Friday, October 19, 2018

Later A6M Zeros

A6M5 Model 52
By the summer of 1943 the Americans in the Pacific were introducing several new fighter types that would eventually simply overwhelm the Japanese, producing huge, rugged machines with 2,000hp+ engines and heavy gun armaments, such as the Grumman F4F Hellcat, Chance-Vought F4U Corsair and Lockheed P-38 Lightning, which were churned out by the tens of thousands from factories totally immune from attack. In a futile attempt to match these new American fighters and also to speed their own production, which was lagging far behind demand due to casualties, the A6M was subjected to a range of improvements and modifications and in the summer of 1943 appeared in a new guise on the combat zone. This was the A6M5 Model 52, which was destined to be the most prolific of all Zero marks. The wings were of the shorter 36ft 2in (11 metre) span, with the clipped wings abandoned and rounded tips re-adopted. The ailerons were lengthened out to the tips of the wings and the flaps were also elongated accordingly to obviate any space in between. In order to improve the known weakness of its poor dive qualities the wings were thickened up with heavier gauge material and this resulted in a 410mph (659.83km/h) diving speed, while overall maximum speed was increased to 351mph (564.88km/h) in level flight by introducing individual exhausts, which protruded aft from notched flaps in the engine cowling (five to port, six to starboard, each vent requiring its own individual fuselage heat shield patch), instead of a single centralized exhaust collector ring. The range of the Type 22 having proved key, the two 12-gallon (45.42 litres) wing tanks were retained in this model giving a range of 1,194 miles (1,921.6km). The power unit remained the 1,130hp Sakae-21 and armament reverted to two 20mm cannon and two 7.7mm machineguns. Production problems at home had seen the introduction of large, tail-finned 66-gallon (300-litre) wooden drop tanks replacing the original under-belly type and the introduction of 33-gallon (150-litre) under-wing drop tanks. The Type 3 Mark 1 radio was installed to replace the Type 98Ku Mark 1 set and the new installation could be detected by a much shorter antenna mast, but the reputation for unreliability apparently continued, the main problem being the unshielded ignition systems, exacerbated by the effect of humid tropical conditions as well as hindered by the difficulty of access for proper maintenance in the field on jungle airstrips. There was also a persistent shortage of copper for the wiring and it was not until 1945 that the Second Research Institute was established to improve electronics in the fleet’s aircraft. Some land-based units were reputed to have discarded their radios completely, although, of course, they were a vital necessity for carrier-based A6Ms who did not abandon them as some have claimed.
A prototype was converted from a Model 22 machine and made its debut in August 1943 achieving a maximum speed of 351.07mph (565km/h). This seemed to satisfy the Navy but this aircraft proved to be less agile than the Model 21, while the endurance was far inferior, three hours against eight. If it were expected that this would render the concept null-and-void this proved not to be the case and production was immediately ordered as the urgent need from the front predominated over any reservations. Although problems were encountered with the new engine exhaust venting design, which meant early models had to go to war without them, Mitsubishi eventually produced a total of 747 in the numerical range #3904 to #4650. These were rushed into service due to the worsening situation.

A further adaptation of the A6M5 was the Model 52 KO (a). The 20mm 100-round drum-fed cannon were upgraded to the Model 2-4, which was belt-fed, each carrying 125 rounds. Further thickening of the wing construction pushed the dive speed up still further, to around 460mph (740.3km/h). Mitsubishi produced 391 of this sub-type but they did not begin to enter front-line service until March 1944. They still lacked armour protection however, but the need to get them into battle quickly dominated. To address this latter problem another variant, the Model 52 OTSU (b) appeared in April 1944. The pilot was given a measure of protection with the introduction of an armoured glass plate almost 2 inches (45mm) thick. To try and solve the notorious tendency of the A6M to burn easily the internal main fuel tank was fitted with a CO2 fire-extinguishing system. These defensive measures were matched by an increase in offensive power and range. The 7.7mm machine-gun mounted in the starboard wing was deleted and a more powerful 13-mm Type 3 machine gun was substituted. This weapon was a licence-built version of the German Rheinmetall-Borsig Maschinengewer MG-131 weapon, and weighed 37lb (17kg). It had a rate of fire of 900 rpm, was belt-fed and was of the percussion-ignition variant, which the IJN apparently considered more suitable for Pacific climate conditions. This enabled a smoothing of the wing under-surface and required an addition fairing where the cannon protruded from the leading edge of the wing. Ammunition access points were also modified. A number of OTSU aircraft also had provision for a pair of 33-gallon (150-litre) under-wing drop tanks. Both Mitsubishi and Nakajima production runs involved the OTSU, 470 being built by the former but no figures seem to be available for the latter plants output.

Subsequently further changes along these lines were progressively introduced to the A6M5, which resulted in the Model 51 HEI (c). The pilot was finally given protection on the scale that Allied flyers had long been used to as standard by the insertion of a sheet of one-third of an inch (8mm) thick armoured plate to the rear of his seat, with over 2 inches (55mm) of armoured glass sheet emplaced to protect his head from above. This added considerable weight but there was more for to further increase internal fuel stowage a 37-gallon (140-litre) fuel tank was inserted directly astern of the pilot, and this tank was made self-sealing. The armament was subject to further drastic revision. The type 13mm machine-gun, with 240rpg, was now fitted to both wings, beyond the wing-mounted cannon; the 7.7mm machine-gun mounted in the nose was eliminated while the 13mm fuselage-mounted weapon remained. Some of this sub-type later had their underwing hard-points for 30–60kg (66–132lb) air-to-air bombs replaced by an air-to-air rocket carrier. Wooden drop-tanks with stabilizer fins and four-point support became the norm. These aircraft began to enter front-line service in November 1944.

Night-fighters (Heisen)
Despite the continuing western article of faith that the Japanese avoided night-fighting (Savo Island and other examples to the contrary notwithstanding) night fighter (Heisen) adaptations were in use both in the south-west Pacific and later in the defence of the home islands. One early attempt to use the A6M in this capacity had been made by the 4 Kōkūtai, which was established on 10 February 1942 at Truk under the command of Captain Gashi Moritama with twenty-seven Type 96 fighters and moved into Vunakanau airfield at Rabaul shortly afterward. American bombers were subjecting this area to nuisance night bombings with some frequency at this time. Three A6Ms arrived on 28 January and attempts were made at interceptions in conjunction with the bases searchlights, but this embryo effort proved futile.

It was not until of the advent of A6M5s properly equipped for night work that real progress was made. The selected Zeros were converted to this mission type by adding a fixed, angled Type 99 Mark 2 Model 4 20mm cannon firing obliquely upward and forward from the rear of the cockpit over the pilots head. This required a modification of the panelling of the after greenhouse, which was plated over astern for strength from the recoil. An additional Type 3 Sight was emplaced on top of the forward windshield frame.

Another ‘in-the-field’ adaptation was the fitting of some A6M5, A6M5a and A6M5bs with a fixed Type 99-2 Model 4 20mm cannon mounted in the aircraft fuselage behind the pilot. This weapon was angled to fire 30° forward and 30° to port when first installed. Later variations had an oblique firing angle of 10° to port and in both cases the barrel of the gun barrel pierced the aircraft’s fuselage. Another modification resulted in the same cannon being recessed into the cockpit canopy rather than the fuselage itself. All methods found some disfavour with the pilots due to the weight penalty this involved and the difficulty in lining up the target. Units that experimented with this type of fixture were not dedicated only to night interceptions of course and among such was the 302 Kōkūtai at Atsugi, near Tokyo, whose experienced commander, Yasuna Kozono, was apparently somewhat fixated on the use of slanted cannon, having successfully deployed it with 251Ku earlier, even considering it useful in fighter duelling (an enthusiasm not generally shared by his pilots or his superiors). Originally organized for the defence of Tokyo on 1 March 1944, with twenty-four night fighters, the 302 Kōkūtai moved to Atsugi to replace the 203-Ku, which had shifted up to the Kuriles. Containing many experienced aviators, including redundant seaplane pilots, reinforced by university student cadets, the unit was given intensive training in the use of oblique-firing cannon. The A6M5s thus equipped, under Lieutenant Kushichirō Yamada and later Lieutenant Hiroshi Morioka, the unit’s strength between January and March 1945 was never more than twenty strong. The additional weight of the emplaced cannon also detracted from the A6M5’s climb performance, which meant that actual interceptions were rare and success almost non-existent. After participating in the defence of Okinawa the force was reduced to ten A6M5s. Along with its other aircraft types the unit was dispersed to be husbanded in readiness for the Allied invasion and flew its last mission on 15 August, when eight A65Ms joined a fight with six F6Fs over their airfield, losing six of their own number to just one Hellcat. When the order came to lay down their arms, the fanatical Kozono refused to accept the inevitable and vowed to carry on the fight regardless. He began to gather aircraft to organize an attack, but was restrained and confined to a mental ward and the disbandment of his unit under a new commander was undertaken.

Other units that specialized in oblique armed fighters, were the 332 Kōkūtai at Iwakuni, in Yamaguchi Prefecture, southern Japan, and on 6 November eight A6M5s equipped with this weapon moved to Atsugi for one month undertaking air defence of the Kantō region. The 131 Kōkūtai was re-organized as a night fighter outfit in March 1945, and based at Kanoya, Kyūshū, under Lieutenant-Commander Tadashi Minobe, flying their first night interceptions on 4 April. After sixty such missions the unit moved to the secret Iwakawa airbase on 20 May with ten A6M5 night fighters, in order to avoid continuous enemy bombing.

Engine boost system
While the Zero herself had been steadily improved, lack of a really powerful engine continued to restrict just what improvements could be made to the existing airframe, which these additions had now increased by 700lb (317.5 kg) additional weight with the same power plant. Mitsubishi requested the Navy Air Arsenal allow them to fit the more powerful Mitsubishi Kinsei-62 radial to this aircraft, but the request was rejected outright. The Navy’s solution was to insist on fitting the existing Sakae-21 with a water-methanol injection system to boost its output for short bursts during combat. However, such was the slow pace of development of this system that the HEI had to go into combat without it. Eventually one trial aircraft appeared late in 1944 fitted with the Sakae 31a radial. It was a far from successful prototype; the injection system was still in its experimental stage and was plagued with continual problems, while the modified engine generated less power not more. The A6M5c sub-type was built at the Nakajima Koizuma Plant, and attained a maximum speed of 346mph (556.83km/h) but only ninety-three ever appeared before the whole project was cancelled in the increasingly frenetic situation the Navy found itself that final winter of the war. Production was complex and still mired by problems, maintenance of the temperamental power plant in the field difficult.

Work on the water-methanol injection system continued, however, utilizing the Nakajima Sakae-31 engine, which was a fourteen-cylinder, aircooled radial, which was rated at 1,130hp (831.11kW) for take-off, 1,100hp (809kW)at 9,350ft (2,850m) and 980hp (720.79kW) at 19,685ft (6,000m). With this power plant the climb rate to 26,250ft (8,001m) was 9 minutes 53 seconds. It provided a service-ceiling of 33,300ft (10,150m), and a maximum range of 956 miles (1,538.5km) at a cruising speed of 230 mph (370.15km/h). These aircraft had a wingspan of just over 36ft (10m), an overall length of almost 30ft (9.118m), a height of 11ft 6in (3.505m) and a wing area of 229.27ft2 (21.300m2). Weights were 4,519lbs (2,049kg) empty, 6,614lb (3,000.59kg) loaded. The armament comprised a single 13mm Type 3 machine-gun in the upper fuselage decking, two wing-mounted 13mm Type-3 machine-guns and two wing-mounted 20mm Type 99 cannon. Provision was also made for eight 22lb (10kg) rockets or two 132lb (60kg) rockets carried beneath the wings. For the first time the A6M was fitted with a CO2 fire-suppression system.

A direct comparison was done later in the war between the A6M5 Model 52 (‘Zeke-52’), the F4U-1D, F6F-5 and the FM-2, which makes for interesting reading. With regard to speed the A6M5 was overall speedier, with the top speeds recorded being 335mph (539.13km/h) at 18,000 feet (5,486m) for the A6M5, against 321mph (516.6km/h) at 13,000ft (3,962.4m) for the FM-2. At sea level the FM-2 was marginally faster than the Zeke-52 by 6mph (9.66km/h) but as the altitude increased the A6M5’s superiority became more and more marked, being 4mph (6.44km/h) faster at 5,000ft (1,524m); 12 mph (19.31 km/h) faster at 10,000ft (3,048m); 8mph (12.87km/h) faster at 15,000ft (4,572m), 19mph (30.58km/h) faster at 20,000ft (6,096m), 22mph (35.41km/h) faster at 25,000ft (7,620m) and 26mph 41.84km/h) faster at 30,000ft (9,144m). To get to those altitudes the two machines were evenly matched, with, at sea-level, the FM-2 exceeding the A6M5’s rate of climb by 400ft/min (2.03m/s). However, at an altitude of 4,000ft (1,219.2m) the two aircraft were level, the FM-2 being 500ft/min (2.54 km/h) superior at 8,000ft (2,438.4m), with the rate of climb equalling out again at 13,000ft (914.49m). The A6M5’s best climb speed was 105 knots (120.832mph) compared with the FM-2’s 120 knots (138.094 mph).

In the field of combat that the A6M had hitherto made her own, the roll rate was found to be equal to the FM-2 at less than 160 knots (184.125mph), but the margin was with the FM-2 at high speeds, which was put down to high control forces. The Japanese fighter could still out-turn the Wildcat gaining one turn in eight at an altitude of 10,000ft (3,048m). In the dive, unexpectedly, the heavier American machine showed no great excess over the A6M5, the latter in fact being better in the initial acceleration and on a par afterward, with zooms following dives also being similar. With regard to pilots’ all-round vision, the A6M5 excelled, notably rear-vision. The manoeuvring of the A6M5 was recorded as ‘remarkable’ at speeds under 175 knots (201.386mph), but this edge lessened as the speed factor went up, again due to high control forces, and above 200 knots (230.156mph/370.4 km/h) the FM-2 was considered to have a marginal advantage.

The final suggested tactics for FM-2 pilots tangling with the Zero fighter remained, as they had for the previous three years, quite simple: ‘DO NOT DOG-FIGHT WITH THE ZEKE 52.’ Other advice was to maintain altitude advantage but, should a Zero appear on your tail, to ‘… roll and dive away into a high speed turn’. The A6M5’s maximum safe dive speed was around 350mph, but she lost her roll agility.

Return to China
Three years after the last A6M units had departed from the Chinese mainland they returned again in a combat rôle. As early as May 1943, following heavy Allied bombing raids, nine A6Ms were once more based at San Ya airfield on Hainan Island to provide limited defensive air protection over the Hainan and Canton airspace. In October this section was beefed up with the establishment of 245 Wing based on Haikou, Hainan Island, with twenty-four A6Ms plus four Kates. Some of the A6Ms were detached for similar duties to Yangzi. Finally, in February 1944, 256 (Air Defence) Unit was established at Longhua airfield, Shanghai, with a strength of twenty-four A6Ms and eight Kates.
With the war drawing ever closer to the home islands and the (perceived) success of the suicide method of attack, a bakusō version of the once nimble A6M became the last combat variant and was termed by some the Kamikaze variant, the A6M7 Model 62/63. Designed for the job instead of being retro-fitted conversions of earlier models, the power plant was the 1,130hp (831.11kW) Sakae-31-KOH water-methanol injected radial, designated as Model 62. Others using the same engine but without the injector boost equipment fitted to it, were termed the Model 63. The injection gear necessitated the enlargement of the engine cowling to accommodate it. Further increases in weight to aid diving capability were incorporated by using heavier-gauge material in the tail assembly. 

Armament was generally as the Model 52c, with the Type 4 gun-sight, but in many the 13mm cannons were removed. The centre wing section was beefed up also to enable a 550lb (250kg) bomb to be carried on a recessed ventral rack instead of the 37-gallon (168.2 litres) drop-tank, and to compensate a 33-gallon (150-litre) drop tank could be accommodated under each wing or alternately air-toair rocket bombs. Introduced as a means of bringing down the hordes of American four-engined bombers laying waste to her cities, the Japanese Navy introduced two type of Tekkō-dan (armour-piercing) rocket bombs toward the end of the war. The Type 3 No. 25 Mk 4 (I-Gou) weighing 315kg appeared as early as 1943 and had a launch speed of 100m/sec with a 3.5kg bursting charge. This was found to be inefficient for the job and was replaced from February 1945 onward by the more practical and efficient Type 3 No. 6 (6-Gou) air-to-air rocket bomb. Weighing just 145lb and with a velocity of 270m/sec, it was in three sections and had a rocket motor with a 5.5lb incendiary shrapnel warhead, consisting of 140 metal pellets embedded with white phosphorus, affixed. The warhead became activated by a clockwork adjustable-delay time fuse, which ejected them in a 60-degree cone. This weapon appeared shortly after the first A6M7 Model 63 appeared in the spring of 1945. In the chaos of the final days of the Pacific War, and also by the deliberate destruction of some records, it is difficult to ascertain exactly how many of this variant were built. However, an entire Model 63 (#82729) has been on exhibition at the Arashiyama Museum, Kyoto City, having been brought up from the cold waters of freshwater Lake Biwa, north of Kyoto in 1976, in relatively good condition.

Training conversions
The A6M was a ‘hot ship’ and, although her reputation had outlived her undisputed merits by 1943, she continued to soldier on to the very end of the war. Like all such fighters the need to train increasing numbers of fresh young pilots, who had already qualified in the primary and intermediate training course, in the face of ever-mounting casualties, finally saw the modification of early types for which little combat value remained. The Model 21 survivors in particular were deemed particularly apt to have their now surplus airframes converted to the training (Tsukuba) role but it was not until March 1944 that they first appeared on the scene becoming the A6M2-K. The Naval Aeronautics Arsenal quickly did the necessary design work, following which the Hitachi Kokuki was allocated the mission of carrying out the task and eventually 273 conversions were carried out. The elongated cockpit canopy was extended aft to accommodate the instructor and dual-controls were emplaced, while the front cockpit for the trainee had an open canopy, accessible from both sides. The radio mast was moved to a middle position between pupil and instructor or even removed altogether. For added stability, an elongated running strake extended from the new rear end of the greenhouse to just before the horizontal stabilizers of the tail fin and the fire wall was shifted aft. To compensate for the additional weight the aircraft were stripped down; the 20mm cannon were removed from the wings but the nose-mounted 7.7mm machine-guns were retained to provide rudimentary gunnery training; the main landing gear doors likewise and the rail wheel was made permanently fixed in the landing position further forward, doing away with the covers, and making for easier maintenance, and with the tail cone often being deleted as well. A thicker tyre was substituted.

Plans were also being considered for a similar derivative of the Model 52, known as the A6M5-K, Interim Type O Training Model 22. The concept basically followed that of the A6M2-K but with the entire nose armament deleted. The Ohmura-based 21 Aircraft Arsenal actually produced a prototype conversion in 1945 but the war terminated before production could get underway at Hitachi. All of these trainers were able to offer less and less air time. By 1 January 1945 the average pilot training hours flown had fallen to a mere 275 hours for the entire fleet. Fresh-faced Imperial Navy pilots were being sent from the training cadres with about forty hours flying time; in contrast the US Navy combat pilots usually had more than 500 hours to their credit. There was no longer much question of teaching the rookies complex dog-fighting techniques; lack of fuel, lack of facilities and, above all, lack of time meant that many were sent off to face the enemy with just the sheer basics. Inevitable also, in those final days, was the sacrifice of numbers of even these training aircraft in Kamikaze missions.

A6M8 HEI(c)
Belated realization that all the many ‘improvements’ had, in general, added weight but slowed down the original concept until it was no longer able to catch, let alone destroy, many opponents, meant a reversion was made to shed some of the excess and get back to basics. The basis was the adoption of the new Mitsubishi Kinsei 14-cylinder air-cooled radial, which developed 1,560hp (1,163.76kW), which was still far less than contemporary American fighter engines of course. This power plant had a bigger diameter (more than 4 inches larger, at 103mm) than its predecessor and therefore entailed a corresponding enlargement of the cowling and the nose guns were deleted, resulting in a bigger profiled but smoother, forward profile. As a consequence the pilot’s windshield was slightly modified to suit. This aircraft was designated as the Model 64, but only two examples were ever completed. Testing of the prototype in April 1945 revealed a maximum top speed of 349.83mph (563km/h), with a climb rate of 19,685ft (6,000m) in just under seven minutes.

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