The Nakajima Ki-84 Hayate (Gale) was the best Japanese fighter aircraft to see front line service in significant during the Second World War, and the last in a series of radial engine powered fighters produced by the Nakajima company.
Work on the Ki-84 began early in 1942, just as the Ki-43 was entering combat. It was designed in response to an ambitious army specification issued on 29 December 1941. This called for an aircraft with the agility of the Ki-43 and the performance of the Ki-44. It was to have a top speed of 398-423mph, a wing area of 204-226 sq ft, a wing loading of no more that 34.8lb/ sq ft and be able to operate at combat rating for 1h 30minutes at a range of 250 miles from its base. The new aircraft was to be armed with two 12.7mm Ho-103 machine guns and two 20mm Ho-5 cannon. Finally it was to have armour protection and self-sealing fuel tanks.
The Ki-84 was designed by T. Koyama, the designer of the Ki-44. During 1941 he had worked on two new fighter designs, the Ki-62 powered by a Japanese version of the Daimler Benz DB 601A and the radial-powered Ki-63. Neither aircraft had moved beyond the design stage, but many of their features found there way into the Ki-84. The basic design was ready by April 1942, and on 27 May 1942 the Japanese army ordered Nakajima to build a prototype.
The first prototype was completed in March 1943 and made its maiden flight in April. The second prototype followed in June, and in the same month the first prototype was taken to the Akeno Fighter Army Flying School, where it was shown to the impressed instructors and students. The flight tests went well, and Nakajima were ordered to build a pre-production series. One of these aircraft was tested against the Ki-44 and a Focke-Wulf Fw 190A-5. The Ki-84 outperformed the Ki-44 on all counts and was more manoeuvrable than the Fw 190, but was slower in level flight and in dives.
Eighty three service trial aircraft were built between August 1943 and March 1944, with modifications introduced all the time. They were followed by 42 pre-production aircraft, built between March and June 1944. After that the aircraft was ordered into production as the Ki-84-Ia Army Type 4 Fighter Model A. The Ki-84 entered full production remarkably quickly, with over 100 aircraft completed in June 1944, over 300 by October and a peak of 373 aircraft reached in December 1944. The aircraft was built on two construction lines by Nakajima and in small numbers by the Mansyu factory in Manchuria. Production slowed down after the US 20th Air Force destroyed the Nakajima engine factory at Musashi. Production of the engine resumed at an underground factory in Asakawa and a new factory at Hamamatsu, but never reached the previous levels. Even so around 3,382 production aircraft were completed in seventeen months. The rapid pace of construction played a part in the Ki-84's main flaw - the build quality wasn't as good as necessary, causing problems in combat conditions.
The Ki-84 was a low-wing monoplane. The fuselage had a level upper fuselage, with a bubble cockpit for good vision. The wing had a straight leading edge and tapered trailing edge, while the tailplane was set ahead of the vertical surface of the tail. In the original version the 12.7mm machine guns were mounted at the top of the engine, while the 20mm cannon were in the wings, just outboard of the main undercarriage. The aircraft had a big exhaust collector pipe on each side of the engine. The production versions could carry either a fuel drop tank or a 551lb bomb under each wing.
The aircraft was designed to be easy to produce, and took around 10,000 fewer hours to complete than the Ki-44. It was designed to use many of the same production jigs as the Ki-43, meaning that there was very little disruption when it replaced the Ki-43 on the production lines.
The Ki-84 was first issued to an experimental flight training company in Japan in October 1943. This unit was used to eliminate a number of problems with the aircraft, and was then disbanded in March 1944. Most of its pilots were transferred to the 22nd Sentai, the first unit to use the Ki-84.
In the summer of 1944 the 22nd Sentai moved to China, where it came up against the P-47 Thunderbolts and P-51 Mustangs of the US 14th Air Force. In five weeks the 22nd achieved remarkable results, partly because of its excellent new aircraft and partly because it contained many of the best Japanese pilots.
The first major test of the Ki-84 came on the Philippines. Seven Ki-84 Sentais had been posted to the islands by mid-October, just in time for the American invasion on Leyte. In theory the Japanese had enough aircraft to cause the Americans some serious problems, but the Ki-84 was about to reveal its main weakness. In ideal conditions it was a reliable aircraft, but in more normal combat conditions the Ha-45 engine proved itself rather unreliable, as did the landing gear. After a promising start the number of serviceable aircraft dropped rapidly, and many of the replacement aircraft were either lost en-route from Japan or had a much poorer performance than expected. In the hands of a good pilot the better aircraft were able to more than hold their own against most Allied fighters, but despite rushing fresh units to the area the Japanese were increasingly outnumbered. By the third week in November the JAAF in the Philippines was down to only 21% of its full strength, and this figure fell even further in December. By the end of January 1945 the battle for the Philippines was over, and the JAAF had suffered catastrophic losses. The Ki-84 had proved itself to be a capable fighter, but was simply overwhelmed.
The Ki-84 was used during the battle of Okinawa, operating in a hit-and-run capacity from southern Kyushu. Once again the Japanese fighters were overwhelmed by Allied numbers, especially after the Allies began to attack the bases in Kyushu.
The Ki-84 was the most effective Japanese single-seat fighter during the air battles over Japan. It could hold its own against the Allied fighters ranging over the Home Islands, and was also capable of inflicting serious damage on the B-29s, at least until they moved from daylight operations to night bombing. Yet again a lack of numbers meant that the Ki-84 didn't have a significant impact on the course of the fighting.
The entire -I series was powered by the 1,900hp Army Type 4 radial engine (Nakajima Ha-45). Early aircraft used the Model 11 or 12, which provided 1,800 or 1,825hp at take off, while later aircraft got the Model 21, with 1,990hp at take off. The Ki-84-Ia was armed with two 12.7mm machine guns and two wing mounted 20mm cannon.
The Ki-84-Ib was armed with two 20mm cannon in the fuselage and two 20mm cannon in the wings. It was produced alongside the Ki-84-Ia.
A small number of Ki-84-Ic bomber destroyers were built, armed with two 20mm cannon in the fuselage and two 30mm cannon in the wings.
The Ki-84-II entered production in the spring of 1945. It was produced in the same three versions as the -I, and in the army was normally know as the Ki-84a, Ki-84b or Ki-84c. The -II had wooden wing tips, control rods and a largely wooden rear fuselage and was produced in an attempt to save duralumin, which was in short supply in Japan.
The Ki-84-III was a design for a high altitude version of the aircraft, powered by a Ha-45 Ru engine and with a turbo-supercharger in the belly of the fuselage. It never progressed beyond the drawing board.
The Ki-106 was an all-wooden version of the Ki-84 designed by the Tachikawa aircraft company. Three prototypes were built, with a smooth lacquered surface over a plywood skin and larger vertical surfaces.
The Ki-113 was built with as much steel as possible to save on scarce lighter metals. One prototype was built but was too heavy.
The Ki-116 was a single Ki-84-I given a lighter 1,500hp Mitsubishi [Ha-33] 62 (Ha-112-II) engine. The reduction in weight made up for the reduction in power, but the war ended before the type could enter production.
The Ki-117 was a high-altitude version of the aircraft that was in the early stages of development at the end of the war.
Engine: Nakajima Ha-45 radial engine
Wing span: 36ft 10.5in
Length: 32ft 6.5in
Height: 11ft 1.5in
Empty Weight: 5,864lb
Maximum take-off Weight: 8.576lb
Max Speed: 392mph at 20,080ft
Service Ceiling: 34,350ft
Range: 1,347 miles
Armament: Two 12.7mm machine guns, two 20mm cannon
Bomb-load: Two 551lb bombs under the wings