TheAichi M6ASeiran (晴嵐, "Clear Sky Storm"^[1]) is asubmarine-launched attackfloatplane designed for theImperial Japanese Navy during World War II. It was intended to operate fromI-400 class submarines, whose original mission was to conduct aerial attacks against the United States.

M6ASeiran
M6A1
General information
Type	Submarine-launcheddive / torpedo bomber
Manufacturer	Aichi Kokuki KK
Primary user	Imperial Japanese Navy Air Service
Number built	28 (inc. 2 M6A1-K landplane prototypes)
History
Manufactured	1943–1945
Introduction date	1945
First flight	November 1943
Retired	1945

From the late 1920s, the Imperial Japanese Navy had developed a doctrine of operating floatplanes from submarines to search for targets.^[2]

In December 1941, Commander-in-Chief of the Japanese Combined Fleet, AdmiralIsoroku Yamamoto, proposed constructing a large fleet ofsubmarine aircraft carriers (also designated STo orsen-toku—special submarine) whose purpose was to mount aerial attacks against American coastal cities. The submarines would surface to launch their aircraft bycatapult, submerge to avoid detection, then surface again to retrieve the aircrews, who would ditch their planes nearby. By June 1942, the plan was to build a fleet of eighteen such submarines. This was later cut to nine, then five, and finally just three, as Japan's wartime fortunes declined.^[3][4]

To equip the submarine aircraft carriers, theImperial Japanese Navy Air Service requested thatAichi design a folding attack aircraft with a range of 1,500 km (810 nmi) and a speed of 555 km/h (300 kn). Aichi was already manufacturing under license theD4Y1Suisei (Judy), a relatively small single-engined carrier dive bomber with exceptionally clean lines and high performance. Detailed engineering studies commenced in an effort to modify the Suisei for use aboard the I-400 submarines but the difficulties in doing so were eventually judged insurmountable and a completely new design was initiated.^[5]

Aichi's final design, designated AM-24 by Aichi and given themilitary designation M6A1, was a two-seat, low-wingedmonoplane powered by a 1,050 kW (1,410 hp)Aichi AE1P Atsuta 30 engine (a licence-built copy of theDaimler-Benz DB 601 liquid-cooledV12 engine). The original specification dispensed with a traditional undercarriage but it was later decided to fit the aircraft with detachable twin floats to increase its versatility. If conditions permitted, these would allow the aircraft to land next to the submarine, be recovered by crane and then re-used. The floats could be jettisoned in flight to increase performance or left off altogether for one-way missions.^[6]TheSeiran's wings rotated 90 degrees and folded hydraulically against the aircraft's fuselage (with the tail also folding down) to allow for storage within the submarine's 3.5 m (11 ft) diameter cylindrical hangar. Armament was a single 850 kg (1,870 lb) torpedo or an equivalent weight in bombs. One 13 mm (0.51 in)Type 2 machine gun was mounted on a flexible mounting for use by the observer.^[7][8][9]

As finalized, each I-400 class submarine had an enlarged watertight hangar capable of accommodating up to three M6A1s. TheSeirans were to be launched from a 26 m (85 ft) compressed-air catapult mounted on the forward deck. A well-trained crew of four men could roll aSeiran out of its hangar on a collapsible catapult carriage, attach the plane's pontoons and have it readied for flight in approximately 7 minutes.^[1]

In order to shorten the launching process and eliminate the need for time-consuming engine warm-ups, theSeirans were to be catapulted from a cold start. This necessitated heating the engine oil for each plane to approximately 60 °C (140 °F) in a separate chamber and pumping it, as well as hot water, through the engine just prior to launch while the planes were still in the hangar. In this way, the aircraft's engine would be at or near normaloperating temperature immediately upon getting airborne. The idea was borrowed from the Germans who planned on using a similar launch method for the aircraft of their unfinished carrierGraf Zeppelin.^[10][11]

The first of eight prototypeSeirans was completed in October 1943, commencing flight testing in November that year. A problem with overbalance of the auxiliary wings was eventually solved by raising the height of the tail fin. Further testing was sufficiently successful for production to start in early 1944. In order to aid pilot conversion to theSeiran, two examples of a land basedtrainer version fitted with a retractableundercarriage were built. These were given the designation M6A1-KNanzan ("Southern Mountain"). Besides the difference in landing gear, thevertical stabilizer's top portion, which was foldable on theSeiran, was removed.^[12][13]

The first production examples of theSeiran were completed in October 1944. Deliveries were slowed byan earthquake near Nagoya on 7 December 1944, and by an American air-raid on 12 March 1945.^[12] Construction of the STo submarines was stopped in March 1945, after two submarine aircraft carriers had been completed and a third finished as a fuel tanker. These were supplemented by two smallerType AM submarines, originally designed as command submarines carrying reconnaissance floatplanes, but capable of carrying twoSeirans.^[14][15] Owing to the reduced carrier submarine force, production of theSeiran was halted, with a total of 28 completed (including the prototypes and the M6A1-Ks).^[13][16]

The new submarines and aircraft were assigned to the1st Submarine Flotilla, comprising the two STo submarines, theI-400 and the flagshipI-401, each carrying threeSeirans together with two type AMs, theI-13 andI-14. The 1st Submarine Flotilla commenced training with theSeirans in January 1945, and the crews gradually learned how to handle the submarines and aircraft. Launching all threeSeirans took longer than expected: 30 minutes if floats were fitted, although this could be reduced to 14.5 minutes if the floats were not used.^[16]

The first mission of theSeiran squadron, which was named theShinryuu Tokubetsukougeki-tai (神龍特別攻撃隊, meaning "God-Dragon Special Attack Squad")^[17] was to be a surprise air strike on theGatun locks of thePanama Canal, to cut the main supply line for US forces in the Pacific. When the force was finally ready to set off on their mission against Panama, Japan's increasingly desperate situation led to a change in plan, with the target for the attack, called OperationHikari (Splendour), being switched to the American base atUlithi Atoll where forces, including aircraft carriers, were massing in preparation for attacks on the Japanese Home Islands. The flotilla departed Japan on 23 July 1945 and proceeded towards Ulithi. On 16 August, the flagshipI-401 received a radio message from headquarters, informing them ofJapan's surrender and ordering them to return to Japan.^[18][19][20] All sixSeirans on board the two submarines, having been disguised for the operation as American planes in violation of the laws of war,^[10] were catapulted into the sea with their wings and stabilizers folded (for theI-401) or pushed overboard (for theI-400) to prevent capture.^[21]

M6A1

Prototypes powered by Atsuta 30 or 31 1,044 kW (1,400 hp) engine and removable floats, 8 built.

M6A1Seiran (Shisei-Seiran)

Special Attack Bomber, 18 built.^[22]

M6A1-KNanzan (南山, "South Mountain")('Shisei-Seiran Kai)

Prototypes of training version, retractable wheeled landing gear, 2 built.

M6A2

Prototype modification, powered by Mitsubishi Kinsei MK8P 62 1,163 kW (1,560 hp) engine, 1 built^[23]

  Japan

• Imperial Japanese Navy Air Service
  • Naval Air Technical Arsenal / 1st Naval Technical Arsenal
  • 631st Naval Air Group

A single M6A1 has been preserved and resides in theUdvar-Hazy Center of the Smithsonian'sNational Air and Space Museum. It is located in theWashington, DC suburb ofChantilly, Virginia nearDulles International Airport. TheSeiran was surrendered to an American occupation contingent by Lt. Kazuo Akatsuka of the Imperial Japanese Navy, who ferried it fromFukuyama toYokosuka. The US Navy donated it to theSmithsonian Institution in November 1962. Restoration work on theSeiran began in June 1989 and was completed in February 2000.^[24]

Data fromJapanese Aircraft of the Pacific War,^[13]and Aichi M6A1 Seiran^[25]

General characteristics

• 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 takeoff weight: 4,445 kg (9,800 lb)
• Powerplant: 1 ×Aichi AE1P Atsuta 30 or Atsuta 31 V-12 inverted liquid-cooled piston engine, 1,000 kW (1,400 hp) for take-off

1,000 kW (1,340 hp) at 1,700 m (5,600 ft)

960 kW (1,290 hp) at 5,000 m (16,000 ft)

• Propellers: 3-bladed constant-speed propeller

Performance

• Maximum speed: 474 km/h (295 mph, 256 kn) at 5,200 m (17,100 ft)
• Cruise speed: 296 km/h (184 mph, 160 kn) at 3,000 m (9,800 ft)
• Range: 1,188 km (738 mi, 641 nmi)
• Service ceiling: 9,900 m (32,500 ft)
• Time to altitude: 3,000 m (9,800 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-mountedType 2 machine gun
• Bombs:
  
  • 1×Type 91 torpedoor
  • 2× 250 kg (551 lb) or 1× 850 kg (1,874 lb) bombs

Aircraft of comparable role, configuration, and era

• Arado Ar 231
• Cox-Klemin XS
• Parnall Peto
• Yokosuka E14Y

Related lists

• List of aircraft of World War II
• List of bomber aircraft
• List of military aircraft of Japan
• List of submarine-borne aircraft

• Burke, Stephen.Without Wings: The Story of Hitler's Aircraft Carrier. Trafford Publishing, 2008.ISBN 978-1-4251-2216-4.
• Francillon, René J.Japanese Aircraft of the Pacific War. London: Putnam & Company Ltd., 1970.ISBN 0-370-00033-1 (2nd edition 1979,ISBN 0-370-30251-6).
• Geoghegan, John J.Operation Storm: Japan's Top Secret Submarines and Its Plan to Change the Course of World War II. Crown Publishers, NY, 2013.ISBN 978-0-307-46480-4.
• Januszewski, Tadeusz.Japanese Submarine Aircraft. Sandomierz, Poland/Redbourn, UK: Mushroom Model Publications, 2002.ISBN 83-916327-2-5.
• Layman, R.D. and Stephen McLaughlin.The Hybrid Warship. London:Conway Maritime Press, 1991.ISBN 0-85177-555-1.
• Mikesh, Robert C.Monogram Close-Up 13: Aichi M6A1 Seiran, Japan's Submarine-Launched Panama Canal Bomber. Bolyston, MA: Monogram Aviation Publications, 1975.ISBN 0-914144-13-8.
• "Nipponese Canal Cracker...Aichi's M6A1 Seiren".Air International, October 1989, Vol.37, No 4. Bromley, UK:Tri-Service Press. ISSN 0306-5634. pp. 182–188, 201.
• Ogawa, Toshihiko.Nihon Kōkūki Daizukan, 1910–1945. Tokyo, 1993.
• Passingham, Malcolm (March 2000). "Les hydravions embarqués sur sous-marins" [Japanese Submarine-embarked Seaplanes].Avions: Toute l'aéronautique et son histoire (in French) (84):25–37.ISSN 1243-8650.
• Sakaida, Henry; Gary Nila and Koji Takaki.I-400: Japan's Secret Aircraft-Carrying Strike Submarine. Hikoki Publications, 2006.ISBN 978-1-902109-45-9

• AirToAirCombat.com: Aichi M6A Seiran