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Not only in the history of the Pacific War, but in the history of military aviation, the Aichi M6A1 Seiran (Mountain Haze) remains, to this day, the only aircraft ever built by any nation with the primary role of submarine-launched attack aircraft. This distinction assures it a place in history despite the Pacific War ending before it could ever be used in combat.

Japan pioneered the idea of basing aircraft on submarines with the Yokosuka E14Y Glen. Although the Glen actually dropped bombs on the North American mainland, it wasn't intended for the offensive role. That was intended for the seaplanes to be based on the projected I-400 class of submarines. These were to be quite large, displacing 4,500 tons each, and were to have a cruising radius of 41,575 nautical miles at 14 knots. Each was to have a large watertight hangar and catapult on the forward deck, and would carry two aircraft. But the plan to build 18 of these submarines was revised due to the more pressing needs of the surface navy, and only five were actually ordered, with an enlarged hangar capable of accommodating three planes. Now, all Japan needed was an airplane to base on them. The Glen was seen as too small and too lightly armed for this duty, so Aichi, late in 1942, was instructed to design a "special attack" bomber to be based onboard the I-400 class (this was well before the term "special attack" was attached to the kamikazes; in this case, "special" described the way the planes were based, rather than their method of attack).

The original specification called for a fast, catapult-launched expendable aircraft with no landing gear (apparently, the aircrew would ditch their plane after the attack and wait for rescue), but this was later revised to a detachable twin-float undercarriage. The prototype, given the company designation AM-24, was designed by a team led by Engineers Ozaki, Ozawa, and Mori. Despite the complexity of designing an aircraft for easy storage aboard a small submarine-borne watertight hangar, progress was smooth, if rather slow, due to other design projects going on simultaneously. Two versions were designed, the actual attack variant, dubbed the M6A1 Seiran, and a land-based trainer with retractable undercarriage, termed the M6A1-K Nanzan (Southern Mountain).

The first prototype M6A1 was completed in November 1943. Interestingly, it was powered by an inline engine, the Aichi Atsuta 30 of 1,400 horsepower driving a three-bladed constant-speed propeller. With the twin floats, the overall sleek streamlining, and the inline engine, the M6A1 rather resembled the famous Supermarine S6B Schneider Cup floatplane racer, with an enclosed cockpit built for two. The wings and tail planes were made foldable. The outer wing panels swiveled on their rear spars to lie along the fuselage when folded, similar to the wing-folding mechanism of the American-made Grumman F4F and F6F carrier fighters and the TBF Avenger torpedo bomber. The tip of the vertical tail folded down to starboard and the horizontal tail surfaces folded downwards as well. The folding mechanisms seemed overly complex, and space would obviously be at a premium aboard the submarine, but the M6A1 could easily be prepared for flight in just seven minutes by a crew of four trained personnel. Since most such assemblies and readying for flight would occur at night, fluorescent paint was applied to all important parts' attachment points.

Five more prototypes were built, powered by the essentially similar Atsuta 31 engine, and 20 production M6A1s and two M6A1-K trainer prototypes were built through July 1945, powered by the Atsuta 32. The Nanzan received an inwardly-folding landing gear and a fixed tailwheel, and because the trainer lacked the floats and had superior directional stability as a result, the upright tail-fin's folding tip was deleted.

A scheme was hatched for an attack on the Panama Canal. Two I-400 class submarines, the I-400 and the I-401, would each carry three Seirans, and the older I-13 and I-14, which normally carried E14Ys, would carry two Seirans apiece. At almost the last minute, though, the target was changed to the American fleet anchorage at Ulithi Atoll in the Carolines. The attack fleet, dubbed the First Submarine Flotilla, sortied in late July 1945, but word of the Japanese surrender prevented what would undoubtedly have been a most unpleasant surprise attack, possibly as startling as Pearl Harbor, if on a much smaller scale.



Data

Type:
Single-engined, two-seat, submarine-borne floatplane attack bomber (M6A1) or land-based combat trainer with retractable landing gear (M6A1-K). All-metal construction with fabric-covered control surfaces.


Accommodation:
Crew of two in tandem enclosed cockpits.


Powerplant:
(Prototype M6A1s) One Aichi Atsuta 30 or 31 twelve-cylinder inverted-vee liquid-cooled engine, rated at 1,400 hp for take-off, 1,250 hp at 5,580 ft., and 1,290 hp at 16,405 ft.

(Production M6A1s and M6A1-K prototypes) One Aichi Atsuta 32 twelve-cylinder inverted-vee liquid-cooled engine, rated at 1,400 hp for take-off, 1,340 hp at 5,580 ft., and 1,290 hp at 16,405 ft.


Armament:
One flexible rear-firing 13mm machine gun; bombload of two 551-lb. bombs, or one 1,764-lb. or one 1,874-lb. bomb.


Dimensions, weights, and performance:

(Production M6A1)
Wingspan, 40 ft. 2 in.;
length, 38 ft. 2 in.;
height, 15 ft. 5/16 in.;
wing area, 290.624 sq. ft.;
empty weight, 7,277 lb.;
loaded weight, 8,907 lb.;
maximum weight, 9,800 lb.;
wing loading, 30.6 lb./sq. ft.;
power loading, 4.6 lb./hp;
maximum speed, 295 mph at 17,060 ft.;
cruising speed, 184 mph at 9,845 ft.;
time to 9,845 ft., 5 min. 48 sec.;
service ceiling, 32,480 ft.;
range, 739 st. miles.

 

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