J2M Raiden

[LostCosmonaut]

Compared to the most well known Japanese fighter of World War 2, the A6M “Zero”, the J2M Raiden (“Jack”) was both less famous and less numerous. More than 10,000 A6Ms were built, but barely more than 600 J2Ms were built. Still, the J2M is a noteworthy aircraft. Despite being operated by the Imperial Japanese Navy (IJN), it was a strictly land-based aircraft. The Zero was designed with a lightweight structure, to give extreme range and maneuverability. While it had a comparatively large fuel tank, it was lightly armed, and had virtually no armor. While the J2M was also very lightly built, it was designed that way to meet a completely different set of requirements; those of a short-range interceptor. The J2M's design led to it being one of the fastest climbing piston-engine aircraft in World War 2, even though its four 20mm cannons made it much more heavily armed than most Japanese planes.

 

 

 

Development of the J2M began in October 1938, under the direction of Jiro Hirokoshi, in response to the issuance of the 14-shi interceptor requirement (1). Hirokoshi had also designed the A6M, which first flew in April 1939. However, development was slow, and the J2M would not make its first flight until 20 March 1942, nearly 3 ½ years later (2). Initially, this was due to Mitsubishi's focus on the A6M, which was further along in development, and of vital importance to the IJN's carrier force. Additionally, the J2M was designed to use a more powerful engine than other Japanese fighters. The first aircraft, designated J2M1, was powered by an MK4C Kasei 13 radial engine, producing 1430 horsepower from 14 cylinders (3) (compare to 940 horsepower for the A6M2) and driving a three bladed propeller. The use of such a powerful engine was driven by the need for a high climb rate, in order to fulfill the requirements set forth in the 14-shi specification.

 

The climb rate of an aircraft is driven by specific excess power; by climbing an aircraft is gaining potential energy, which requires power to generate. Specific Excess Power is given by the following equation;

 

(Airspeed*(Thrust-Drag))/Weight

 

 

 

It is clear from this equation that weight and drag must be minimized, while thrust and airspeed are maximized. The J2M was designed using the most powerful engine then available, to maximize thrust. Moreover, the engine was fitted with a long cowling, with the propeller on an extension shaft, also to minimize drag. In a more radical departure from traditional Japanese fighter design (as exemplified by aircraft such as the A6M and Ki-43), the J2M had comparatively short, stubby wings, only 10.8 m wide on the J2M3 variant, with a relatively high wing loading of 1.59 kN/m2 (33.29 lb/ft2) (2). (It should be noted that this wing loading is still lower than contemporary American aircraft such as the F6F Hellcat. The small wings reduced drag, and also reduced weight. More weight was saved by limiting the J2M's internal fuel, the J2M3 had only 550 liters of internal fuel (2).

 

Hirokoshi did add some weight back into the J2M's design. 8 millimeters of steel armor plate protected the pilot, a luxurious amount of protection compared to the Zero. And while the J2M1 was armed with the same armament as the A6M (two 7.7mm machine guns and two Type 99 Model 2 20mm cannons), later variants would be more heavily armed, with the 7.7mm machine guns deleted in favor of an additional pair of 20mm cannons. Doubtlessly, this was driven by Japanese wartime experience; 7.7mm rounds were insufficient to deal with strongly built Grumman fighters, let alone a target like the B-17.

 

The first flight of the J2M Raiden was on March 20^th, 1942. Immediately, several issues were identified. One design flaw pointed out quickly was that the cockpit design on the J2M1, coupled with the long cowling, severely restricted visibility. (This issue had been identified by an IJN pilot viewing a mockup of the J2M back in December 1940 (1).) The landing speed was also criticized for being too high; while the poor visibility over the nose exacerbated this issue, pilots transitioning from the Zero would be expected to criticize the handling of a stubby interceptor.

 

Wrecked J2M in the Philippines in 1945. The cooling fan is highly visible.

 

However, the biggest flaw the J2M1 had was poor reliability. The MK4C engine was not delivering the expected performance, and the propeller pitch control was unreliable, failing multiple times. (1) As a result, the J2M1 failed to meet the performance set forth in the 14-shi specification, achieving a top speed of only 577 kph, well short of the 600 kph required. Naturally, the climb rate suffered as well. Only a few J2M1s were built.

 

The next version, the J2M2, had several improvements. The engine was updated to the MK4R-A (3); this engine featured a methanol injection system, enabling it to produce up to 1,800 horsepower for short periods. The propeller was switched for a four blade unit. The extension shaft in the J2M1 had proved unreliable, in the J2M2 the cowling was shortened slightly, and a cooling fan was fitted at the the front. These modifications made the MK4R-A more reliable than the previous engine, despite the increase in power.

 

However, there were still problems; significant vibrations occurred at certain altitudes and speeds; stiffening the engine mounts and propeller blades reduced these issues, but they were never fully solved (1). Another significant design flaw was identified in the summer of 1943; the shock absorber on the tail wheel could jam the elevator controls when the tailwheel retracted, making the aircraft virtually uncontrollable. This design flaw led to the death of one IJN pilot, and nearly killed two more (1). Ultimately, the IJN would not put the J2M2 into service until December 1943, 21 months after the first flight of the J2M1. 155 J2M2s would be built by Mitsubishi (3).

 

By the time the J2M2 was entering service, the J2M3 was well into testing. The J2M3 was the most common variant of the Raiden, 260 were produced at Mitsubishi's factories (3). It was also the first variant to feature an armament of four 20mm cannons (oddly, of two different types of cannon with significantly different ballistics (2); the 7.7mm machine guns were replace with two Type 99 Model 1 cannons). Naturally, the performance of the J2M3 suffered slightly with the heavier armament, but it still retained its excellent rate of climb. The Raiden's excellent rate of climb was what kept it from being cancelled as higher performance aircraft like the N1K1-J Shiden came into service.

 

 

The J2M's was designed to achieve a high climb rate, necessary for its intended role as an interceptor. The designers were successful; the J2M3, even with four 20mm cannons, was capable of climbing at 4650 feet per minute (1420 feet per minute) (2). Many fighters of World War 2, such as the CW-21, were claimed to be capable of climbing 'a mile a minute', but the Raiden was one of the few piston-engine aircraft that came close to achieving that mark. In fact, the Raiden climbed nearly as fast as the F8F Bearcat, despite being nearly three years older. Additionally, the J2M could continue to climb at high speeds for long periods; the J2M2 needed roughly 10 minutes to reach 30000 feet (9100 meters) (4), and on emergency power (using the methanol injection system), could maintain a climb rate in excess of 3000 feet per minute up to about 20000 feet (about 6000 meters).

 

 

 

 

 

 

 

 

 

 

 

Analysis in Source (2) shows that the J2M3 was superior in several ways to one of its most common opponents, the F6F Hellcat. Though the Hellcat was faster at lower altitudes, the Raiden was equal at 6000 meters (about 20000 feet), and above that rapidly gained superiority. Additionally, the Raiden, despite not being designed for maneuverability, still had a lower stall speed than the Hellcat, and could turn tighter. The J2M3 actually had a lower wing loading than the American plane, and had flaps that could be used in combat to expand the wing area at will. As shown in the (poorly scanned) graphs on page 39 of (2), the J2M possessed a superior instantaneous turn capability to the F6F at all speeds. However, at high speeds the sustained turn capability of the American plane was superior (page 41 of (2)).

 

The main area the American plane had the advantage was at high speeds and low altitudes; with the more powerful R-2800, the F6F could more easily overcome drag than the J2M. The F6F, as well as most other American planes, were also more solidly built than the J2M. The J2M also remained plagued by reliability issues throughout its service life.

 

In addition to the J2M2 and J2M3 which made up the majority of Raidens built, there were a few other variants. The J2M4 was fitted with a turbo-supercharger, allowing its engine to produce significantly more power at high altitudes (1). However, this arrangement was highly unreliable, and let to only two J2M4s being built. Some sources also report that the J2M4 had two obliquely firing 20mm Type 99 Model 2 cannons in the fuselage behind the pilot (3). The J2M5 used a three stage mechanical supercharger, which proved more reliable than the turbo-supercharger, and still gave significant performance increases at altitude. Production of the J2M5 began at Koza 21^st Naval Air Depot in late 1944 (6), but ultimately only about 34 would be built (3). The J2M6 was developed before the J2M4 and J2M6, it had minor updates such as an improved bubble canopy, only one was built (3). Finally, there was the J2M7, which was planned to use the same engine as the J2M5, with the improvements of the J2M6 incorporated. Few, if any, of this variant were built (3).

 

A total of 621 J2Ms were built, mostly by Mitsubishi, which produced 473 airframes (5). However, 128 aircraft (about 1/5^th of total production), were built at the Koza 21^st Naval Air Depot (6). In addition to the reliability issues which delayed the introduction of the J2M, production was also hindered by American bombing, especially in 1945. For example, Appendix G of (5) shows that 270 J2Ms were ordered in 1945, but only 116 were produced in reality. (Unfortunately, sources (5) and (6) do not distinguish between different variants in their production figures.)

 

Though the J2M2 variant first flew in October 1942, initial production of the Raiden was very slow. In the whole of 1942, only 13 airframes were produced (5). This included the three J2M1 prototypes. 90 airframes were produced in 1943, a significant increase over the year before, but still far less than had been ordered (5), and negligible compared to the production of American types. Production was highest in the spring and summer of 1944 (5), before falling off in late 1944 and 1945.

 

The initial J2M1 and J2M2 variants were armed with a pair of Type 97 7.7mm machine guns, and two Type 99 Model 2 20mm cannons. The Type 97 used a 7.7x56mm rimmed cartridge; a clone of the .303 British round (7). This was the same machine gun used on other IJN fighters such as the A5M and A6M. The Type 99 Model 2 20mm cannon was a clone of the Swiss Oerlikon FF L (7), and used a 20x101mm cartridge.

 

The J2M3 and further variants replaced the Type 97 machine guns with a pair of Type 99 Model 1 20mm cannons. These cannons, derived from the Oerlikon FF, used a 20x72mm cartridge (7), firing a round with roughly the same weight as the one used in the Model 2 at much lower velocity (2000 feet per second vs. 2500 feet per second (3), some sources (7) report an even lower velocity for the Type 99). The advantage the Model 1 had was lightness; it weighed only 26 kilograms vs. 34 kilograms for the model 2. Personally, I am doubtful that saving 16 kilograms was worth the difficulty of trying to use two weapons with different ballistics at the same time. Some variants (J2M3a, J2M5a) had four Model 2 20mm cannons (3), but they seem to be in the minority.

 

 

 

In addition to autocannons and machine guns, the J2M was also fitted with two hardpoints which small bombs or rockets could be attached to (3) (4). Given the Raiden's role as an interceptor, and the small capacity of the hardpoints (roughly 60 kilograms) (3), it is highly unlikely that the J2M was ever substantially used as a bomber. Instead, it is more likely that the hardpoints on the J2M were used as mounting points for large air to air rockets, to be used to break up bomber formations, or ensure the destruction of a large aircraft like the B-29 in one hit. The most likely candidate for the J2M's rocket armament was the Type 3 No. 6 Mark 27 Bomb (Rocket) Model 1. Weighing 145 pounds (65.8 kilograms) (8), the Mark 27 was filled with payload of 5.5 pounds of incendiary fragments; upon launch it would accelerate to high subsonic speeds, before detonating after a set time (8). It is also possible that the similar Type 3 No. 1 Mark 28 could have been used; this was similar to the Mark 27, but much smaller, with a total weight of only 19.8 pounds (9 kilograms).

 

 

 

The first unit to use the J2M in combat was the 381^st Kokutai (1). Forming in October 1943, the unit at first operated Zeros, though gradually it filled with J2M2s through 1944. Even at this point, there were still problems with the Raiden's reliability. On January 30^th, a Japanese pilot died when his J2M simply disintegrated during a training flight. By March 1944, the unit had been dispatched to Balikpapan, in Borneo, to defend the vital oil fields and refineries there. But due to the issues with the J2M, it used only Zeros. The first Raidens did not arrive until September 1944 (1). Reportedly, it made its debut on September 30^th, when a mixed group of J2Ms and A6Ms intercepted a formation of B-24s attacking the Balikpapan refineries. The J2Ms did well for a few days, until escorting P-47s and P-38s arrived. Some 381^st Raidens were also used in defense of Manila, in the Phillipines, as the Americans retook the islands. (9) By 1945, all units were ordered to return to Japan to defend against B-29s and the coming invasion. The 381st's J2Ms never made it to Japan; some ended up in Singapore, where they were found by the British (1).

 

 

 

least three units operated the J2M in defense of the home islands of Japan; the 302^nd, 332^nd, and 352^nd Kokutai. The 302nd's attempted combat debut came on November 1^st, 1944, when a lone F-13 (reconaissance B-29) overflew Tokyo (1). The J2Ms, along with some Zeros and other fighters, did not manage to intercept the high flying bomber. The first successful attack against the B-29s came on December 3^rd, when the 302^nd shot down three B-29s. Later that month the 332^nd first engaged B-29s attacking the Mitsubishi plant on December 22^nd, shooting down one. (1)

The 352^nd operated in Western Japan, against B-29s flying out of China in late 1944 and early 1945. At first, despite severe maintenace issues, they achieved some successes, such as on November 21^st, when a formation of B-29s flying at 25,000 feet was intercepted. Three B-29s were shot down, and more damaged.

In general, when the Raidens were able to get to high altitude and attack the B-29s from above, they were relatively successful. This was particularly true when the J2Ms were assigned to intercept B-29 raids over Kyushu, which were flown at altitudes as low as 16,000 feet (1). The J2M also had virtually no capability to intercept aircraft at night, which made them essentially useless against LeMay's incendiary raids on Japanese cities. Finally the arrival of P-51s in April 1945 put the Raidens at a severe disadvantage; the P-51 was equal to or superior to the J2M in almost all respects, and by 1945 the Americans had much better trained pilots and better maintained machines. The last combat usage of the Raiden was on the morning of August 15^th. The 302nd's Raidens and several Zeros engaged several Hellcats from VF-88 engaged in strafing runs. Reportedly four Hellcats were shot down, for the loss of two Raidens and at least one Zero(1). Japan surrendered only hours later.

At least five J2Ms survived the war, though only one intact Raiden exists today. Two of the J2Ms were captured near Manila on February 20^th, 1945 (9) (10). One of them was used for testing; but only briefly. On its second flight in American hands, an oil line in the engine failed, forcing it to land. The aircraft was later destroyed in a ground collision with a B-25 (9). Two more were found by the British in Singapore (1), and were flown in early 1946 but ex-IJN personnel (under close British supervision). The last Raiden was captured in Japan in 1945, and transported to the US. At some point, it ended up in a park in Los Angeles, before being restored to static display at the Planes of Fame museum in California.

 

 

 

 

Sources:

 

 

1. https://www.docdroid.net/gDMQra3/raiden-aeroplane-february-2016.pdf#page=2

2. F6F-5 vs. J2M3 Comparison

3. http://www.combinedfleet.com/ijna/j2m.htm

4. http://www.wwiiaircraftperformance.org/japan/Jack-11-105A.pdf

5. https://babel.hathitrust.org/cgi/pt?id=mdp.39015080324281;view=1up;seq=80

6. https://archive.org/stream/corporationrepor34unit#page/n15/mode/2up

7. http://users.telenet.be/Emmanuel.Gustin/fgun/fgun-pe.html

8. http://ww2data.blogspot.com/2016/04/imperial-japanese-navy-explosives-bombs.html

9. https://www.pacificwrecks.com/aircraft/j2m/3008.html

10. https://www.pacificwrecks.com/aircraft/j2m/3013.html

11. https://www.pacificwrecks.com/aircraft/j2m/3014.html

 

 

Further reading:

 

An additional two dozen Raiden photos: https://www.worldwarphotos.info/gallery/japan/aircrafts/j2m-raiden/

 

 

[LostCosmonaut]

Added some pictures

[LostCosmonaut]

Some brief shots of J2Ms on the ground and taking off in the first minute of this video;

 

 

Additional footage (untranslated):

 

 

Brief footage of testing of J2M (very brief appearance at the end) and other captured Japanese aircraft;

 

 

 

 

[Collimatrix]

I had read that the J2M had laminar-flow airfoils.  Do you have any more information on that?

[watch_your_fire]

It's quite interesting that the J2M was faster than the Hellcat above 6km, I wonder if it's due to the pressure generated by the engine's inlet fan?

The Fw-190 is the only other plane that comes to mind when I think of this sort of inlet fan, so it might have been one of those "super special axis wonderwaffles" that gave these planes an edge at higher altitudes. What's strange to me, though, is that the main production J2Ms (J2M2 and J2M3) didn't even have superchargers AFAIK, which is.... awful, frankly, for a plane that was fighting this late into the war. And yet, even without superchargers, the flight performance seems reasonably competitive even at those higher altitudes.

In fact, the F6F did have a two speed supercharger through most of it's variants, so the fact that the J2M was faster at higher altitudes (where superchargers become increasingly important) is doubly paradoxical.

 

To me, and of course this is just a guess, I would have to imagine that Jiro Horikoshi must have predicted that Japan's aircraft industry would be incapable of producing proper modern supercharged radial engines, while still early in the design process of the J2M. This would make sense, as his most famous design, the A6M, was basically built around this idea of making the absolute most out of Japan's terrible aircraft radials. So, rather than building the J2M along the lines of American or European radial powered fighters, he must have seriously spent some time figuring out how to maximize ram air with that huge engine cowling and that large inlet fan. The end result was a fighter that could maintain manifold pressure up to high altitudes better than some planes that had the luxury of turbo/superchargers.

 

I would propose then that while the FW-190s inlet fan was included more for cooling and feeding it's supercharger than anything else, the inlet fan on the J2M was included explicitly to maximize manifold pressure at higher altitudes.

 

It's a shame the Japs burned all their documents after the war, I would be really interested in exactly how fast that fan spins, and how much air pressure there is inside that engine cowling as compared to the outside.

 

[Sturgeon]

@watch_your_fire Welcome to the forum!

[Collimatrix]

On 4/25/2021 at 2:52 PM, watch_your_fire said:

It's quite interesting that the J2M was faster than the Hellcat above 6km, I wonder if it's due to the pressure generated by the engine's inlet fan?

The Fw-190 is the only other plane that comes to mind when I think of this sort of inlet fan, so it might have been one of those "super special axis wonderwaffles" that gave these planes an edge at higher altitudes. What's strange to me, though, is that the main production J2Ms (J2M2 and J2M3) didn't even have superchargers AFAIK, which is.... awful, frankly, for a plane that was fighting this late into the war. And yet, even without superchargers, the flight performance seems reasonably competitive even at those higher altitudes.

In fact, the F6F did have a two speed supercharger through most of it's variants, so the fact that the J2M was faster at higher altitudes (where superchargers become increasingly important) is doubly paradoxical.

 

To me, and of course this is just a guess, I would have to imagine that Jiro Horikoshi must have predicted that Japan's aircraft industry would be incapable of producing proper modern supercharged radial engines, while still early in the design process of the J2M. This would make sense, as his most famous design, the A6M, was basically built around this idea of making the absolute most out of Japan's terrible aircraft radials. So, rather than building the J2M along the lines of American or European radial powered fighters, he must have seriously spent some time figuring out how to maximize ram air with that huge engine cowling and that large inlet fan. The end result was a fighter that could maintain manifold pressure up to high altitudes better than some planes that had the luxury of turbo/superchargers.

 

I would propose then that while the FW-190s inlet fan was included more for cooling and feeding it's supercharger than anything else, the inlet fan on the J2M was included explicitly to maximize manifold pressure at higher altitudes.

 

It's a shame the Japs burned all their documents after the war, I would be really interested in exactly how fast that fan spins, and how much air pressure there is inside that engine cowling as compared to the outside.

 

Hello, and welcome to the forums.


The standard J2M definitely had a mechanically driven supercharger; basically all WWII piston engines do.  I believe that little accessory case strapped to the back of the engine has the supercharger in it somewhere:



The heat dissipation finning on WWII radial engines is truly a magnificent form of art.

AIUI, the engine cowling cooling fan reduces power at low airspeeds, since it's strapped to the engine crankshaft and is therefore taking some power to generate cooling airflow, but that this power loss basically goes away at high airspeeds as the ram air pressure of the incoming airstream forces the fan around and offloads and power loss it would otherwise cause.  I don't think it does much, if anything, for manifold pressure.  The FW-190 and the Raiden both have exceptionally tightly wrapped radial engine cowlings.  In order to ensure adequate airflow for cooling at low speeds, that fan needs to be there to actively shove more air over the cylinder fins.

As for why high altitude favors the Jack over the Hellcat, according to this site, the Jack has 1,800 horsepower at takeoff and 1,410 horsepower at 15,700 feet.  Per this very good book, the F6F3 has 2000 horsepower at takeoff, 1800 at 13,500 feet and 1650 at 22,500 feet.  In other words, the Hellcat has a slightly higher percentage of its takeoff power rating (82.5% vs 80%) 6,800 feet higher than the Jack.  So, that big P&W mill is clearly capable of maintaining a better percentage of its power at altitude, on top of being a more powerful (and larger) engine to begin with.

However, the Raiden was a land-based fighter, you know, despite being operated by the Imperial Japanese Navy.  The F6F has gigantic barn door wings to ensure good handling during carrier approaches.  The Raiden has teeny tiny little wings, since as a land-based fighter it can afford much higher landing speeds.

[Jeeps_Guns_Tanks]

Those fins are not up to Pratt&Whitney standards, very chonk.

[Collimatrix]

On 4/26/2021 at 6:32 PM, Jeeps_Guns_Tanks said:

Those fins are not up to Pratt&Whitney standards, very chonk.

 

True!