The Imperial Japanese Navy's A6M4
By Rob Graham
During the Second World War, the Imperial Japanese Navy's main carrier fighter, the Type Zero, received numerous design enhancements in an attempt to match the ever-improving aircraft that the Allied forces were developing. Almost all versions of the Zero were widely used in combat and have been well documented. However, one version, which has been a source of speculation and elusive wonder to aviation historians, was the A6M4. It has been said over the years that the A6M4 was an experimental turbosupercharged version. Recent research has offered several twists to this and other theories. The history of the claims and fruits of this research are presented here.
Several different theories regarding what the A6M4 was can be debunked fairly easily, reducing the theories to about two. Theory number 1 is that the A6M4 was a turbosupercharged variant, and that two were built experimentally before the project was canceled; within this theory, there are several different beliefs regarding the airframe, engine, and layout. Theory number 2 states that the A6M4 was a very brief transitional design between the A6M3 and A6M5 models, although many factors remain unknown. Almost as a news flash, very recently, acclaimed aviation historian Jim Long discovered some translated Japanese documentation that supports the turbosupercharger theory by referencing an intercooler, a device commonly used with a turbocharger that works like a radiator to cool hot compressed air. However, as he points out, it's not a lock, and questions remain as to the layout.
The A6M4 was very likely a turbosupercharged Zero.
Modelers have not had many leads to follow to model the A6M4, and when considering the chronological order of these sources (as they are arranged in this document), it appears as though René Francillon's text was the initial and oft-quoted information. Its accuracy has been the hinge for this whole argument, with many people citing that Dr. Francillon had perhaps misquoted and / or used an inaccurate source.
This contribution courtesy of historian Jim Broshot:
The Mitsubishi A6m3 Zero-Sen ("Hamp"), by René J. Francillon, Aircraft In Profile No. 190 , ©1967
"THE TURBO-SUPERCHARGED SAKAE
"The A6M4 version of the Reisen has long been conspicuously missing from the various historical studies yet published on this aircraft and even the designer of the Reisen, Mr. Jiro Horikoshi, could not remember what the A6M4 was! However, Mr. Horikoshi had the kindness to inquire among his friends of the former Imperial Japanese Navy and, recently, was able to confirm to the present writer that the A6M4 designation was applied to two A6M2s fitted with an experimental turbo-supercharged Sakae engine. The design, modification and testing of these two prototypes was the responsibility of the Dai-Ichi Kaigun Gijitshusho (First Naval Air Technical Arsenal) at Yokosuka and took place in 1943. Lack of suitable alloys for use in the manufacture of the turbo-supercharger and its related ducting resulted in poor operation marred by numerous ruptures of the ducting, and fires. Consequently further development of the A6M4 was cancelled, the aircraft still providing useful data for further aircraft, and the manufacture of the more conventional A6M5, already under development by Mitsubishi Jukogyo K.K. was accelerated."
Other various sources state, as indicated:
Japanese Aircraft of the Pacific War, by René J. Francillon, Naval Institute Press, ISBN 0-87021-313-X, ©1970. Pages 369-371 have a couple of paragraphs that say:
"In Japan at that time Mitsubishi and the Navy were attempting to improve the Reisen. At low altitude it could still hold its own against Allied aircraft, but at medium and high altitude it was hopelessly outclassed by the Lightnings and Corsairs. In an attempt to correct this situation two A6M2s were modified by Dai-Ichi Kaigun Koku Gijitsusho at Yokosuka and, designated A6M4s, were powered by an experimental turbosupercharged Sakae engine. Major teething troubles with the experimental engine precluded the placing of a production order and the Navy had to settle for an interim version of the aircraft, the A6M5, pending availability of the new Mitsubishi A7M Reppu... To improve diving speed Mitsubishi modified the 904th A6M3 in August 1943 by fitting a new set of wings with heavier gauge skin and with redesigned non-folding rounded wingtips..."
Zero Fighter, by Martin Caidin, Ballantine weapons book #9 ©1970. Page 158-159 shows a table that says:
"Experimental version with turbo-supercharger. Only two built. Basically an A6M2.
A6M1-2-2N Zero-Sen by Richard Bueschel, Schiffer, ISBN 0-88740-754-4, ©1995 (This book is a reprint with some minor updating from the original ©1970 Osprey and ARCO-AIRCAM publication). Pages 62-63 have a chart that shows the A6M4 and a footnote that says:
"Model 21 with turbosupercharger"
Zero Fighter, by Robert C. Mikesh and Rikyu Watanabe, Crown, ISBN 0-517-54260-9, ©1981. Page 32 has a paragraph that states:
"The assignment of this designation to a Zero model had been questionable for a long period of time since there was no record of its use. The Japanese use the number "4" with the same reservations that Westerners use the number "13", and therefore it was presumed not to have been used for this reason although not avoided in numbering systems for other aircraft. (The number "4" which is shi in Japanese also has the meaning of the word "death.") It was not until 1968 that Horikoshi revealed that a Model 32 was equipped with an experimental turbo-supercharged engine, and this designation A6M4 was reserved for this configured model had it gone into production."
Zero: Japan's Legendary Fighter, by Robert C. Mikesh, Motorbooks, ISBN 0-87938-915-X, ©1994. Page 89 has a paragraph of text that says:
"The assignment of this designation to a Zero model has been in question for a long time, since no authoritative records have ever been found to prove its use. The designation may have been set aside for a proposed model that never materialized. Some think that it was associated with an A6M3 that was to be equipped with a turbo-supercharged engine, as suggested in 1968 by the Zero's designer Jiro Horikoshi. But the reason is not really known."
Famous Airplanes of the World - A6M models 22-63, #56, 1996. Page 14 has a brief paragraph, basically translated, that the A6M4 design has not been adequately researched. Interestingly, there are virtually NO references to the A6M4 in Japanese texts.
Off the Internet:
"By late 1942 and early 1943, the Zero Fighter was beginning to be confronted with newer, more-capable Allied fighters. At high altitude, the A6M2 and A6M3 were hopelessly outclassed by newer Allied fighters such as the P-38 Lightning and the F4U Corsair. In an attempt to correct this deficiency, two A6M2s were modified by Dai-Ichi Kaigun Koku Gijitsusho at Yokosuka to take an experimental turbosupercharged Sakae engine. The short designation A6M4 was assigned to this project.
"However, major teething troubles were encountered with the A6M4, and no production order was placed. As a substitute, the A6M5 interim version was introduced pending availability of the A7M Reppu."
"A6M4 (2 modified A6M2's as prototypes) Turbosuper charged Sakae engine "
Correspondence from an expert:
Jim Long of AIR'TELL Publications & Research Service says,
"At the time that I first read this explanation, I was skeptical of its validity, and as the years passed and no proof was forthcoming, I became even more uncertain. But after all of these years, I finally found a scrap of evidence to support René Francillon's pronouncement. It is fragmentary, but I think it is enough to make us all believe that there was something to the report of the A6M4. But what I've found is small and only gives evidence of the A6M4 designation in connection with an aircraft that had an intercooler, and which probably means that it had a turbosupercharged engine. There are no other details of that sort, however. We'll all be left with questions, I'm afraid.
"What I've found comes from microfilm reel JP-26 which contains images of Bulletins 67-45 through 78-45. These documents are CINCPAC-CINCPOA or JICPOA intelligence bulletins issued by the Intelligence Center, Pacific Ocean Area, or the Joint Intelligence Center, Pacific Ocean Area, or the Commander in Chief Pacific and Pacific Ocean Area. They are available to the public on microfilm from the Department of the Navy, Naval Historical Center, Washington Navy Yard, Washington, D. C. 20374-0571.
"I have excerpted the material of interest to you, and it runs as follows:
"Excerpt from CINCPAC-CINCPOA BULLETIN NO. 67-45, 30 MARCH 1945 QUARTERLY REPORT ON RESEARCH EXPERIMENTS VOLUME 1 SPECIAL TRANSLATION NUMBER 52
"QUARTERLY REPORT ON RESEARCH EXPERIMENTS
(Naval Air Technical Depot); dated 1 October 1942, Captured on SAIPAN.
13. Type 0 Mark 1 Carrier Fighter (TN: ZEKE)
PROGRESS REPORT: Research on wing flutter is in progress.
FUTURE DISPOSITION: Continue
14. Type 0 Mark 2 Carrier Fighter (TN: ZEKE)
A. Study of engine cooling performance:
The efficiency of each design was tested by using actual engines in medium-sized wind tunnels. As a result, it was found that putting the cylinders and the inter-cylinder baffles close together and fairing the inner surface of the openings in the engine cover were useful in increasing the cooling efficiency.
B. Study of wing flutter:
Measurements of the limiting speed for flutter with and without bomb load has been completed, and results are being compiled.
C. Study of the form of carburetor intake tube:
Comparative tests are being made of the A6M2 and A6M3 using the intake tube on an actual engine.
D. Study of intercooler:
The amount of air cooled and the uniformity of distribution of the cooled air in a cross section of the A6M4 intercooler were examined. Among the things examined, it is best to eliminate the inter-cylinder baffles from the cover of the original Airplane Dept intercoller [sic] design.
A. Present report
D. Formulate wind tunnel test results
[TN means translator's note]"
This latest information from Jim Long has the most compelling information to make a case for the turbosupercharged A6M4, though Mr Long's comment, "We'll all be left with questions, I'm afraid," is absolutely correct.
Researchers may glean that the A6M4 was likely not based on an A6M3 Model 22, as this report was dated October 1, 1942, which was two months before the first Model 22 was built. Mitsubishi's A6M2 production had been replaced in June of 1942 with A6M3 Model 32 production, although the A6M3 Model 32 had been under development for a year prior to its production. It is certainly possible the A6M4 was based on either the A6M2 Model 21 or the A6M3 Model 32. It would likely have had the larger propeller and spinner of the A6M3. Since the plane has been said to have a turbosupercharged Sakae 12, it likely would have had the short cowl of the A6M2. Since the turbosupercharger and / or intercooler components could not have been mounted in the belly or other location without considerable re-engineering, it is possible the plane had the intercooler mounted between the engine and the firewall, much as the turbosupercharged J2M4 Raiden. In order to accommodate the turbosupercharger and intercooler in this location, the further aft firewall of the A6M3 airframe was a likely choice, and long engine mounts similar to the setup used on the A6M2-K. That said, it is also entirely possible the turbosupercharger and intercooler was installed in a package beneath the cowl, as was done on the C6N2 "Myrt."
Please note that throughout this document, the word turbosupercharged refers to a process where a turbocharger is used. This differs from a supercharger in that a turbocharger uses an impeller and shaft that is spun by the passing of exhaust gases that spin the compressor; a supercharger uses a mechanically driven compressor, usually taken indirectly from the crankshaft. The Sakae 12 used in the early Zeros used a single speed supercharger, which was mounted on the rear of the engine, and the Sakae 21 used in later Zeros used a two speed unit that boosted higher altitude performance. Sakae -12 and -21 superchargers were mechanically driven by a planetary gear set and turned at a higher speed than the crankshaft.
For the Modeler:
As a modeler, my personal choice to date to hypothetically model the A6M4 is to use an A6M3 Model 32 with a modified A6M2 Model 21 cowl and Sakae 12 engine, and a turbosupercharger mounted on the side of the fuselage, aft of the engine. Probably, I'd put the intercooler below the cowl, replacing the oil cooler with a larger dual-purpose oil cooler / intercooler scoop. I'd paint it "prototype orange" with a black cowl, and my choice of the tail code would be "Ko"A6-41.
So, theory Number 2, "It was essentially an early A6M5," is almost certainly not valid. Configuration is still uncertain, and we are still left to model our desires to fill the gap of the A6M4 in our collections. As researchers and historians, we must dispel the notion that research can't fill gaps on this subject because so many records were destroyed. The truth is out there.
The Mitsubishi A6M3 Zero-Sen ("Hamp") by R. J. Francillon, Aircraft In Profile No. 190 , ©1967
Japanese Aircraft of the Pacific War by R. J. Francillon, Naval Inst. Press, ISBN 0-87021-313-X, ©1970
Zero Fighter by Martin Caidin, Ballantine weapons book #9 ©1970
A6M1-2-2N Zero-Sen by Richard Bueschel, Schiffer, ISBN 0-88740-754-4, ©1995 (This book is a reprint Zero Fighter by Robert C. Mikesh and Rikyu Watanabe, Crown, ISBN 0-517-54260-9, ©1981
Zero Japan's Legendary Fighter by Robert C. Mikesh, Motorbooks, ISBN 0-87938-915-X, ©1994
Famous Airplanes of the World - A6M models 22-63, #56, 1996
The Siege of Rabaul by Henry Sakaida, ©1996, ISBN 1-883809-09-6
Acknowledgements go to all the researchers and historians who have brought this information to be published, including (but not limited at all to) the late Richard Bueschel, René Francillon, Robert Mikesh, Shigeru Nohara, Jim Broshot, and many others. A special thanks to Jim Long and Jim Lansdale, whose encouragement and assistance have made this a fruitful exercise.
©2000 Rob Graham