The Army Type 97, better known as the Nakajima Ki-27 – or “Nate” under Allied nomenclature – was the first modern fighter to serve with the Imperial Japanese Army Air Force (IJAAF). This all-metal cantilever low-wing monoplane featured classic fuselage lines, an enclosed cockpit and fixed landing gear. It proved its excellent qualities during combat service in China and along the Khalkhin-Gol river area.
At the outbreak of the Pacific war, it was the basic IJAAF fighter, seeing service in Indochina, Malaya, the Philippines, and Burma, as well as in defense of the Japanese Islands. On being superceded in first line units by more state-of-the-art successors, it continued in service until the end of the war in training units and pilot schools. Outside Japan, it served with the air forces of Manchukuo and Thailand. Origins
The first fighter aircraft competition
It is necessary to go back to the mid-twenties in order to trace the origins of the Nakajima Ki-27. In March 1927, Koku Hombu (Army Headquarters) invited bids from Mitsubishi, Nakajima, Kawasaki and Ishikawajima to design a Japanese fighter aircraft which could replace the basic fighter currently in service, the Nakajima Type Ko 4 (Ko-Shiki Yon-Gata Sentoki – in the IJAAF designation scheme, the Ko stood for Nieuport-produced aircraft, while the number 4, i.e. yon, the successive type number). The Ko 4, a license-based variety of the French Nieuport 29C1, was the first fighter to be manufactured in quantity by Japan for her IJAAF. A total of 608 machines left the Nakajima assembly line between December 1923 and January 1932. The Ko 4 was a replacement for the Type Hei 1 fighter (imported SPAD XIII) and Type Ko 3 (licensed Nieuport 24C1 and 27C1). However, the origin of the Ko 4, like the older Hei 1 and Ko 3, can be traced as far back as WW1, the French progenitor having been first flown in June 1918. There was thus an urgent requirement to replace it with a more advanced construction.
The Ishikawajima company very quickly withdrew from the project, the first such competition announced by Koku Hombu. The very early stage of the design work showed that their aircraft would not be up to the requirements specified by Koku Hombu. The remaining three competitors completed their designs and built prototypes, which subsequently underwent comparison tests.
Mitsubishi’s two prototypes of an aircraft named Hayabusa-go (Type Hayabusa, factory designation 1MF2) were built in 1928, the first one being ready in May. It was designed by a team led by Germany’s Professor Alexander Baumann and Japan’s Nobushiro Nakata, with the help of engineers Jiro Horikoshi and Jiro Tanaka. The initial assumption was for the aircraft to be a low-wing monoplane, but the Army requested a strutted high-wing monoplane with a parasol-type wing (interestingly, without a single bracing wire). The Hayabusa was of mixed construction (metal fuselage and wooden wings) with fabric covering. The power plant was the liquid-cooled twelve-cylinder V-type Mitsubishi Hispano-Suiza rated at 450 hp and delivering a maximum of 600 hp for takeoff. (450/600 hp)
Kawasaki’s construction, designated KDA-3 (Kawasaki Dockyard, Army, Model 3 – many Japanese aircraft of the ‘20s and ‘30s were designated using English abbreviations), was designed by one of the factory’s German employees, Dr Ing. Richard Vogt. It was a strutted high-wing parasol-type monoplane of metal-and-wood wing construction with fabric covering; the fuselage was all metal, modeled on the German Dornier Do H “Falke”. In March 1928 the factory built the first of three prototypes, powered by the BMW VI 6.3 engine of 450/630 hp. This machine was soon lost in a landing accident at Kagamigahara. The two following prototypes were completed in May. These were propelled by Hispano-Suiza engines of the same power rating; both plants were liquid-cooled twelve-cylinder V-type units.
May and June 1928 also saw the completion of two fighter prototypes from the Nakajima factory designated NC. This aircraft was the brainchild of a team of Japanese engineers headed by Shigejiro Ohwada and Yasushi (Yasumi?) Koyama, in cooperation with French engineers of the Dewoitine company, André Marie and Maxime Robin. This was also a strutted high-wing parasol monoplane, but of all-metal construction with fabric covering. Unlike both competitors, it was propelled by the air-cooled nine-cylinder Bristol Jupiter VI radial engine delivering 450/520 hp, later built under license by Nakajima as the Kotobuki. The NC’s construction was undoubtedly influenced by the French designers, inspired by the Nieuport Delage and Dewoitine aircraft.
All three contestants were single-seaters with open cockpits and two-wheel fixed landing gear with tail skids. Each was armed with two 7.7-mm machine guns housed in the fuselage, the standard Japanese fighter armament until the early ‘40s. Also, all embodied the strutted parasol wing layout as per the requirements of Koku Hombu. The direct cause of this requirement was the most recent trend in European fighter designs, particularly with regard to German and French constructions, of which a few had been bought for testing by the IJAAF, making a very strong impression on the Japanese military.
After the first factory tests, the prototypes of the three competitors were delivered to the IJAAF research establishment at Tokorozawa in the Saitama prefecture for comparison tests. However, flight tests were abandoned after the first Mitsubishi Hayabusa prototype literally fell apart in a diving attempt to exceed 400 kph. Ground testing on all the aircraft continued for some time. As a result, it was concluded that the strutted parasol layout was impractical and did not ensure the required strength of the construction, especially in the case of the heavy loadings experienced by fighter aircraft. In addition – even though the competition was largely inconclusive, given that none of the fighters met the requirements of Koku Hombu – the NC from Nakajima was found to be the best design. The Kawasaki KDA-3 was runner up, whereas the unfortunate Mitsubishi 1MF2 Hayabusa-go was abandoned.
Nakajima Type 91 monoplane
Despite the inconclusiveness of the ‘fly-off’, the Koku Hombu would not entirely give up the idea of a parasol fighter monoplane, and Nakajima was instructed to develop the basic NC design. The airframe was significantly stengthened and five more prototypes were built in 1929-1931. After the introduction of final modifications dictated by the results of flight tests, the aircraft was eventually approved by Koku Hombu in the fall of 1931 as capable of operational service; intended to replace the Ko 4, the fighter entered volume production under the designation Kyuichi-Shiki Sentoki, Army Type 91 Fighter (the 91 standing for the second part of the design year number according to the Japanese calendar – 2591). The production Type 91s were different from the original NCs chiefly in that they featured a redesigned wing of a smaller lifting surface, a redesigned fuselage, tail unit and wing struts, and a Jupiter VII engine of 450/520 hp enclosed by the Townend ring. In total, by September 1934 the Nakajima factory built 320 such aircraft, designated Type 91-1, plus 22 aircraft of the Type 91-2 version. These were powered by Nakajima Kotobuki 2 engines of 460/580 hp. Additionally, from September 1932 till March 1934 a further 101 (or 115 according to other sources) Type 91-1s were manufactured under license by Ishikawajima.
It should also be mentioned that in 1930, after the NC’s first failure but prior to its redesign, Nakajima purchased a license from Bristol in Britain for the production of the Bulldog Mk II fighter. This classic biplane was being considered as a potential replacement for the Ko 4 in the case of Koku Hombu’s definitive withdrawal from the development of a redesigned NC monoplane. However, upon the approval of the latter for production as the Type 91, further work on the Japanese Bulldog was abandoned after the construction of only two prototypes.
Kawasaki Type 92 biplane
Kawasaki was not discouraged by the lack of success of its KDA-3, and no later than June 1929 Dr Ing. Richard Vogt and Takeo Doi began working on the design of a new aircraft designated KDA-5. This was a light all-metal biplane with fabric-covered wings, open cockpit, and fixed two-wheel landing gear and a tail skid, powered by a BMW VI 6.3 rated at 500/630 hp and manufactured under license by Kawasaki. The aircraft was armed with two 7.7-mm machine guns. Assembled in July 1930, the first prototype attained a maximum speed of 320 kph and a ceiling of 10,000 m in tests. These results exceeded anything achieved by Japanese fighters of the time, and potentially placed the KDA-5 among the best fighter aircraft in the world. Built in January 1931, the second KDA-5 prototype achieved an even higher top speed of 335 kph. Impressed by this performance, Koku Hombu showed much interest in the new Kawasaki fighter and ordered volume production of the machine in January 1932 under the designation Kyuni-Shiki Sentoki, Army Type 92 Fighter. This decision was ratified not only on the basis of the fighter’s high performance (better than that of the only recently approved Nakajima Type 91) but also as a direct result of the military conflict with China developing in Manchuria, which called for a greater involvement on the part of the IJAAF.
In total, Kawasaki built five KDA-5 prototypes in 1930-1931, 180 production Type 92s in 1932 (the designation Type 92-1 can also be found in the literature but the IJAAF did not officially recognize such a version), and a further 200 in 1933. The aircraft from this latter batch, sometimes distinguished from the former by the designation Type 92-2, were powered by the higher-rated Kawasaki-BMW VII of 600/750-800 hp (despite the identical designations, this engine differed from the original German BMW VII). Initial combat sorties in China revealed the Type 92’s poor performance at low-speed (particularly during takeoff and landing) and an array of teething problems, especially in the difficult winter conditions. In addition, its performance was inferior to the latest foreign machines such as the British Hawker Fury or the American Boeing P-26. The outcome was a demand formulated in June 1933 by Koku Hombu tasking Kawasaki with the development of a more modern successor.
Kawasaki Ki-5 monoplane
The initial design of the new aircraft, which Koku Hombu assigned the designation Ki-5 (according to the new designation scheme known as Ki Go), was developed by Takeo Doi under the supervision of Richard Vogt. Following Dr Vogt’s return to Germany further project work was directed by Doi alone. The first of the four prototypes (five according to some sources) to be built was completed in February 1934. This was a machine totally different from earlier Japanese fighters, being an all-metal cantilever low-wing monoplane with partial fabric covering. The wings had an inverted gull configuration. The cockpit was of the open type, and the classic fixed landing gear featured a tail skid. The main wheels were protected by wide fairings. The Ki-5 was powered by the liquid-cooled twelve-cylinder V-type Kawasaki Ha-9-I (aka BMW IX) rated at 720/800-850 hp, a developmental version of the German BMW engine manufactured by Kawasaki. Armament consisted of two 7.7-mm fuselage-mounted machine guns.
All the Ki-5 prototypes, which differed from one another in various constructional details (e.g. wing dihedral angle, pilot’s seat placement, landing gear and radiator construction), underwent exhaustive flight tests. However, in September 1934 the IJAAF stopped further development of the aircraft, as it was considered unlikely to make a satisfactory fighter despite its quite good performance (e.g. maximum speed was 360 kph, ceiling 9,000 m, and the aircraft climbed to 5,000 m in eight minutes). Unfortunately, the Ki-5 had poor low speed characteristics and suffered from engine vibrations. The cooling system was also a source of problems, while maneuverability was judged to be poor. The latter feature was a basic prerequisite with the Japanese air forces. Indeed a fighter that was assessed as lacking in the agility stakes had no chance of gaining the approval of the Koku Hombu..
The biplane prevails
With the abandonment of the Ki-5 project, the Koku Hombu again turned to Nakajima and Kawasaki in the fall of 1934 with a request for a replacement for the Type 91 and Type 92. This time, having learned the Ki-5 lesson, engineers Takeo Doi and Isamu Imachi of Kawasaki returned to a tried and tested configuration. Their new fighter, for which Koku Hombu chose the designation Ki-10, was a classic strutted biplane with an open cockpit and fixed landing gear with a tail skid. It was an all-metal structure with partial fabric covering. Armament consisted of two 7.7-mm Type 89 fuselage-mounted machine guns. The Ki-10 was powered by the liquid-cooled twelve-cylinder V-type Kawasaki Ha-9-IIa (Ha-9-II Ko) of 720/850 hp. Four prototypes of the type were built in the spring of 1935.
The Ki-11 from Nakajima (factory designation PA) was much more competitive, designed by a team directed by Yasushi Koyama and Shinroku Inoue. It was a low-wing monoplane with wire bracing, modeled on the American Boeing P-26. It had an all-metal fuselage and the wings were of mixed, metal-and-wood construction with plywood and fabric covering. The aircraft incorporated a fixed two-wheel landing gear with a tail skid, and featured an armament of two 7.7-mm machine guns. The cockpit was open on the first three prototypes, whereas the fourth was equipped with a rearwards sliding canopy, a quite novel innovation for a Japanese fighter. The machine was powered by an air-cooled nine-cylinder radial, the Nakajima Kotobuki 3 rated at 550/710 hp. Four prototypes were built between April and December 1935; they were subjected to comparison tests with the Ki-10.
Both competing machines began flight tests in mid-1935. The trials were carried out by military test pilots of Rikugun Kokugijutsu Kenkyusho (Army Technical Air Research Institute, contracted as Kogiken or Giken) at Tachikawa near Tokyo. They revealed that although the Ki‑10 biplane was faster than the Kawasaki Ki-5 monoplane, it was still inferior in terms of speed to the Nakajima Ki-11 monoplane. As to the rate of climb, the Ki-10 considerably outperformed the Ki-11. Combat simulations showed that the former design had superior agility, thus easily outperforming its rival in the type of dogfighting favored by Japanese pilots. This feature was the decisive factor, and in September 1935 Koku Hombu chose Kawasaki’s Ki‑10 for production, giving it the official designation Kyugo-Shiki Sentoki, Army Type 95 Fighter. As the result of the contracts that were signed over the following three years, from December 1935 until December 1938 Kawasaki assembled 300 production Ki-10-Is (Army Type 95 Model 1 Fighter) and 280 production Ki‑10‑IIs (Type 95 Model 2).
Kogiken experts drew attention in their test report to the fact that the Ki-11’s Kotobuki engine was of a simpler construction, easier to handle, and less susceptible to failure and combat damage than the liquid-cooled Ha-9 of the Ki-10. The high speed of the monoplane could also be seen as an advantage, even at the cost of a loss of agility, provided that different fighter combat tactics were employed. These essentially comprised a fast diving or climbing attack and an equally fast exit avoiding a close-in dogfight (the so-called “hit-and-run” attack). However, these qualities were only recognized by Koku Hombu and later exploited in the next generation of army fighters, whereas the unsuccessful Ki-11 was to be the spur for a later Nakajima fighter that would enjoy some success. On the other hand, the operational employment of the Ki-10 biplane was undoubtedly a retrograde step in Imperial Army fighter development.
In 1935 another fighter type underwent trials with the IJAAF. Impressed by the performance of the prototypes of Mitsubishi’s Ka-14 fighter, intended as a new shipboard fighter for the IJNAF (9-Shi, later known as the A5M), Koku Hombu ordered one prototype designated Ki-18. It was a cantilever low-wing monoplane of all-metal construction with an open cockpit and fixed landing gear (and tail wheel). Built in August 1935, the Ki-18 was basically similar to the second Ka-14 prototype construction-wise, but its power plant was the Nakajima Kotobuki 5 rated at 600 hp with reduction gear. The engine cowling was wider, and the rudder and wheels were larger. The aircraft was armed with two 7.7-mm machine guns.
“Advanced fighter” competition
During the fall and winter of 1935 the Ki-18 underwent exhaustive trials at Rikugun Kokugijutsu Kenkyusho, Tachikawa, and at the research facility of Rikugun Hiko Gakko (Army School of Aviation) at Akeno in the Mie prefecture. As was to be expected, the Ki-18 – a fast monoplane lacking in agility compared with biplanes – failed to convince conservative Army pilots although Koku Hombu could not have overlooked the growing trend in the aircraft industries of the world to design fast cantilever monoplane fighters of metal construction, featuring retractable landing gear and an enclosed cockpit. With this in mind, no later than mid-1935 Koku Hombu drafted guidelines for a new, “advanced” (or simply monoplane) fighter aircraft boasting high performance (better than the monoplanes tested thus far), while at the same time retaining the requirement for outstanding agility (or at least only slightly worse than the biplanes in service at the time). These specifications were supplied in early 1936 (some sources say about April) to the three biggest Japanese aircraft manufacturers – Mitsubishi, Nakajima and Kawasaki – the factories being expected to each provide two prototypes. The three companies were soon ready with designs, presenting them as prototypes delivered for comparison testing.
First to respond to the new request was Mitsubishi, which in August 1936 built the first of the two Ki-33 prototypes (designated accordingly Ki-33.01). It was designed by Jiro Horikoshi and was a direct modification of the earlier Ka-14 and Ki-18. The reason for this was that Mitsubishi was at the time busy with production for Kaigun Koku Hombu (IJNAF Headquarters) of the A5M fighter and the G3M bomber, and the company’s managers decided that they could not afford to develop an entirely new construction. The Ki-33 differed from the Ki-18 mainly in that it had a new, air-cooled nine-cylinder Nakajima Ha-1a radial (an elaborated Kotobuki) of 620/745 hp, a different engine cowling, slightly redesigned fin and rear fuselage, and a half-enclosed cockpit with a backwards sliding canopy consisting of one top and two side glass panels. It was a cantilever low-wing monoplane of all-metal construction, with fabric control surfaces and fixed three wheel landing gear featuring a tail wheel. The main wheels were protected by fairings. The aircraft’s armament consisted of two 7.7-mm machine guns. The second prototype, Ki-33.02, was completed in late 1936.
The new Kawasaki fighter, designated Ki‑28, was designed by Eng. Takeo Doi. Overall, the idea of this machine resembled the factory’s earlier type, the Ki-5. It was an elegant cantilever low-wing monoplane of all-metal construction and partial fabric covering. The cockpit was half-enclosed by a rearwards sliding canopy. The landing gear was fixed and featured two main wheels with fairings and a tail skid. The aircraft offered a few novel solutions such as manual operation of landing flaps (used for the first time on this company’s fighter) and a manually operated radiator located under the fuselage. Armament consisted of two 7.7-mm fuselage-mounted machine guns. Unlike the other contestants, and typically for Kawasaki, the Ki‑28 was powered by the liquid-cooled twelve-cylinder V-type Kawasaki Ha-9-IIa of 720/850 hp, the same as used with the Ki-10. The two prototypes, Ki-28.01 and Ki-28.02, were built in November and December 1936 respectively.
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