The Nakajima Ki-43 Hayabusa (隼, "Peregrine Falcon") was a single-engine land-based tactical fighter used by the Imperial Japanese Army Air Force in World War II. The army designation was "Type 1 Fighter" (一式戦闘機); the Allied codename was "Oscar". Like the Japanese Navy's A6M, the radial-engined Ki-43 was light, maneuverable and easy to fly. The Ki-43 was legendary for its combat performance in East Asia in the early years of the war. Its lightweight construction, lack of armour and limited firepower, however proved to be deficient in comparison to later, more powerful Allied fighters. Nevertheless, the Ki-43 shot down more Allied aircraft than any other Japanese fighter. Total production amounted to 5,919 aircraft.
 Design and development
The Ki-43 was designed by Hideo Itokawa, who would later become famous as a pioneer of Japanese rocketry. The Ki-43 prototype was produced in response to a December 1937 specification for a successor to the popular Nakajima Ki-27. The specification called for a top speed of 500 km/h (311 mph), a climb rate of 5,000 m (16,400 ft) in five minutes and a range of 800 km (500 mi). Maneuverability was to be at least good as the Ki-27.
When first flown in January 1939, the Ki-43 prototype was a disappointment. Japanese test pilots complained that it was less maneuverable than the Ki-27 and not much faster. In order to solve these problems, Nakajima produced a series of progressively modified prototypes through the 1939 and 40. These changes involved a major weight saving programme, a slimmer fuselage with the tail surfaces moved further aft and a new canopy. Crucially, the 11th prototype introduced unique "butterfly" (or Fowler-type) maneuvering flaps, which dramatically improved performance in tight turns. The 13th prototype combined all these changes, and tests of this aircraft resulted in an instruction for Nakajima to place the Ki-43 into production, the Ki-27 jigs being transferred to the Mansyu factory at Harbin in Japanese occupied Manchukuo.
The initial production version was given the designation Ki-43-I. Deliveries from Nakajima's Ota factory commenced in April 1941. In addition to outstanding maneuverability, the Ki-43-I had a very impressive rate of climb due to its light weight. Power was provided by the Nakajima Ha-25 engine turning a two bladed, two-pitch metal propeller. Top speed was 495 km/h (308 mph) at 4,000 m (13,160 ft). The Ki-43 was equipped with two cowling machine guns in various configurations, with either two 7.7 mm (.303 in) Type 97 machine guns, one 12.7 mm (.50 in) Ho-103 machine gun (machine cannon) and one 7.7 mm (.303 in) gun, or two 12.7 mm (.50 in) Ho-103 guns; the aircraft was given various sub-designations to reflect these differences. The configuration that appears to have been most prevalent at the outset of the war was the latter configuration with two 12.7 mm (.50 in) Ho-103 machine guns, sometimes given the official designation Ki-43-I (Mark Ic). The Ho-103 was often loaded with explosive ammunition to increase target effect; its penetrative effect against later Allied aircraft armor appears to have been marginal.
Prototypes for the Ki-43-II flew in February 1942. The Ha-25 engine was replaced by the more powerful Nakajima Ha-115 engine, which was installed in a longer-chord cowling. The new engine turned a three bladed propeller. The wing structure, which had suffered failures in the Ki-43-I, was strengthened and equipped with racks for drop tanks or bombs. The Ki-43-II was also fitted with 13 mm armor plate for the pilot's head and back, and the aircraft's fuel tanks were coated in rubber to crude form of self-sealing tank. The pilot also enjoyed a slightly taller canopy and a reflector gunsight in place of the earlier telescopic gunsight. Nakajima commenced production of the Ki-43-II at its Ota factory in November 1942. Production was also started at the Tachikawa Hikoki and the 1st Army Air Arsenal, also at Tachikawa. Although Tachikawa Hikoki sucessfully managed to enter into large scale production of the Ki-43, the 1st Army Air Arsenal was less successful, being hampered by a shortage of skilled workers, being ordered to stop production after 49 Ki-43s were built. Nakajima eventually ceased production in mid-1944 in favor of the Ki-84, but the Tachikawa Hikoki continued to produce the Ki-43.
Tachikawa also produced the Ki-43-III, which utilized the more powerful Ha-115-II engine. Maximum speed increased to 358 mph. Tachikawa produced 2124 Ki-43-II and -III aircraft between April 1944 and the end of the war. Total production of all versions amounted to 5,919 aircraft.
 Operational history
The Ki-43 was the most widely-used Army fighter, and equipped 30 sentai (groups) and 12 chutais (squadrons). The first version, Mark I, entered service in 1941, the Mark II in December 1942, the II-Kai in June 1943, and the Mark IIIa in summer 1944. The aircraft fought in China, Burma, the Malay Peninsula, New Guinea, the Philippines, South Pacific islands and the Japanese home islands.
Like the Zero, the Ki-43 initially enjoyed air superiority in the skies of Malaya, Netherlands East Indies, Burma and New Guinea. This was partly to do with the better performance of the Oscar and partly due to the relatively small numbers of combat-ready Allied fighters, mostly the P-36 Hawk, Curtiss P-40, Brewster Buffalo, Hawker Hurricane and Curtiss-Wright CW-21 in Asia and the Pacific during the first months of the war. As the war progressed, however, the fighter suffered from the same weaknesses as the "Nate" and the Zero; light armor and less-than-effective self-sealing fuel tanks, which caused high casualties in combat. Its armament of two machine guns also proved inadequate against the more heavily armoured Allied aircraft. As newer Allied aircraft were introduced, such as the P-47 Thunderbolt, P-38 Lightning, P-51 Mustang, F4U Corsair, F6F Hellcat and late-model Supermarine Spitfire/Seafire, the Japanese were forced into a defensive war and most aircraft were flown by inexperienced pilots. Towards the end of the war, many Hayabusas were expended in kamikaze raids.
The Ki-43 also served in an air defense role over Formosa, Okinawa and the Japanese home islands. Some examples were supplied to the pro-Japanese countries of Thailand, Manchukuo and Wang Jingwei Government as well. The Thai units sometimes fought against the USAAF in southern China.
Hayabusas were well liked in the JAAF because of the pleasant flight characteristics and excellent manouevreability, and almost all JAAF fighter aces claimed victories with Hayabusa in some part of their career. At the end of the war, most Hayabusa units received Ki-84 Hayate "Frank" fighters, but some units flew the Hayabusa to the end of the war. The top-scoring Hayabusa pilot was Sergeant Satoshi Anabuki with 59 victories.
- Prototypes and operative prototypes.
- Variant armed with 2 × 7.7 mm (.303 in) Type 97 machine guns
- Hayabusa Fighter Type 1 of Army (Mark 1).
- Ki-43-Ib (Mark Ib)
- Variant armed with one 12.7 mm (.50 in) Ho-103 machine gun and 1 × 7.7 mm (.303 in) Type 97
- Ki-43-Ic (Mark Ic)
- Variant armed with 2 × 12.7 mm (.50 in) Ho-103
- Prototypes and evaluative models.
- Ki-43-IIa (Mark 2a)
- Ability to carry up to 500 kg (1,100 lb) of bombs
- Ki-43-IIb (Mark 2b)
- Radio equipment added
- Fitted with ejector exhaust stacks
- Prototypes powered by Nakajima Ha-115-II engine of 920 kW (1,230 hp)
- 2 × 170 L (45 gal) drop tanks (~3 hour full-throttle endurance)
- Ki-43-IIIa (Mark 3a)
- Series model
- Ki-43-IIIb (Mark 3b)
- Variant armed with 20 mm cannons.
- Ki-62 Project
- Advanced interceptor version of Nakajima Ki-43 with a powerful engine and armed with 30 mm (1.18 in) or 40 mm (1.57 in) cannons.