construction and maintenance and it became increasingly difficult to maintain the type's designed performance as the Allies advanced toward the Japanese homeland. To compound reliability problems, the Allied submarine blockade prevented delivery of crucial components, such as the landing gear. Many landing gear units were compromised by the poor-quality heat treatment of late-war Japanese steel. As a result, many Hayates suffered strut collapses on landing. Further damage was caused by inadequately trained late war pilots, who sometimes found it difficult to transition to the relatively "hot" Ki-84 from the comparatively docile Ki-43, which had a significantly lower landing speed.
caption: A prototype of the Nakajima Ki-84 with a unique exhaust stack, similar to a Ki-43's
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