The mid-1930s saw the ever increasing numbers of fast monoplane fighters entering service with many of the world’s air forces.
This was not the case in naval aviation, where biplane designs still reigned supreme. One exception to the rule was the French Navy with its aircraft carrier Bearn and embarked Dewoitine D.371T1 fighters in parasol configuration, hardly a promising design for carrier-based aircraft. Prevalence of biplanes among naval fighters of the time was due to very strict limitations on landing speeds imposed by small flight decks of contemporary aircraft carriers. It was in the middle of that decade, in 1935, that the Mitsubishi A5M entered the stage – a low-wing, monoplane carrier-based fighter, which set new standards for aircraft of its class. Having said that, the Claude wasn’t the first fighter in this configuration designed for the Imperial Japanese Navy.
A5M was born
In the early 1930s the Imperial Japanese Navy Air Service underwent radical overhaul which included the launch of a special aircraft design development program, championed by admirals Matsuyama and Yamamoto. The program’s goal was to build an indigenous and fully independent aviation industry to fulfil the Navy’s needs following lessons learned in the Sino – Japanese conflict of 1932.
The first and foremost goal of the program (designated 7 Shi) was to deliver new designs for the IJN based on the technical requirements provided by Kaigun Koku Hombu in several basic categories of carrier-based aircraft: fighter, dive bomber, torpedo bomber and reconnaissance floatplane. Official requirements set the bar very high for new aircraft designs, which were expected to be superior to the most advanced machines in each category in service with foreign navies. In order to ensure success of the program, Japanese aviation industry received the highest priority status and generous government funding.
Required specifications for the carrier-based fighter under the 7 Shi program were as follows: maximum speed at 3,000 m – from 335 to 370 km/h; time to climb to 3,000 m – no more than four minutes; wingspan – not exceeding 10.25 m. The latter was dictated by the dimensions of aircraft elevators used on IJN carriers. The future aircraft was to be a successor to the Nakajima A1N fighter – the mainstay of the Japanese carrier aviation at the time. Nakajima and Mitsubishi were approached to submit their bids in the design competition, with the winner to receive a contract for a full-scale production.
For Mitsubishi the competition was of vital importance as it offered the company a chance to regain its status of the lead contractor to the Imperial Japanese Air Service which it had lost when Nakajima A1N entered service. As soon as the formal invitation was received Mitsubishi management formed a design team led by Jiro Horikoshi, who had been working for the company since 1928. Horikoshi’s design was to vindicate Mitsubishi’s loss of prestige and bring the company good fortune. Horikoshi had spent years studying in Europe and in the USA, where he gained invaluable insight into the most advanced aircraft design technologies. Although still a relatively inexperienced designer, Horikoshi wasted no time and quickly produced several preliminary design concepts of biplanes and high-wing monoplanes, but in the end he settled for a modern cantilever low-wing monoplane, since only that configuration could possibly achieve the required speed performance.
The new aircraft was visually similar to the Boeing P-26 – a cantilever, low-wing monoplane with elliptical inverted gull wing. The low-drag airfoil was also relatively simple to manufacture. In order to minimize the wing’s bending moments in recovery from dives, the American M-6 airfoil was used, which later became a staple of many Japanese designs.
Fabric-skinned fuselage featured a welded tube frame. The wing’s structure was also made of metal and skinned with fabric, which was tauter than fuselage skin. This arrangement was “borrowed” from the French Dewoitine D1C1 fighter, which the Japanese acquired in 1924. The aircraft featured fixed landing gear with spats. The new fighter was to be powered by the 700 hp Mitsubishi A-4 engine and armed with two 7.7 mm Type 89 machine guns mounted on top of the engine. The machine received its official designation “Experimental Navy Fighter 7 Shi” and factory designation 1MF10.
Horikoshi and his team ran into serious difficulties designing the prototype. Although the machine was ready in February 1933, it quickly became apparent that it fell short of the expectations. In relation to the wingspan the fuselage was too long and the airframe was in need of aerodynamic refinement. Despite those obvious shortcomings, the prototype began its flight test program just a month later with Yoshitaka Kajima serving as the program’s test pilot.
During tests the machine demonstrated a time to climb to 3,000 m of just over four minutes and a top speed at that altitude of 320 km/h. This less than impressive performance was mainly due to huge amounts of drag produced by the bulky radial and its cowling. Poor choice of wing fabric skins, underpowered engine and poorly designed propeller didn’t help either. In addition, flight test program revealed worrying stability issues and poor forward visibility from the cockpit, which all but disqualified the machine as a carrier-based fighter.
Comparative tests of the NK1F (Nakajima’s entry into the program – a mixed construction fighter in parasol configuration with a monocoque all-metal fuselage and fabric-skinned wings and tailplane) and the Mitsubishi fighter proved that both machines had various design flaws. The 1MF10 was 24 km/h faster than its competitor, but in the end it didn’t matter much: Kaigun Koku Hombu rejected both projects since neither of them fulfilled the 7 Shi requirements.
Undeterred, Mitsubishi continued the flight test program of their fighter. In June 1933 the aircraft’s vertical stabilizer broke in flight during a recovery from a dive forcing the pilot to bail out. Amazingly, the aircraft didn’t crash, but glided back to the airfield for a more or less uneventful landing and suffered only minor damage. Following the incident the vertical stabilizer was strengthened and its fabric skins were made tauter, but that didn’t prevent the very same failure from happening again during another test sortie.
To further experiment with the new fighter’s design Mitsubishi built a second 1MF10 prototype with redesigned landing gear featuring carefully streamlined legs. The machine completed a full flight test program, but eventually proved to be a disappointment with its top speed 48 km/h below the requirements. In July 1933 the aircraft was lost in a crash after it had entered an unrecoverable flat spin forcing the pilot, Lt. Motoharu Okamura, to bail out. Finally the unsuccessful 1MF10 project was abandoned, but Horikoshi used the lessons learned in designing the machine in his work on the highly successful A5M fighter.
9 Shi requirements – Nakajima A5N1
It appeared that all hopes for a quick acquisition of a new fighter and modernization of Japanese carrier aviation were lost with the fiasco of the 7 Shi program. Trying their best to save the day, Kaigun Koku Hombu contracted Nakajima to deliver a modernized version of the A2N fighter, which became the A4N – hardly an improvement on the original. The new fighter was indeed faster and more maneuverable than its predecessor, but its performance was still a far cry from what the Navy was hoping for. Nonetheless, reeling from the failure of the 7 Shi program and trying to respond to growing tensions in China, the Navy brass gave a green light in January 1936 for a full-scale production of the A4N.
Following the unsuccessful 7 Shi program Koku Hombu published new requirements (8 Shi), which were once again submitted to Mitsubishi and Nakajima in 1933. The program spawned a pair of two-seat biplane fighters: Nakajima NAF-2 whose prototype was delivered in March 1934 and Mitsubishi Ka-8 (first prototype built in January 1934). Not surprisingly, both fell short of the Navy’s requirements. Even the Nakajima A2N fighters already in service had better performance characteristics than either of the proposed machines and what the Navy was looking for was a modern fighter of a significantly better performance.
Koku Hombu, struggling to secure locally-manufactured modern carrier-based fighters, was forced to adapt stop-gap measures, such as continuing production of obsolete Nakajima A2N aircraft and its upgraded A4N version. That, however, couldn’t go on forever. In February 1934 a new set of requirements was published – 9 Shi, which was mainly the work of Lt.Cdr. Hideo Sawai. Yet again Mitsubishi and Nakajima were invited to take part in the program.
This time the requirements were fairly straightforward and included little more than expected top speed, climb rate, fuel load, armament, types of radios to be used and dimensions dictated by the size of aircraft elevators. Interestingly, the program specified a single-seat fighter aircraft, rather than a carrier-based type, which freed the designers to build a machine without the constraints inherent to aircraft specifically designed for carrier operations. The idea was that the new design was supposed to be first and foremost an aircraft of superb performance, which then could be adapted as necessary to carrier operations. Lt.Cdr. Sawai firmly believed that the only way forward in aircraft design was to focus on the machine’s tactical characteristics, without imposing unnecessary limitations of weight, particular technological features or powerplants. This was a radical departure from a standard approach in which the designers’ ingenuity was often stifled by such limitations. Thus the 9 Shi requirements contained little more than expected performance parameters: top speed at 3,000 m – 350 km/h; time to climb to 5,000 m – no more than 6.5 minutes; internal fuel – no less than 240 l; armament – two 7.7 mm machine guns; communication radio; wingspan – no more than 11 m; length – not exceeding 8 m.
Mitsubishi’s entry into the program was PA-Kai, also known under its military designation “Experimental Single-Seat Navy Fighter 9 Shi” (A5N1). Nakajima’s entry into the earlier 7 Shi competition was a carrier fighter design based on the company’s Type 91 machine built for the Army. Nakajima used the same tactics in the single-seat fighter program submitting the aircraft based on their Ki-11 fighter designed for the Imperial Japanese Army.
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