The Nakajima Ki-84 Hayate, known as "Frank" in the Allied jargon, was one of the best IJAAF (Imperial Japanese Army Air Force) fighters during the final year of the Pacific war. Featuring an enclosed cockpit and retractable landing gear, this all-metal cantilever low-wing monoplane of an elegant body was the successor of the previous Nakajima fighters – the Ki-43 Hayabusa and Ki-44 Shoki.
The Ki-84 proved its value fighting in China and the Philippines, although the highest number of these machines were lost there, too. When in the hands of an experienced pilot, it was a good match for the best of Allied fighter aircraft. After the war a small number of Ki-84s served with the Chinese air forces, taking part in the civil war of 1945-1949.
Nakajima Hikoki Kabushiki Kaisha
The history of the Nakajima company, one of the oldest and best known Japanese aircraft manufacturers, dates back to December 6, 1917, when a former Chui of the Imperial Navy, Chikuhei Nakajima, established a company named Hikoki Kenkyusho (Aircraft Research Institute) near the Daikoin Temple in Ota. When still a navy officer, Nakajima had been sent to the United States in 1912 to study aircraft construction and complete pilot training at the Glenn Curtiss factory and school of aviation. In April 1918 Chikuhei Nakajima renamed his company Nakajima Hikoki Seisakusho (Nakajima Aeroplane Manufacturing Works). A little later, when the industrialist Seibei Kawanishi (who would later establish his own aircraft company, Kawanishi Kokuki KK) joined the venture, the name was changed to Goushikaisha Nihon Hikoki Seisakusho (Japan Aeroplane Works Co. Ltd.). The beginnings were modest, but in April 1919 the new company received first big orders for a self-designed two-seat training biplane, the Nakajima-Shiki Go-Gata (Type Nakajima Model 5). By May 1921 the company had built 118 of these aircraft for both the Imperial Army and non-military customers.
Owing to differences of opinion, Seibei Kawanishi had in the meantime left the company, with which regard in December 1919 Chikuhei Nakajima restored the former company name, Nakajima Hikoki Seisakusho. In 1920 the company received the first order from the Imperial Navy. In 1924 Nakajima decided to undertake engine manufacturing, for which purpose he established a factory at Ogikubo in the suburbs of Tokyo. The further development of the company was made possible owing to a big (for those times) contract to build Ko-Shiki Yon-Gata Sentoki aircraft (Typ Ko Model 4 Fighter), with more than 600 of these built over the years 1923-1932. They were actually licensed French Nieuport 29C1s, but they provided Nakajima with the resources and experience to design in the late 1920s its first own fighter, the 91-Shiki Sentoki (Type 91 Fighter). This construction was born with the help of engineer Yasushi Koyama, the father of almost all the future Nakajima fighters built for the Army Air Force.
Nakajima once more renamed the company in December 1931, this time Nakajima Hikoki Kabushiki Kaisha (Nakajima Aeroplane Co. Ltd.). At the same time he retired and became involved in politics, while still retaining the honorary title of the company's president. Control of the business was taken over by his younger brother, Kiyoichi Nakajima. In the following years the company designers began to experiment with new ideas and technologies developing in fighter construction. This gave birth to the metal monoplane Ki-8, Ki-11 and Ki-12 fighters (the latter featuring retractable landing gear and armed with a 20-mm cannon).
The constantly developed aircraft and engine factories of Nakajima, including the subsidiary Nakajima Koku Kinzoku KK (Nakajima Aero-Metal Co. Ltd.), grew into a practically self-sufficing industrial unit which manufactured almost all of the more important parts and sub-assemblies used in aircraft production (with the exception of propellers). This made Nakajima the Second World War's greatest Japanese aircraft manufacturer and placed it as the second best right behind Mitsubishi in aircraft engine production. Throughout 1941-1945 the company built 28 percent of all the aircraft produced in Japan (or no less than 37.1 percent if we only consider military aircraft) and 31.3 percent of engines.
The origin and development
As the war broke out in the Pacific, the fighter component of the IJAAF was still based on the Ki-27, a light low-wing monoplane with fixed landing gear designed in 1937 (see Monograph no. 11). It was being replaced by the more modern Ki-43 Hayabusa with retractable landing gear, while another fighter, the Ki-44 Shoki – heavier and less agile but faster, with a more powerful engine and stronger armament – was undergoing operational trials. All the three types were genuine Nakajima designs.
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.
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.
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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