- A6M3 Type 0 Model 32: With the introduction in 1941 of the Nakajima Sakae engine, which was equipped with a two-speed supercharger, for better altitude performance and yielded 1.130 hp of power, a prototype of a Zero fitted with this engine was made and flew for the first time on 15th July 1941.
This new engine was slightly heavier and longer because of the superchargers. This extra weight changed the center of gravity and moved it too forward on the airframe. To fix this, the engine mountings were cut back by 185 mm (7.3 in) in order to move the engine toward the cockpit. This created the side effect of reducing the size of the main fuselage fuel tank, which was placed between the engine and the cockpit, from 518 L (137 US Gal) to 470 L (120 US Gal). The cowling was revised to make cowl flaps larger, the oil cooler was revised and the carburettor's air intake was moved to the upper half of the cowling.
Wings were also redesigned to reduce in order to reduce span, eliminate folding wingtips and square off the tips. The inboard edge of the aileron was placed outward by one rib and wing fuel tanks were enlarged to 420 L (110 US Gal). The two 20 mm wing cannons which were present in previous versions, were upgraded to the Type 99 Mark II, which required a bulge in the sheet metal of the wing below each cannon. The wings also included larger ammunition boxes increasing the amount of ammo to 100 rounds per cannon.
The Sakae 21 engine increased maximum speed by just 11 km/h (6.8 mph) when compared to the previous A6M2 Model 21 and sacrificed nearly 1.000 km (620 mi) of range. However, the Imperial Japanese Navy Air Service (IJNAS) accepted the type and it entered into production in April 1942.
Thanks to its shorter wing, it had better rolling capabilities and, considering its reduced drag, the diving speed was increased to 670 km/h (420 mph). However, turning and range, which were the strong points of the A6M2 Model 21, suffered due to the smaller ailerons, decreased lift and greater fuel consumption. The shorter range proved to be a serious handicap during the Solomon Islands Campaign as the Zeroes stationed in Rabaul had to travel nearly to the their maximum range to reach Guadalcanal and return. Therefore the A6M3 was declared as unsuitable for that campaign and was used for shorter range offensive and interception missions.
When the A6M3 appeared the Americans codenamed it 'Hap'. However that codename didn't live for long time as USAAF Commander Henry "Hap" Arnold protested and forced the change of the codename to "Hamp". Later, it was realized that it was a new variant of the Zero, which already had the codename of 'Zeke' and was accordingly re-codenamed as 'Zeke 32'.
This variant wasn't flown by many IJNAS units and only 343 units were manufactured by Mitsubishi in Nagoya and Nakajima in Koizumi, both in Japan. It was manufactured from April to December 1942. - A6M3 Type 0 Model 22: To correct the flaws of the previous Model 32, a new version with folding tips and redesigned wing was put into service with the IJNAS. The fuel tanks were moved to the outer wings, fuel lines for a 330 L (87 US Gal) drop tank were installed under each wing and the internal fuel capacity was increased to 570 L (150 US Gal). These changes made the model to regain the range numbers seen on the previous A6M2 Model 21.
This model, however, was put into service before being accepted formally by the Navy and, in December 1942 it was de-facto introduced. Around 560 machines of this type were manufactured from December 1942 until late 1943 by both Mitsubishi and Nakajima.
Both models 32 and 22 were powered by the Nakajima Sakae. - A6M4 Type 0 Model 41/42: This designation is not an official Mitsubishi's one. However, in some captured documents from a Naval Air Technical Arsenal dated 1st October 1942, there are some references to a "cross-section of an A6M4 intercooler" which was being designed. Some researched believe that this designation was given to two prototypes fitted with an experimental turbo-supercharger Sakae engine designed for high altitude performance. Mitsubishi's involvement in the project was quite limited or even nul, as the unmodified Sakae engine was exclusively made by Nakajima. The design and testing was made by First Air Naval (Technical) Arsenal in Yokosuka. There's one photo of this variant showing a turbo unit mounted in the forward left fuselage, however we couldn't find it.
The lack of suitable alloys for manufacture of a turbo-supercharger and its related ducting caused numerous ruptures, resulted in fires and poor performance. Therefore any further development of a turbo-supercharged A6M was cancelled and discontinued. As the IJNAS didn't accept any A6M4 into service and didn't bestow models 41/42 formally. However, it seems that the Arsenal did use the designation of A6M4.
Sources:
1. https://en.wikipedia.org/wiki/Mitsubishi_A6M_Zero#Variants
2. https://www.valka.cz/Micubisi-A6M-Reisen-Zeke-Zero-t15680 (translated)
3. AJ-Press - Monografie Lotnicze 7 - Mitsubishi A-6M Zero (translated)
4.Salamander Books - The Complete Book of fighters
5. Signal Squadron - Aircraft In action 59 - Mitsubishi A6M Zero in Action
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