Nakajima A6M2-N "Rufe" pt. 1

Not surprisingly, last month's modelling contest turned to a large degree to a "Rufe" contest with no less than half of the 16 models that took part being "Rufe" models. So, since she is one of the most popular Japanese floatplanes, being kited more than a dozen times, I set out to see what's out there regarding the development of the type, unit histories, first hand accounts and other important material commonly found on most other Zero-sen types. To my surprise there is very little apart from brief histories mostly based on R. J. Fancillon's "Japanese Aircraft of the Pacific War" published by Putnam, a few photos and many many models. It should be noted that except for the "Mitsubishi A6M2-N Rufe" written by Krzystzof Janowicz and published by Kagero there is no other single publication, in any language including Japanese, dedicated exclusively to the "Rufe" and usually the entries in books dedicated to the Zero fighter and its types are no more than 2-3 pages long.

So, in this series of postings we will try to put together as much as possible information on the Nishiki Suijo Sentoki (Type 2 Seaplane Fighter) or Nakajima A6M2-N especially from Japanese sources. 

 

Design and development

The Wikipedia entry is very brief:

"The A6M2-N floatplane was developed from the Mitsubishi A6M Zero Type 0, mainly to support amphibious operations and defend remote bases. It was based on the A6M-2 Model 11 fuselage, with a modified tail and added floats. A total of 327 were built, including the original prototype."

 

The second reason for the development of the "Rufe" (to defend remote bases) is correct while the first (to support amphibious operations) is not. Japanese floatplanes like the Nakajima E8N "Dave" and later the Mitsubishi F1M "Pete" were indeed employed in the ground attack role during landing operations but these two types were found not strong enough in the fighter role. Therefore  the Navy as late as 1940 decided that they needed a dedicated fighter seaplane that could provide air cover, not in amphibious operations, but in captured remote islands of the Pacific until airfields were finished and land based fighters could be adequately deployed. For this, Kawanishi was instructed to design and produce what would be the best seaplane fighter of WWII, the Kawanishi N1K "Kyofu". Since Kawanishi chose to experiment with various too advanced features like retractable stabilising floats and contra-rotating propellers and was taking too long to complete the first prototype, let alone put the new type into full production, the Navy decided to quickly put together an interim type of floatplane fighter based on the highly successful Mitsubishi A6M2. This is how the "Rufe" was originally conceived as a project. Mitsubishi was deemed way too busy with the production of the Zero and the development of the J2M "Raiden", so instead Nakajima was chosen as they were already building Zero fighters.   

The official order from the Navy to Nakajima for the development of the "Rufe" was issued in the beginning of 1941 and the project became to be known within Nakajima as AS-1. The main design engineer was Mitake Shinobu an experienced member of the Nakajima designing team who was involved in the development of the E4N, the Experimental 12-shi and the E8N  Reconnaissance Seaplanes (in Francillon he is mentioned as "Niitake").

The very first test flight of the first prototype was on December 8, 1941, on the first day of the Pacific War. Pilot was Captain Nishihata Kiichiro of the Kugisho and location was lake Kasumigaura. There were almost no stability or handling problems but during alighting the aircraft was sinking a little too much but this could be resolved by changing the flap angle during approach. Another concern was that during taxing when the seaplane would have to make a tight turn, there would be very high water resistance on the stabilizing floats if there was wind. Turning the aircraft using only the rudder was not enough and this turning problem on water could be rectified with the installation of a controllable fin on the main float. The aircraft was delivered to the Navy the same day.

The second prototype was delivered on February 9,1942 and five more pre-production aircraft were delivered by the end of March. 

 

Photo source: here.

Bunrindo's "Japanese Military Aircraft Illustrated" Vol. 3, mentions that the aircraft in the photo above is "an A6M2-N undergoing tests with Yokosuka Kokutai. The aircraft is a preproduction machine, given a provisional designation of Model 1 Fighter Seaplane. The serial number is Nakajima No.913 and the production date recorded on the rear fuselage is 2-4-23, signifying April 23, 1942. Unlike the Reisen, the oil cooler was placed inside the central float pylon. The main float itself housed a 320 l. fuel tank. Note the small ventral fin added to the A6M2-N. Note also that, unlike the A6M2, the rudder has been extended to the bottom of the rear fuselage." 

Of particular interest is the following information mentioned in source 4. While Nakajima had all the detailed drawings of the Zero fighter from Mitsubishi, going through them and making modifications was deemed too complicated and time consuming. So instead a Zero fighter that had received battle damage and was sent to Nakajima for repairs was chosen and all modifications were done upon this aircraft which when completed became the first "Rufe" prototype. A very economical solution.

Another very interesting piece of information comes from source 3. Apart from the first prototype, another prototype called "0" was built and delivered to Kugisho to be used for strength tests. The aircraft was hoisted to a specified height and was left to fall on a pool used for testing. Only one member of the Nakajima design team, Tajima Atsushi, was sent to observe the strength tests and was mortified to see the seaplane handled in such rough manner. As source 4 says he "felt the pain to his own body".

During one of these tests, the main pylon was damaged and there was contestation whether it was due to poor design or defective material or poor welding during construction. The last was deemed to be the reason so it was a problem that could be easily fixed.

Another problem was discovered during testing of the second prototype in the end of March 1942. Some parts that were made by magnesium alloy (sources mention this material as "electron") were found to have rusted. These were the fuel tank access panels on the wing undersides and various covers of access hatches. These were replaced with duralumin and that problem too was resolved.

In February 1943 while the production aircraft were deployed it was found necessary to strengthen the fourth fuselage rib as well as the engine mount since after rough alightings engine mounts were often found cracked and the shock absorber rubber had to be replaced quickly.

Finally, in cold climates it was found that the 20mm cannons froze easily. So modifications were made to waterproof the inspection hatches.

 

In general the development of the Nakajima A6M2-N progressed very smoothly, the first completed aircraft being delivered only 11 months since the official order was issued. Production of the "Rufe" was undertaken in the Ota plant of Nakajima and later in the Koizumi plant.

 

Sources

1. Encyclopedia Vol. 5

2. "All the Regular Formed Aircraft in Japanese Navy" by Akimoto Minoru, Kantosha 2000

3. Maru Extra Vol. 51. Article by Suzuki Junjiro (member of Kugisho), 1977

4. "Kaigun Suijoki-tai" (Navy Seaplane Units), article by Tajima Atsushi, Kojinsha 2013