Nakajima Ki-84 Hayate

Shortly after the IJAAF was equipped with the Ki-43, the Rikugun Koku Hombu (Army Air Headquarters) and the Nakajima board of directors began to prepare initial guidelines for the design of a new fighter aircraft which could in the future succeed both the Ki-43 and Ki-44. The IJAAF staff members were aware that new types of Allied fighters would soon appear at the front that the Ki-43 would not be able to defeat. As to the Ki-44, it was better suited to fighting bombers than outmaneuvering enemy fighters in combat. What the IJAAF was looking for was a universal offensive fighter aircraft of a possibly long range that would combine the best features of the Ki-43 (agility) and the Ki-44 (good performance, heavy armament). It was also significant that the construction be simplified for a shorter production time (a single Ki 43 requiring no less than 25,000 working hours to finish, whereas a Ki-44 took 24,000 hours).
On December 29, 1941 the Koku Hombu gave Nakajima instructions as to the tactical and technical requirements from the new heavy (which then simply meant "cannon-armed") fighter, known at this stage as the Shisaku Jusentoki (Army Experimental Heavy Fighter). The construction was to be all-metal, with self-sealing fuel tanks, cockpit armor and a heavy armament of two 12.7-mm machine guns and two 20-mm cannons (actually, the IJAAF terminology at that time considered 12.7-mm machine guns as cannon, too). The aircraft was to be propelled by the new eighteen-cylinder Nakajima Ha-45 radial, a version of the NK9A Homare, the latter being developed for the Navy Air Force. It was a very promising power plant expectedly rated at around 2000 hp. This engine construction took advantage of the cylinders and other parts of the tried fourteen-cylinder Ha-25 and Ha-115 (which propelled, among others, the Ki-43-I and Ki-43-II, and were known to the IJNAF as the NK1 Sakae). The engine was adapted for injection of a mixture of water and methanol into the cylinders for short power boosts. Performance-wise, the Koku Hombu was expecting the fighter to attain a maximum speed of 680 km/h, climb to 5000 m in less than 4.5 minutes, be able to operate within a radius of 400 km from the base, and carry enough reserves to combat for 1.5 hours. For good agility, it was also required that wing loading be not higher than 170 kg/m2. One additional requirement was formulated in April 1942: the amount of production work on a single aircraft should not exceed 14,000 hours.

Hayate tabela

In order to meet the requirements Nakajima formed a design team led by the experienced engineer Yasushi Koyama, the designer or co-designer of all the three previous fighter types used by the IJAAF (Ki-27, Ki-43 and Ki-44). The team consists for example of engineers Setsuo Nishimura, Masaru Iino and Yoshio Kondo. The first draft of the new Shisaku Jusentoki fighter was ready in the spring of 1942 and approved by the Koku Hombu on May 27. This new fighter received the designation of Ki-84.
In his work on the Ki-84 engineer Koyama utilized the experience gained while working on the Ki-43, Ki-44 and the unfinished Ki-63 fighter (developed in 1940-1941 along with the Ki-62 as an alternative for Kawasaki's Ki-60 and Ki-61). The Ki-84 was a cantilever low-wing monoplane with retractable landing gear, classic tail unit and enclosed cockpit. The construction was all metal except for the fabric covering of ailerons and tail control surfaces. The three-wheeled landing gear with a tail wheel was entirely retractable and protected by covers. The main wheels, folding toward the fuselage, retracted into wells in the wings, whereas the tail wheel did into a well in the rear fuselage. The tail had a characteristic shape, with the horizontal stabilizer set forward of the fin. This was to prevent mutual disturbances of these surfaces in performing rapid combat maneuvers. The semimonococque fuselage was typical of Nakajima and resembled the one used earlier for the Ki-27, Ki-43 and Ki-44. The fuselage was divided into two parts joined at frame no. 9. The front section formed a single unit with the wings, which, too, was a hallmark of Nakajima aircraft (and a few other Japanese fighters). This solution was supposed to ensure a greater strength of the construction, at the same time reducing the weight of links and ferrules.