Chapter 2

Striving for the Impossible

Mitsubishi’s Chief Designer, Horikoshi, had wide experience of the military fighter field, having worked at cutting-edge aeronautical companies in Germany, the United Kingdom and the United States, in addition to which he could draw on his own personal experience of having designed the most successful Type 96, which was still the principal Japanese interceptor. In the latter aircraft he had the advantage of on-going military combat against the latest foreign types against which to evaluate his ideas for the new aircraft that the Navy demanded. Continual information from the front line was fed in and the Type 96 was constantly being modified and amended in the light of this war experience. Nobody in the world, therefore, was better placed to tackle the task, than Horikoshi and his team. He espoused the view, which was generally held to be correct for an era of rapid change, that no matter how innovative a fighter aircraft design was, it would be outmoded by rapid progress; his estimate was this process would only take four years even in times of peace when there were no pressures from the front, but that this would reduce to a mere two years under the hothouse of war conditions. It was an excellent estimation for indeed two years was to prove almost exactly the period of time that the future A6M was to totally dominate the Pacific air war without serious challenge. With the China ‘Incident’ raging on without any end it sight, it was under war conditions that the new Prototype 12 was to be finalized.

Horikoshi has recorded how he initially concentrated on the power plant for the new fighter; this being the area that he knew would most restrict his airframe design as the choices were so limited.¹ Initially then the choice that confronted Horikoshi was a straight choice between his own company’s Kinsei Type 46 engine and the Zuisei Type 13. The Model 14 Zuisei (‘Holy Star’) was developed in 1931 as a 14-cylinder, supercharged, air-cooled, double-row radial with various marks, typically rated at 1,080hp (805kW), 2,700rpm for take-off. It weighed 1,190lb (540kg). The Kinsei (Venus) was a 1934 developed Navy engine also a 14-cylinder, air-cooled, double-row radial, with a dozen marks, rated at 1,075hp (802kW) at 2,500 rpm at 5,560ft (2,000m). It had a weight of 1,202lb (545kg).²

To choose the Kinsei would give the design an immediate power boost, but being larger it was therefore heavier and took up more space, which impacted the size of the airframe, wing and tail size and fuel considerations. This would result in an estimated 6,614lb (3,000kg) aircraft, entirely unsuitable to the Navy’s specification. To opt for the smaller Zuisei would reduce these dimensions and give a smaller, lighter aircraft, a more acceptable 3,527lb (1,600kg) weight, far more suitable for carrier operations and closer to existing Navy fighters and would also increase mobility. Eventually the weight factor won and Horikoshi accepted the loss of power that would result from this choice. Dr Den-ichiro Inouse set up a special design sub-team to deal with the problems of propulsion, which were far from settled yet. Lack of power was to ever dog the A6M’s otherwise brilliant career, more so later when increases in performance were constantly required to counter newer Allied types as the Pacific War progressed.

Having made that decision Horikoshi then made outline sketches that incorporated his thinking about streamlining, which meant a fully retractable landing gear for low drag,³ but more weight, a new low-wing planform with a straight taper and narrow tip replacing the currently fashionable elliptical shape with matching horizontal tail. The wing format was crucial as it had to be thick enough to accommodate port and starboard integral fuel tanks and also the 20mm cannon. The 7.7mm machine-guns were housed in the engine cowling and were based on a British Vickers design being licence-built, each LMG having a 680rpg capacity and an effective range of about 600 yards (548.64m). Being so close together limited their effectiveness as to their cone of fire, however, and there were vibration problems. The low wing loading was essential in ensuring the design’s superior turn radius was achieved and was aided by the large ailerons. The actual upper surface of the wing formed the cockpit floor giving the aircraft considerable integral strength and also aided ease of construction and maintenance. Horikoshi consulted with his team. Sadahiko Kato was assigned design of the retractable undercarriage and stowage; to Yohimi Hatakenaka was given the task of planning the 20mm cannon mountings and Yohitoshi Sone was allocated to sort out the wing dimensions and structure. Having decided on these basics with his team, during the last months of 1937 Horikoshi wrestled with how to fit in all the Navy’s requirements to this outline design. He was then only just recovering from stress through overwork and the problems seemed insurmountable.

On 17 January 1938 a joint Navy/Company meeting was held at Yokosuka Naval Aeronautical Establishment with representatives of both Nakajima and Mitsubishi, the latter including the Project Head, Jyoji Hattori, as well as Jiro Horikoshi, Yoshitoshi Sone and Sadahiko Kato on the one hand, and the Navy representatives, all under the auspices of Vice-Admiral Koichi Hanajima, the director. Present were Lieutenant-Commander Misaro Wada, Head of the Fighter Section, and a score of leading naval aviation experts, including leading fighter aces from the China front line, including Lieutenant-Commander Minoru Genda who was serving on the Aviation Staff with the Second Combined Kōkū Sentai. The latter gave graphic descriptions of the air fighting then underway while the senior officer emphasized the probable future direction and extension of the war, the deterioration of the international situation⁴ and the urgent need for the new fighter. When asked for designer opinions Horikoshi requested that at least one or more of the extreme requirements be withdrawn, but was met with a steely response – there was to be no compromise on the Navy’s part and this he was forced to accept. Genda himself, many years later, admitted that even he had reservations about how ‘… the impossible, incompatible, requirements …’ could be met.

As we have seen the result of this conference was that Nakajima threw in the towel, but the reaction of Mitsubishi was sombre determination. A five-section design team, covering computing, structure, propulsion, armament and landing gear, was immediately established under Hattori and Horikoshi to push forward with Prototype 12. The latter spent the next ten months agonizing about how to get the lowest weights and the best performance from his outline design. In that period more than 3,000 detailed drawings were made, checked, modified and approved. The engine for the prototype being agreed upon, he applied himself to the propeller and decided that, as with most things associated with this aircraft, a radical approach was required. It was decided to adopt a constant-speed unit, whereby instead of having fixed pitch propellers automatically adjustable ones were used that lowered the blade angle at low speed and increased it as speed rose, changing the pitch by way of a governor to meet the continually changing speeds encountered in operations, and in fighter combat conditions in particular. This was one area were Japan was deficient and Horikoshi revealed that the Navy ‘… pressured industry to import engineering know-how from the United States’. This was done and as a result, although the early A6M had a two-bladed variable-speed propeller, when these aircraft later suffered from vibration problems in service, subsequent marks used the three-bladed variable-pitch metal Hamilton Standard Constant Speed propeller, which was built under licence by the Osaka-based Sumitomo Metals.

Dr Isamu Igarashi and Engineer Yasuyuki Ozeki of this same corporation had developed a new lighter-weight but higher-strength zinc aluminium alloy (T-7178), which was dubbed Extra Super Duralumin (ESD) and patented in 1936. It was a decade ahead of western practice, although, again, many American historians claim that Japan merely copied US materials. It was, in truth a development of a German invention used in the construction of frames for Zeppelin Airships in World War I. One of these airships was shot down near London and a Japanese military liaison officer stationed there sent a fragment from the wreckage back to Japan for it to be analysed.⁵ After receiving Navy approval, Horikoshi eagerly incorporated this metallurgical advance into his design, not for ‘skinning’ but principally for the main wing sparring and internal forgings.⁶ The designer estimated he saved over 66lb (30kg) of weight with this material.

This innovation was a blessing to the A6M designers when weight considerations were paramount throughout every stage and aspect of the project and a constant headache for the Mitsubishi team. Again normal usage had to be thrown aside and even the 1932 established Summary of Airplane Planning went by the board, with Horikoshi opting for a stress safety multiple of 1.6 times the overall aircraft’s maximum load weight with certain members constructed as ‘slender’ units instead of ‘stubby’ units, rather than the 1.8 factor laid down as standard. Where it did not affect strength detrimentally, widespread use of ‘meat trimming’ was practised, whereby surplus material was bored out of solid fitments. The use of flush riveting on the semi-monocoque structure combined minimum weight with a aerodynamic airflow, which along with the fully enclosed cockpit and retractable undercarriage, folding inward from a wide base, and even the tail-wheel retracted into a recess under the tail-plane, all made agility in the air machine. This hydraulic tail-wheel retraction gear was prevented from providing egress to dust and debris from land-based operations, with the provision of a canvas cover. Both the Mitsubishi- and the Nakajima-built A6Ms utilized the same design for this cover, which was in two parts to allow free movement of the wheel. Whereas the Type 11s, being land based were rated as ‘Base Defence Fighters’ the Type 21s were pure and simple carrier-based fighters. With these aircraft destined principally for sea service all were in-built designed to be fitted with a ‘stinger’ tail hook, located just ahead of the tail-wheel.

The A6M1,⁷ due to its lightness, although an incomparable mount in turning combat, was notoriously slow into a dive, and this aroused much criticism post-war from armchair warriors warring from behind their PCs. They accused the designer of an oversight here. In fact, Horikoshi examined this in detail when designing the wing form and area. He wrote:

… I decided to provide an ample wing area with a wing span of almost twelve meters, the maximum allowable according to both our experience and foreign data. This permitted me to achieve the Navy’s requirements with respect to maneuvering, turning radius, landing and takeoff performance, but it reduced the aircraft’s performance with respect to diving, level flight speed, and lateral maneuverability, because of the increased drag and weight of a larger wing. On balance, I decided to proceed with this plan, since turning radius and takeoff performance had a higher priority.

He planned to compensate for the speed issue through the maximum elimination of drag factors, as described above. The A6M’s clean lines were an indication of his care. The old maxim that an aircraft looked right then it flew right, applied. The wings had a slight dihedral but the cigar-like fuselage was unmarred save for the for oil cooler air-scoop. In planform the leading edge of the wing tapered more than the trailing edge, while the pilot’s view was excellent with a greenhouse style canopy with sliding access that sat above the fuselage giving all-round vision. Behind the (unprotected) pilot’s seat was the Direction Finding Antenna loop to enable him to ‘home-in’ on his parent aircraft carrier in the wastes of the Pacific and he also had a radio compass to further aid ocean navigation. The pilot’s fresh air intake was via an elliptical intake located at the starboard wing root, but from airframe #227 onward it was changed to become a smaller square aperture for ease of construction. The carrier-borne A6M pilots were also equipped with both parachutes and life rafts.

Two more controversial features were the large vertical and horizontal tails, which tapered to a point to aid airflow; these added extra weight but were considered essential to give lateral and longitudinal stability. A lengthened fuselage also assisted pilot control when firing the 20mm cannon. The square cannon apertures in either wing were overlarge to minimize blast damage and had curved corners, but the subsequent drag effect resulted in them being reduced in size and made circular, becoming flush with the leading edges of the wing from airframe #326 onward and they were retrofitted on some earlier aircraft. On the credit side the innovative ‘wash out’, a subtle downward twist to the tapered wings leading-edge extremities, was carried over from the Type 96 design to reduce stall at high angles of attack.

The incorporation of the wings into the fuselage structure and constructing them as one overall unit rather than in outboard and inboard sections, gave added strength here in compensation. Here Horikoshi acknowledged that although he had considered using it earlier, he was following Nakajima’s lead here with their Type 97, Ki-27 fighter design.⁸ The most glaring omissions to western minds was the omission of armour protection for the pilot and self-sealing fuel tanks, but neither factor had been asked for by the combat pilots themselves, nor in the Navy’s specification. It ran against their offensively orientated mind set. However, the war was to show that these increased the A6M’s vulnerability, although even Allied aircraft with self-sealing tanks frequently failed to survive the experience of taking cannon shells into them. In fact the Mitsubishi team did give serious consideration to fitting such tanks in the wings, but rejected it because the existing aluminium tanks could hold more fuel. To give the Prototype the extra range demanded a streamlined low-drag auxiliary fuel tank to be attached below the fuselage as had been done with the Curtiss Hawk III and was first used in combat by the Luftwaffe’s Kondor Legion during the Spanish Civil War.

Even with these radical measures Horikoshi and the Prototype team, under Naokazu Yui, still, in March 1938, lacked confidence that they could meet the original Navy specifications. The team returned to Yokosuka once more on 13 April and when Horikoshi revealed his continual worries about which aspect, fighting agility, speed or range, should take priority, it resulted in a fierce argument between Naval Aviation representatives, Lieutenant-Commander Minoru Genda arguing for the former to be stressed, while Lieutenant-Commander Takeo Shibata was equally as vehement for the latter attribute to be emphasized, arguing that combat fighting could be improved by training but speed and range could not. There seemed no possible compromise in the two viewpoints and Horikoshi reached the conclusion that only by adopting a much improved power plant and bringing in the adoption of the constant-speed propeller could he even begin to please both requirements.

The full-scale wooden mock-up was already completed at the Oye-cho Minatoku, Nagoya plant, as per normal practice in those days, with dummy engine and guns emplaced. This was duly inspected by a Navy team under Captain (later Rear-Admiral) Sunichi Kira, and included the irascible Shibata; the design was criticized for its size with a further 100 amendments being recommended. With that, work commenced on the first two actual Prototype 12s themselves. Further theoretical criticisms of the design were received from front-line aviators in China, which led to yet more heart-searching. The 20mm cannon were criticized for their low muzzle-velocity, their limited magazine capacity and low hitting rate. Also criticized was, in an apparent volte-face, the long-range and, again, the physical size of the new fighter. Lieutenant-Commander Eiichi Iwaya, controller of fighter production at the Tokyo Naval Aeronautical Headquarters, consulted with Horikoshi and the latter worked hard on an alternative design to meet these requests, but, fortunately, no more was heard of it and work on the Prototype 12 continued unchanged. The first test airframe was duly delivered (in two segments) by rail to the Aeronautical Establishment to be expended in a series of trials to destruction. Known as the Naval Experimental 12-Shi Carrier Fighter, this aircraft was to revolutionize air warfare being the first carrier-based fighter to totally out-perform all her land-based opponents. Until the advent of the A6M it was an article of faith among air forces that naval aircraft would always be inferior, due to the demands of ship-born operations in the case of the United States and Japanese navies, and total neglect by their RAF masters in the case of the British FAA. The A6M turned this ‘given’ on its head.

Under the auspices of Dr Kiyoshi Matsudaira and Engineer Tsugumaro Imanaka, the results of the vibration tests were pronounced as very good as were the wind tunnel and flutter tests figures with stress tests to follow. The only modifications insisted upon at this stage were that the tail should be re-positioned further aft; with a small ventral fin added to the Prototypes in the interim. Meanwhile, the completion of the first Prototype led to its own inspection on 17 March 1939. Done on site by a Navy Inspector, the principal concern for Horikoshi was, as always, the weight factor. The aircraft had already been criticized purely on size, but weight was crucial. In fact, the aircraft came in at 3,452.2lb (1,566kg) against the original estimate of 3,382lb (1,534kg), but this extra 342lb (55kg) could be explained away by the excess weight of the engine, propeller and landing gear, all of which had been sourced by the Navy itself, and therefore the Mitsubishi team were thus fully exonerated. Horikoshi revealed that when it came to the first Prototype’s initial flight from Kawasaki’s Kagamigahara, Gifu Prefecture, on 23 March 1939, the weight came out at 3,571lb (1,620kg), an excess of 190lb (86kg), but with the same unavoidable deductions applied the Navy still considered this satisfactory. On 1 April 1939 the Prototype A6M1 Type 0 carrier fighter made its first official test flight.

Some indication as to why westerners believed that no such aircraft could possibly be designed and built, let alone operated by a nation such as Japan, (her acknowledged military and naval humiliation of both China and Tsarist Russia just a few decades earlier, notwithstanding), can be gleaned by the mode of transportation of this high-tech machine from plant to the closest flight test ground. Disassembled into two sections, the transportation was conducted by ox-cart over unpaved roads! The A6M was to retain this detachable rear fuselage feature in order to facilitate transport but also to simplify maintenance. It was found that this useful function had the drawback of entailing damage to the glazed rear segment of the cockpit canopy on occasions. To overcome this, a totally metallic after segment was substituted from airframe #47 onward. The date of the first test flight was 1 April 1939 and Kumataro Takenaka ensured all was ready before being joined by many of the design and construction teams. Two test pilots were on standby for this flight, Harumi Aratani, a graduate of Tokyo Engineering University who had undergone Navy flight training, and the more experienced Katsuzo Shima, a retired Navy Petty Officer with a long track record of successful flight testing at the Aeronautical Establishment. In the end Shima was selected for the initial ground testing and brief ‘jump’ aerial flight. On conclusion Shima expressed satisfaction with the control surfaces, but dissatisfaction with the machine’s braking system. Further flights with the undercarriage down were carried out by both test pilots duly over the next twelve days, overseen by Commander Satoshi Imada and Lieutenant-Commander Hiroyoshi Nishizawa. Vibration was experienced in the climb, level flight and gliding with the engine shut down. This problem remained in subsequent flight testing with the undercarriage fully retracted; this was found to be due to resonance and a change from a two-bladed propeller to a three-bladed one in tests conducted on 17/18 April reduced this vibration by 50 per cent. A second difficulty involved the elevators, which proved to be over-efficient at higher speeds. This Horikoshi solved by a major re-design aimed at reducing the stiffness in the control system and introducing more springiness, while at the same time reducing the weight of the control cables without sacrificing strength. Extensive wind tunnel testing revealed the validity of this solution, which was adopted forthwith.⁹ On 25 April the aircraft was laden to a weight of 5,139lb (2,331kg), the planned regular operations norm, and flight tested. The recorded speed was 304.47mph (490km/h), which Yoshimura stated was less its true speed by about 11.75mph (18km/h) due to a faulty Pitot tube placement.¹⁰ If true then the planned-for speed of 316.68mph (500km/h) was actually exceeded that day, a highly satisfactory result.

As part of the solution, the Navy had granted permission for the Zuisei engine fitted in the first two A6M1 aircraft with a Nakajima Sakae Type 12, Navy designation NK1C, for the third test model. The Sakae (‘Prosperity’) was a 950hp, double-row, 14-cylinder, air-cooled radial engine licence-built from the French Gnome-Rhone 14k. With this power plant the aircraft was expected to easily meet and exceed the original IJN 12-Shi specifications

Meanwhile, with the first Prototype there still remained issues with the stiffness of the ailerons, however, and the Navy pressured Mitsubishi to solve these problems quickly. After adjustments to the control system flight testing by four different test pilots, Harumi Aratani, Seiichi Maki, Chujiro Nakano and Katuzo Shima, on 6 July, improvements to the elevators were shown, but the trials continued. On 23 and 24 August and again 10 September, yet further flight tests were conducted at Kagamigahara and, after minor tweaking, the condition was deemed satisfactory. Speeds of 316.28mph (509km/h) were recorded at 11,811ft (3,600m) altitude. The two A6M1 aircraft (Serials #201 and #202) remained the only ones of their type.

The arrival of the Sakae engine was accompanied by other modifications and ultimately brought about a change of designation to A6M2. The new prototype machine was duly fully successfully flight-tested at Kagamigahara field on 5 June in front of Vice-Admiral Koichi Hanajima by test pilots Aratani and Shima. Further tweaking of the new control system, and the replacement of the first engine with a more reliable one of the same type, followed and on 6 and 7 July Navy test pilots Lieutenant-Commander Chujiro Nakano and Lieutenant Seiichi Maki joined the flight testing. As a result all further attempts at using the two-bladed propeller were abandoned. A total of forty-seven test flights were carried out at this time, followed by further official tests on 23/24 August and a final Acceptance Flight was concluded on 13 September. Total flight-testing for the Zero was therefore only 43 hours and 26 minutes over 119 flights which, considering the revolutionary aspect of the aircraft, was quite impressive. Service testing of various aspects continued under the auspices of Lieutenant Mambeye Shimokawa of the Yokosuka Kōkūtai. Thus, after ground tests and 215 flight tests, the Prototype 12-Shi was formally handed over on 14 September.

Demands from the China front continued to be received and continued to be the same conflicting pulls between manoeuvrability and range, with the latter now predominating as the Chinese Air Force had unilaterally withdrawn from engaging regular fighter combat at this time. Still, the overriding call was for the new fighter to reach the front as urgently as possible. The Naval Aeronautics Headquarters reacted to this by initiating a plan to introduce at least some preliminary A6Ms into the combat zone ahead of schedule. The tests of the second Prototype 12 followed at Kagamigahara on 25 September and at Yokosuka from 18 October.

On 18 and 19 January 1940 the third Prototype was flight-tested with the Sakae-12 engine. As predicted, results were highly satisfactory, and testing continued at Yokosuka from 25 January onward. Three more aircraft followed, each with minor changes, many involving the introduction of the constant-speed propeller.

However, plans for early introduction into service were threatened when, on 11 March 1940, the second prototype, carrying out a series of dives to test the constant speed propeller’s variable pitch operation, with experienced test pilot Masumi Okuyama of the Establishment, had apparently disintegrated in mid-air over Oppama field. According to eyewitness reports there was an explosion at an altitude of 16,460.5 to 1,312ft (500–400m), which blew the wing off, the engine and propeller separated and the fuselage disintegrated. Although Okuyama successfully baled out at 1,312ft to 984ft (300–400m) he became separated from his parachute and fatally plunged into the shallow waters just off the coast. On the conclusion of the prolonged Naval Aeronautical Engineering Establishment (as it had now become) trials and testing, the Navy concluded that the mass balance weight (mbw – which was an adjustable attachment to the control surface of an aircraft in an effort to eliminate or reduce flutter) attached to the elevators had become fatally weakened by repeated operational shocks and had sheared off, causing uncontrollable elevator flutter and, in turn, terminal vibration through the airframe. Measures were taken to redesign and strengthen this area, but it was to result in an inevitable delay to the operational use of the aircraft, which was postponed to July by which time the name Reisen had started to come into general usage in Japan itself, while Zero was beginning to be accepted elsewhere.