Precursor to the Zero – The Mitsubishi A5M

With the A5M Japan moved from dependence on Western imports (aircraft, aircraft components, design concepts and designers) to a completely indigenous product in every way the equal of the best of its Western equivalents. The origins of the type can be found in the April 1932 Continuous Development programme created by the Imperial Japanese navy air force for the re-equipment of its units with modern warplanes of Japanese design and manufacture. One of the first requirements was an advanced carrierborne fighter, and Mitsubishi and Nakajima responded with a low-wing monoplane and parasol-wing monoplane respectively. Neither design fully met the requirement, however, and the service therefore ordered the Nakajima A4N, developed from the current A2N carrierborne fighter, as an interim type pending the development of an advanced type to an updated requirement issued in February 1934.

Mitsubishi entrusted the task of designing its contender to a team led by Jiro Horikoshi; the result was a low-wing cantilever monoplane of all-metal construction with an inverted-gull wing and fixed tailwheel landing gear including faired and spatted main units as well as an arrester hook for carrier compatibility. Maximisation of performance was aided by minimisation of drag through the use of the smallest possible airframe with a stressed skin of flush-riveted light alloy.

An A5M fighter, from the Akagi carrier, in flight in 1938 or 1939

Prototype revisions
The Ka-14 prototype recorded its maiden flight on 4 February 1935 with the Nakajima Kotobuki 5 air-cooled nine-cylinder radial engine rated at 550 hp (410 kW) for take-off. The prototype exceeded all estimates for performance, but was found to suffer from longitudinal instability and a tendency to balloon on landing. The second prototype was therefore revised to a somewhat different standard with the Kotobuki 3 engine rated at 640 hp (477 kW) for take-off, and a revised wing with a flat centre section carrying trailing-edge split flaps and dihedralled outer wing panels carrying ailerons. There followed another four prototypes based on the second machine, and these differed from each other minor details as well as being tested with different engines. Service trials confirmed that the basic aeroplane was excellent, and the Ka-14 was ordered into production with the Kotobuki 2 Kai 1 engine rated at 580 hp (432 kW) for take-off.

This initial production model was known by the short designation A5M1, and more fully as the Navy Type 96 Carrier Fighter Model 1. The A5M1 entered service in the spring of 1937, and proved both popular and successful. The type was dimensionally identical to the A5M4 with the exception of its length of 25 ft 3.5 in (7.71 m), and otherwise differed from the last variant in its empty weight of 2,370 lb (1075 kg), maximum take-off weight of 3,307 lb (1500 kg), maximum speed of 219 kt (252 mph; 405 km/h) at 6,890 ft (2100 m), and climb to 16,405 ft (5000 m) in 8 minutes 30 seconds.

The A5M1a was a single prototype with the armament revised to a pair of 20 mm Oerlikon FF fixed forward-firing cannon.

Reversion to an open cockpit
By the time it was first committed to combat during the early stages of the 2nd Sino-Japanese War (1937/45), the A5M1 had been replaced in production by the A5M2a (Navy Type 96 Carrier Fighter Model 2-1). This differed from the A5M1 mainly in its Kotobuki 2 Kai 3A engine rated at 610 hp (455 kW) for take-off, and was itself soon supplanted in production by the A5M2b (Navy Type 96 Carrier Fighter Model 2-2) with an enclosed cockpit and the Kotobuki 2 Kai 3A engine driving a three-blade metal propeller and installed inside a NACA ring cowling with cooling flaps. The revised engine installation improved the pilot’s fields of vision, but the enclosed cockpit was heartily disliked by Japanese pilots who often removed the canopy and were happy when late-production aircraft were completed with the original type of open cockpit.

All three A5M variants were used in the first part of the 2nd Sino-Japanese War, and were largely instrumental in gaining complete Japanese air superiority, thereby giving the Japanese bombers the freedom to roam and attack wherever they wanted. Moreover, the Japanese still took the fighter war to the Chinese, establishing intermediate bases between Shanghai and Nanking at which fighters could refuel and rearm before patrolling deep into areas once thought safe by the Chinese.

The A5M3a was a single prototype with the Hispano-Suiza 12Xcrs liquid-cooled V-12 engine rated at 610 hp (455 kW) for take-off and fitted with a 20 mm Hispano-Suiza HS-404 cannon in a moteur-canon installation to fire through the hollow propeller shaft. The combination offered marginally higher performance and considerably greater firepower, but was not ordered into production as agility suffered from the additional weight of the cannon and the liquid-cooled powerplant.

Lengthened range
Despite the adoption of the intermediate landing field system, the Imperial Japanese navy air force decided that a longer-range version of the A5M2 would be an operational advantage. Mitsubishi responded with the A5M4 development of the open-cockpit A5M2b with the Kotobuki 41 engine rated at 710 hp (529 kW) for take-off and driving a three-blade metal propeller. The additional power improved the A5M4’s performance, and range was improved by provision for one 35.2 Imp gal (42.25 US gal; 160 litres) drop tank. The A5M4’s full designation was Navy Type 96 Carrier Fighter Model 4, later changed to Navy Type 96 Carrier Fighter Model 24 when a new system of identifying airframe and engine modifications was introduced. The A5M4 was built in larger numbers than any other A5M variant, and began to reach China-based Japanese air units in 1938. The new model’s capabilities soon forced the Chinese into a further withdrawal of their already hard-hit fighter units.

The A5M4 remained in production up to 1940 at Mitsubishi’s Nagoya plant, where 783 A5M fighters were built, and the last aircraft from this source had the full designation Navy Type 96 Carrier Fighter Model 34 as they had the improved Kotobuki 41 Kai engine as well as a number of minor alterations found advisable as a result of operational experience. Another 200 A5M4 aircraft were delivered by K.K. Watanabe Kekkosho, which completed 39 machines between 1939 and 1942, and by the Dai-Nijuichi Kaigun Kokusho (21st Naval Air Arsenal) at Omura, which completed 161 machines between 1939 and 1941. This ended production of the A5M fighter at a total of 982 aircraft.

The A5M was phased out of service in favour of the Mitsubishi A6M Reisen ‘Zero’ during 1941, but the Allies wrongly thought that the type still formed the bulk of the fighter force available to the Imperial Japanese navy air force and allocated the type the reporting name ‘Claude’, with ‘Sandy’ reserved for the imagined production version with an inverted-gull wing. In fact the A5M was still serving in a first-line carrierborne capacity only on the carriers Hosho, Ryujo and Zuiho, which were tasked with the provision of fighter cover for bombers attacking the Philippines from land bases on Formosa (now Taiwan). The A5M was not needed in this task, however, for its astounding range allowed the more advanced A6M to undertake the task. Only a very few operational missions were flown by the A5M in the Pacific War (1941/45) of World War II, therefore, and most of the surviving aircraft were based in Japan for use as trainers and, if required, emergency home-defence fighters. In 1944, the last surviving A5M4 fighters were expended in kamikaze attacks on Allied warships operating off the coast of the Japanese home islands (see post on Japanese defence of Iwo Jima).

The A5M4-K was an advanced trainer version of the A5M4 without wheel spats fighter, with small longitudinal stakes on the sides of the rear fuselage to improve spin recovery characteristics, and the central fuselage revised to provide a separate instructor’s cockpit. Production of this Navy Type 96 Carrier Fighter Trainer Model 24 amounted to 103 aircraft delivered between 1942 and 1944.

Specification

Mitsubishi Navy Type 96 Carrier Fighter Model 24 (A5M4)

Type: carrierborne and land-based fighter

Accommodation: pilot in the open cockpit

Fixed armament: two 0.303 in (7.7 mm) Type 89 fixed forward-firing machine guns with 500 rounds per gun in the upper part of the forward fuselage with synchronisation equipment to fire through the propeller disc

Disposable armament: up to 132 lb (60 kg) of disposable stores carried on two underwing hardpoints and generally comprising two 66 lb (30 kg) free-fall bombs

Powerplant: one Nakajima Kotobuki 41 or Kotobuki 41 Kai air-cooled 9-cylinder radial piston engine rated at 710 hp (529 kW) for take-off and 785 hp (585 kW) at 9,845 ft (3000 m)

Internal fuel: 72.6 Imp gal (87.2 US gal; 330 litres)

External fuel: up to 46.2 Imp gal (55.5 US gal; 210 litres) in one 46.2 or 35.2 Imp gal (55.5 or 42.25 US gal; 210 or 160 litre) drop tank

Dimensions: span 36 ft 1.125 in (11.00 m); area 191.60 sq ft (17.80 m²); length 24 ft 9.875 in (7.565 m); height 10 ft 8.75 in (3.27 m)

Weights: empty 2,874 lb (1263 kg); normal take-off 3,763 lb (1707 kg); maximum take-off 4,017 lb (1822 kg)

Performance: maximum level speed 235 kt (270 mph; 435 km/h) at 9,845 ft (3000 m) declining to 195 kt (225 mph; 362 km/h) at sea level; cruising speed 140 kt (161 mph; 259 km/h) at 9,845 ft (3000 m); climb to 9,845 ft (3000 m) in 3 minutes 35 seconds; service ceiling 32,150 ft (9800 m); maximum range 755.5 nm (870 miles; 1400 km) with 42.25 US gal (35.2 Imp gal; 160 litre) drop tank; typical range 571 nm (657.5 miles; 1058 km) with standard fuel

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