The designation “]-AM-2” (Ko-AM-2) can be explained as follows: ]- prototype (or rather, a machine delivered to the Navy’s experimental center for testing), A – fighter, M – Mitsubishi company, -2 – second unit. Formally, in documents, this aircraft was referred to as the A6M1 “12-Shi,” which translates to: Mitsubishi fighter aircraft built to meet the Navy’s 12-Shi specifications. M1 indicates Mitsubishi’s first configuration of the aircraft. Pre-production and production aircraft were already designated A6M2 and model numbers (Model 11, Model 21). The name “Zero” only emerged after the aircraft was officially put into production in 1940.
Prototypes 1 and 2 differed from the A6M2 Model 11 mainly in the engine (hence the M1 designation). The A6M1 versions were equipped with the Mitsubishi Zuisei 13 engine (not the Nakajima Sakae 12 found in later versions). As a result, there were significant differences in the engine cowling, air intakes, etc. The Zuisei 13 was primarily smaller in diameter and shorter than the Nakajima engine. The illustration shows a two-blade propeller. Prototype No. 2 initially used such a propeller during its early flights but soon switched to a three-blade Sumitomo (Hamilton Standard) propeller with an early spinner. This change was ordered after the initial tests of Prototype No. 1 in April 1939 when the two-blade propeller proved inefficient. The A6M1 versions also had a different shape for the vertical stabilizer and rudder, as well as a different profile for the oil cooler intake beneath the engine. Additionally, under the rear fuselage, there was a stabilizing fin, not used on production machines but later present (in a slightly modified form) in the floatplane versions of the Zero, the A6M2-N Rufe.
The factory flight of Prototype No. 2 took place on October 18, 1939, and by October 25, it was at the Yokosuka base and tested by military pilots from Yokosuka Kokutai. The main focus of Prototype No. 2’s trials was armament, as it was the first unit equipped with fuselage-mounted 7.7 mm machine guns and wing-mounted 20 mm cannons. The tests were “outstanding.” In initial ground target shooting tests on a 19-square-meter surface, 45-50% accuracy was achieved with the cannons (9-10 hits out of a 20-round burst—10 from each barrel). Further trials of Prototype No. 2 yielded excellent results, but due to the engine change and some other minor adjustments, further flight tests were temporarily suspended. Prototype No. 2 returned to testing in March 1940. It participated in engine over-rev stress tests during steep dives. On March 11, 1940, Lieutenant Okuyama piloted it from Oppama airfield. From an altitude of 1,500 meters, he dived at a 50-degree angle toward the ground. Around 900 meters, the engine began howling horribly before exploding. The pilot was ejected from the cockpit by the force of the explosion, likely already deceased by then. The parachute opened automatically at 300 meters, but the pilot’s body slipped out of the harness and fell into the sea. The exact cause of the accident was never determined. Additional inspections were ordered for the A6M2 Model 11s already in production, delaying their entry into service from May to July 1940. No issues with their design were found, and the causes of the tragedy remain unknown to this day.
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