Nakajima Ki-84 Hayate

The cantilever tapered wings with rounded tips were single-piece except for the wingtips, which were mounted on. The wing construction was based on one main spar, one auxiliary spar, and 24 ribs. The wings featured ailerons and flaps, the latter being another hallmark of Nakajima – they were the so-called "butterfly combat flaps", constructed a few years before by professor Hideo Itokawa, and had been first used for the Ki-43. They were retractable slotted flaps very much like the regular Fowler flaps; the basic difference, however, was in the way they were extended, i.e. not by the same amount over their entire length but more at the wing root than at the tip. This outline in a way resembled the spread wings of a butterfly, hence their characteristic name. The flaps could be deflected for combat by an angle of 15 degrees, which greatly improved the agility of the aircraft. For take-off, they could be deflected by 35 degrees, whereas for landing it was 53 degrees. The operation was steered hydraulically and activated by a button placed on the control stick. The cockpit was enclosed by a three-piece glass canopy. The windscreen was made of 65 mm thick bulletproof glass. The seat back and headrest were protected by 13-mm armor plates. Fuel was stored in five sheet metal tanks covered with a self-sealing material. The main tank was in the fuselage, the other four in the wings (two between the spars, and two in the leading edge, forward of the main spar). The overall amount of fuel was 697 liters. Additionally, a drop tank of 300 liters could be carried in a rack under the fuselage. The aircraft were normally equipped with a radio set and oxygen system.


The armament of the aircraft consisted of two synchronized 12.7-mm Ho-103 machine guns mounted in the upper fuselage forward of the cockpit (they fired through the propeller; the muzzles were located in the engine cowling) and two 20-mm Ho-5 cannons in the wings (with the muzzles projecting from the leading edges). Interestingly, both types of weapon were based on solutions borrowed from the 12.7-mm American Browning M2 machine gun (which became the basis for spiteful remarks that if it was not for the Americans, the IJAAF fighters would be only armed with 7.7-mm guns throughout the war). Interestingly, the Ki-84 was the first IJAAF fighter which was intended to be armed with 20-mm cannons when it was still an idea on the designers' drawing boards. By then, it was typical for IJAAF fighters to be armed with either two 7.7-mm machine guns (Ki-27, Ki-43) or two such guns and two 12.7-mm ones, which was already considered heavy armament (Ki-44, Ki-61).
Technologically speaking, the Ki-84 construction was also a breakthrough in the existing trends. Engineer Koyama made an effort to simplify and unify the construction as much as possible in order to reduce the number of necessary components and hardware (e.g. bolts, rivets etc.). The aircraft could be assembled on the same lines as the Ki-43, that only requiring minor modifications. Lastly, the production was to be done taking advantage of the so-called standard drilling templets (kijunko shuseiho), a kind of patterns which would eliminate the need to take measurements manually during the production and processing of components – that would have ensured the repeatability of manufacturing operations and exchangeability of parts. All those factors together were supposed to keep the overall production time within the limits imposed by the Koku Hombu.


The elaborated design of the Ki-84 was finished in November 1942, whereupon the building of a prototype immediately followed. In the last week of March 1943, ten months after the design had been approved, the first Ki-84 prototype no. 01 (Ki-84.01) rolled out of the assembly room of the Nakajima factory at Ota. The machine was powered by the Ha-45-11 engine (Ha 45 11-Gata, or Ha-45 Model 11) rated at 1342 kW for take-off (1800 hp). The engine rotated a four-bladed metal constant-speed propeller of 3 m in diameter (some authors, e.g. [2, 22, 23], give 3.05 m). This prototype was probably still unarmed.
The Ki-84.01 was very secretly transported to Ojima airfield, where it was probably flown for the first time toward the end of April. Right after that it was sent to the Koku Shinsabu (Army Flight Test Centre) at Fussa near Tokyo for extensive in-flight testing. The test pilots responsible for the testing of the Ki-84 were led by an experienced IJAAF pilot, Shosa Jozo Iwahashi. The test pilots of Dai-Ichi Rikugun Kokusho (First Army Air Arsenal, or Kosho in short) at Tachikawa near Tokio also made some trial flights in the new aircraft, confirming the enthusiastic opinions given by the Nakajima factory pilots. In August 1943 Shosa Iwahashi flew the prototype to the Rikugun Hiko Gakko (Army Flying School) at Akeno in Mie prefecture in order to present the new fighter to the instructors and cadets of this school, which produced the best fighter pilots in the IJAAF. Iwahashi made a show of aerobatics and landed, stopping the machine after only a little more than 400 m of landing run. In Iwahashi's opinion, the Ki-84 was easy and pleasant to fly, and learning to control it should not pose any problems to young pilots who had completed only 200 flight hours. The presentation of the new fighter greatly impressed the audience and left the future IJAAF fighter pilots with a much higher morale.
The situation at the front had become much worse around that time. The 4th Kokugun (Air Army), which was operating in New Guinea, had sustained heavy losses, and the IJAAF was exerting pressure for the aircraft to enter production due to an urgent need of machines capable of opposing the Allied air forces. With this regard, the Koku Hombu made an unprecedented decision to order a total of no less than 125 pre-production or supplementary prototypes aircraft in two batches. Modifications and improvements were to be made on these machines according as the concurrent extensive flight tests dictated. This was on the one hand to quickly deal with the aircraft's teething troubles and speed up the arrival at a point where the Ki-84 would be ready for quantity production; on the other hand, the personnel would sooner become acquainted with the aircraft and combat tactics would be developed for the new fighter, which would in turn facilitate the introduction of the aircraft into first line units. In the meantime a second prototype was finished in June; the Ki-84.02 was flown for the first time in August. Some authors (e.g. [5, 7, 22, 23]) claim that a third prototype, the Ki-84.03, was also built. Unfortunately, it is not possible to precisely establish the real look of the prototypes. Even though there exist photographs of the first prototype, the Ki-84.01, they do not show all the details of its construction. What is certain is that initially the first prototype had a wing lifting surface of 19.00 m2, the same as the prototype Ki-44-III, which was increased to 21.00 m2 on the second prototype for a lower wing loading.

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