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 the following month. 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, with
conventional 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 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 several versions of the Homare engine, including the
carbureted model 21 and the
fuel-injected model 23 versions of the engine. Most Homare engines used
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. In 1946, US Technical Intelligence bench-tested a Homare 45, Model 21 engine and verified the engine's maximum horsepower output using 96 octane AvGas, plus methanol injection. The complicated Ha-45-21 carbureted engine was a compact design (only larger in diameter than the Ki-43's 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. ==Operational service==