The Aichi M6A Seiran, designed as a submarine-launched attack floatplane for the Imperial Japanese Navy during World War II, was meant to be deployed from I-400 class submarines.
These submarines were originally tasked with carrying out aerial assaults on the United States.
Since the late 1920s, the Imperial Japanese Navy has adopted a strategy of using floatplanes launched from submarines for target scouting.
In December 1941, the Japanese Combined Fleet’s Commander in Chief, Admiral Isoroku Yamamoto, suggested the creation of a substantial fleet of submarine aircraft carriers aimed at launching air attacks on American coastal cities.
The submarines were designed to surface and launch their aircraft via catapult, then submerge to evade detection, and resurface to recover the aircrews who would ditch their aircraft in proximity.
Initially, in June 1942, the intention was to construct a fleet of eighteen such submarines.
However, this number was subsequently reduced to nine, then five, and ultimately just three as Japan’s wartime situation worsened.
To outfit these submarine aircraft carriers, the Imperial Japanese Navy Air Service commissioned Aichi to develop a folding attack aircraft capable of 1,500 km (810 nmi) range and a speed of 555 km/h (300 kn).
Aichi had already been producing the D4Y1 Suisei (Judy), a compact, single-engine carrier dive bomber known for its sleek design and high performance, under licence.
Efforts to adapt the Suisei for deployment from I-400 submarines began with detailed engineering studies; however, the challenges proved to be overwhelming, leading to the decision to develop an entirely new design.
The culmination of Aichi’s efforts was the AM-24, internally designated by Aichi and officially named M6A1 by the military, a two-seat, low-wing monoplane equipped with a 1,050 kW (1,410 hp) Aichi AE1P Atsuta 30 engine.
The initial design omitted a conventional undercarriage, opting instead for detachable twin floats to enhance versatility.
These floats enabled the aircraft to land besides the submarine, where it could be hoisted aboard by crane for reuse.
For improved performance, the floats could be discarded during flight or omitted entirely for single-use missions.
The Seiran’s wings were designed to rotate 90 degrees and fold hydraulically against the fuselage, facilitating storage in the submarine’s 3.5-metre (11-foot) diameter cylindrical hangar.
Its armament consisted of either a single 850 kg (1,870 lb) torpedo or a comparable payload of bombs.
Additionally, a 13 mm (0.51 in) Type 2 machine gun was installed on a flexible mount for the observer’s use.
In its final form, each I-400 class submarine featured an expanded watertight hangar that could house up to three M6A1s.
The Seirans were designed to be launched from a 26-metre (85-foot) compressed-air catapult on the forward deck.
A skilled crew of four could swiftly prepare a Seiran for flight by rolling it out of its hangar on a foldable catapult carriage, attaching its pontoons, and readying it in about seven minutes.
To expedite the launch and avoid lengthy engine warm-ups, the Seirans were launched cold.
This required pre-heating the engine oil to around 60°C (140°F) in a separate chamber and circulating it, along with hot water, through the engine just before launch while the aircraft remained in the hangar.
Thus, the aircraft’s engine would reach or be close to normal operating temperature as soon as it became airborne.
This concept was adapted by the Germans, who intended to use a similar method for launching aircraft from their incomplete carrier, the Graf Zeppelin.
The first of eight Seiran prototype aircraft was finished in October 1943, and flight testing began the following month.
An issue with the auxiliary wings’ overbalance was resolved by increasing the tail fins height.
After further successful testing, production commenced in early 1944.
To facilitate pilot transition to the Seiran, two land-based trainer versions with retractable landing gear were constructed, designated as M6A1-K Nanzan.
The only other difference was the removal of the foldable top portion of the vertical stabiliser, present on the Seiran.
Specifications
Crew
2
Length
11.64 m (38 ft 2 in)
Wingspan
12.262 m (40 ft 3 in)
Height
4.58 m (15 ft 0 in)
Wing area
27 m2 (290 sq ft)
Empty weight
3,301 kg (7,277 lb)
Gross weight
4,040 kg (8,907 lb)
Max take-off weight
4,445 kg (9,800 lb)
Powerplant
1 × Aichi AE1P Atsuta 30
Or
1 x Atsuta 31, V-12 inverted liquid cooled piston engine
1,000 kW (1,400 hp) for take-off
999 kW (1,340 hp) at 1,700 m (5,577 ft)
962 kW (1,290 hp) at 5,000 m (16,404 ft)
Propellers
3-bladed constant-speed propeller
Performance
Maximum speed
474 km/h (295 mph, 256 kn) at 5,200 m (17,060 ft)
Cruise speed
296 km/h (184 mph, 160 kn) at 3,000 m (9,843 ft)
Range
1,188 km (738 mi, 641 nmi)
Service ceiling
9,900 m (32,500 ft)
Time to altitude
3,000 m (9,843 ft) in 5 minutes 48 seconds
Wing loading
149.6 kg/m2 (30.6 lb/sq ft)
Power/mass
0.3574 kW/kg (0.2174 hp/lb)
Armament
Guns
1 × 13 mm cabin mounted Type 2 machine gun
Bombs
1× Type 91 torpedo
Or
2× 250 kg (551 lb)
Or
1× 850 kg (1,874 lb) bombs.
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The Xplanes of Imperial Japanese Army & Navy 1924-1945-Illustrated Warplane History.
Wings of the Rising Sun Uncovering the Secrets of Japanese Fighters and Bombers of World War II-Mark Chambers.
Japanese Imperial Army Navy Aircraft Color Markings-Koku Fan 42.
Japanese Code Names-Richard M Bueschel.
Japanese Aircraft-John Stroud.
Japanese Aircraft of the Pacific War-Rene Francillon.
Japanese Aircraft Interiors, 1940-1945-Robert C. Mikesh.
Japanese Aircraft Equipment 1940-45-Robert C Mikesh.
Famous Airplanes of the World-47, Imperial Japanese Navy Reconnaissance Seaplanes.
