Performance difference between the A6M5a and A6M5b/c

Hello All,

There certainly has been a lot of traffic here since I last stopped by.
Hopefully I can cover a few of the points that have been brought up.

Regarding 85 octane fuel used by the Japanese:
This is certainly true, but it was not the standard. It was the end result of a nation that could no longer even feed itself and sustain any kind of industry because of the blockade and bombing raids. The standard was still 91 octane for the Army and 92 octane for the Navy.
As for Army versus Navy, one can basically consider that the Army and Navy were two entirely separate and COMPETING forces that took inter service rivalry to a new standard. The Army did not use Navy weapons or aircraft and vice versa.

The way that the Japanese got around the low octane limitations was to use a LOT of anti-detonant. Just about any power setting over "cruise" was supplemented by Water-Methanol injection.
I will use the Nakajima Ki 84 with its Homare engine as an example:
Emergency Power
2000 HP @ 3000 RPM @ Sea Level -- +500 mm Boost (49.2 inches Hg)
Rated Power
1890 HP @ 3000 RPM @ 1700 Meters -- +350 mm Boost (43.3 inches Hg)
Cruise Power
1300 HP @ 2600 RPM @ 3000 Meters -- +100 mm Boost (33.5 inches Hg)

Note that Water-Methanol injection come on at +180 mm Boost (36.6 inches Hg).
Fuel:
Fuselage: 217 Liters
Wing Main: 346 Liters
Wing Aux: 134 Liters
Water-Methanol: 130 Liters
So, for 700 Liters of fuel, the Hayate carries 130 Liters of Water-Methanol which is a pretty high percentage and starts using it just barely above cruise settings.

The problem with the Nakajima Homare engine was that although it was capable of developing around 2000 HP if everything worked as designed, in service things generally did not work as well. Manufacturing quality of accessories was poor and typical power was more like 1300-1500 HP.
The Ki 84 tested at Middletown which hit 427 MPH had to have accessories like a fuel pump and ignition harness replaced or rebuilt because the originals did not work well. With all that done, the engine developed the power levels claimed in Japanese specifications on US 92 octane fuel.

Regarding the idea of the Japanese building the Messerschmitt 109F under license:
The basic problem was that first, the Germans only licensed the DB 601Aa to the Japanese.
The Japanese managed to improve the supercharger almost to the level of the DB 601A non-export version.
The big problem was that they could not manufacture them so that they were reliable.
If they could, they already had an airframe in the Ki 61 that was arguably superior to the Me 109F.

The Mitsubishi Kasei was a reliable and relatively powerful engine but its frontal area was much too large.
It was intended for lower speed bombers and transports and would probably not have worked well in the 109.
Please note the way that the J2M Raiden was designed to fit that engine.

Regarding development of the A6M Type Zero:
The A6M was designed was as little excess structural strength as possible in order to reduce weight.
Its load limit was 6G and while most other aircraft were designed to be able to withstand up to 50% extra load before structural failure, the A6M was not. (25% IIRC)
This 6G maximum load was for a fighter that in its first service variant weighed about 5300 pounds loaded.
With the early A6M5, normal gross weight had already reached 6000 pounds. Additional weight in the way of engines and armament would only further reduce the load limits on the weak airframe.

By contrast, contemporary fighters were usually designed to 6G to 8G with 50% safety margin to structural failure and had the extra "stretch" for additional weight in engines, armour, and armament that the A6M did not have.

Regarding reworking the controls:
The A6M was intentionally designed with some flexibility in its controls. Horikoshi commented that this was to avoid overstressing the airframe and while normally this would result in control flutter, it did not in this case. My belief is that a redesign for better high speed control would not have been an easy task.

- Ivan.

The Zeke was forced into a role for which it was unsuited. All of the modifications that were made to it from 1943 onward were designed around the concept of increasing its survivability as a defensive fighter. It had been designed initially as a lightweight long range fighter able to reach all the way from the Japanese held bases in central china to the KMT bases in western china, and out perform and land based opposition it was likely to encounter. It was designed to carry a composite armament, in which its cannon armament was intended to be used only for a very limited amount of time but to be devastating in the firepower it could deliver. At a time when other nations were equipping their fighters with a maximum of two light MGs this was probably true to say about the early Zekes.

The zeke took advantage of some new materials that enabled the zeke to be built to very light construction standards. At a time when the F4f had a maximum loaded weight of 7000 lbs (with only 750 lbs of fuel, equating to 110 gallons of fuel) , the zeke (32) weighed in at 6200 lbs, of which 1200lbs or 175 gallons was for fuel.

So the early zekes were designed with four main criteria, adequate speed and climb performance to deal with enemy land based fighters, high degrees of manouverability to take advantage of the high skill levels of its pilots, above average firepower levels, but above all these elements, long range to reach across the seas to engage enemy targets at long range to high levels of firepower.


We all know that as the war progressed and the war turned, some of these advantages were either lost outright, or lost their usefulness in a defensive war.

Japanese fighters struggled against the heavily armed and armoured allied bombers. Range was no longer needed as much when forced on the defensive, firepower gradually was found inadequate, particularly the lightweight 7.7mm LMGs. Divespeed was a particular weakness and the fragility of the Zeke is well known.

Japanese efforts after 1942 were designed to partially address the shortcomings evident in the Zeke. Firepower gradually crept up and ammunition supply increased for the 20mm cannon after it was changed to a belt fed weapon. The lightweight 7.7mm guns were exchanged for some fairly hard hitting 13.2mm guns with armour piercing capabilities. Dive speed was improved, to just under 450mph, mostly by strengthening of the wings to withstand the stresses. Engine power crept up firstly to 1130hp, then with water methanol injection, and in the final marks to a 1560hp Kinsei, which would have been interesting.

Some grudging efforts at improving protection and fire retardance were made in the A6M5. The early types had some rather pathetic attempts at improving protection whilst later subtypes of the A6M5 were given comprehensive protection schemes but were found to be grossly underpowered. It would have been interesting to see how the final mark , the A6M8 would have stood against the hellcat. The A6M8 had higher climb rate, inferior top speed, better range, better firepower, inferior dive characteristics (but not as hopeless as the A6M3), better turn capability, similar levels of protection and far less propensity to catch fire. It was still only about half the weight, suggesting it would still crumple under fire fairly easily.

The A6M8 would have been far more suited to japanese situation later in the war, and could have been in service at least from may 1944 except for the effort wasted on the a7M Reppu, but even under the most optimistic of assessment, would still have been hard pressed against the later war US types. The truth is, the A6M was not very suitable as a defensive fighter. Something better was needed, though it was never going to be the army Ki-61. The Navy types with potential to replace the zeke were the George and the J2M. Either of these types had the potential to make the IJN air force more competitive in its defensive battles but they were never seriously considered in this way.

parsifal said:

At a time when other nations were equipping their fighters with a maximum of two light MGs this was probably true to say about the early Zekes.

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Much of your post is well thought and good summation but this part may be in error. In the period between the start of work on the Zero (1937) and first flight (April 1939) only the Japanese Army was sticking with two light machine guns. The British had moved to eight, even the Gladiator had four. The French had moved to an engine mounted 20mm cannon in service aircraft in 1935 supplemented by a LMG in each wing. The Germans had been using 4 gun 109s in Spain in 1938. The Italians were using a pair of 12.7mm guns. The Russians while using a pair of LMGs on a lot of their fighters were equipping some batches with four guns and the Russian gun fired much faster than everybody else's, perhaps twice as fast as the Japanese LMG. Only the Americans had really retained two guns but one of them was a heavy machine gun and by the time of the 1939 fighter trials heavier armament was certainly under consideration. XP-40 carried two HMGs and the XP-39 and XP-38 having cannon and a pair of HMGs at a minimum. The order for the four gun P-36C had been placed in Jan 1939.
The Zero may have been keeping up (vs the Ki 43 not keeping up) but it wasn't really ahead of the trend.

What obsolete aircraft various nations shuffled off to their colonies to face the Japanese vs keeping at home my be different but the Japanese knew they would not be facing two LMG fighters for very long.

Ivan1GFP said:

Regarding development of the A6M Type Zero:
The A6M was designed was as little excess structural strength as possible in order to reduce weight.
Its load limit was 6G and while most other aircraft were designed to be able to withstand up to 50% extra load before structural failure, the A6M was not. (25% IIRC)
This 6G maximum load was for a fighter that in its first service variant weighed about 5300 pounds loaded.
With the early A6M5, normal gross weight had already reached 6000 pounds. Additional weight in the way of engines and armament would only further reduce the load limits on the weak airframe.

By contrast, contemporary fighters were usually designed to 6G to 8G with 50% safety margin to structural failure and had the extra "stretch" for additional weight in engines, armour, and armament that the A6M did not have.

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This has been adressed by both Shinpachi and me quite a few times on this boards. Shinpachi even posted a table from A6M3 maintnance manual clearly indicating that A6M Zero load limit was 7 G with 1.8 safety factor. This applies equally to A6M2 and A6M5 models.

The wing loading on even the "small wing" A6M5 was very low compared to late war fighters, even the A6M5 looks like a stick with wings! Could small wings ala USSR Yak-9 have given A6M5 ~380mph?

I only just realised that the A6M8 Zero prototypes used the Mitsubishi Kinsei engine which was the same as in the Ki-100 which is spoken of very highly in 1945 combat (army designation for late Kinsei was Ha-115-II). The other shock is the Zero designer Horikoshi had wanted to use the Kinsei from the very start in 1940! This would have given the Zero models a proven path from 1075-1300-1560hp over WW2 rather than been stuck with the little Sakae motor 980-1130hp.

Hiromachi said:

This has been adressed by both Shinpachi and me quite a few times on this boards. Shinpachi even posted a table from A6M3 maintnance manual clearly indicating that A6M Zero load limit was 7 G with 1.8 safety factor. This applies equally to A6M2 and A6M5 models.

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Thanks for the correction, Hiromachi.
I do remember reading an article to this effect but obviously it (and I) was incorrect.
I had a bit of a chore finding my copy of "Eagles of Mitsubishi" but found it a few days ago.
It actually states that while the requirements were for a Load limit of 7G and 1.8 safety factor, Horikoshi actually designed many of the parts to only a 1.6 safety factor. There is a pretty detailed discussion as to why long thin parts that were compression loaded could be designed this way because they would take a proportionately higher load before being permanently deformed.

- Ivan.

taly01 said:

I only just realised that the A6M8 Zero prototypes used the Mitsubishi Kinsei engine which was the same as in the Ki-100 which is spoken of very highly in 1945 combat (army designation for late Kinsei was Ha-115-II). The other shock is the Zero designer Horikoshi had wanted to use the Kinsei from the very start in 1940! This would have given the Zero models a proven path from 1075-1300-1560hp over WW2 rather than been stuck with the little Sakae motor 980-1130hp.

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Hello Taly01,
In Horikoshi's book "Eagles of Mitsubishi", he also discussed the choice of engine for what became the A6M Zero fighter.
He considered the Kinsei engine and the Zuisei engine. The Kinsei would have resulted in a larger and heavier fighter overall, so his choice was the Zuisei engine with a two blade propeller. The propeller was replaced with a three blade version and the engine was replaced with the Nakajima Sakae before the definitive A6M2 production model.

- Ivan.

Ivan1GFP said:

Hello Taly01,
In Horikoshi's book "Eagles of Mitsubishi", he also discussed the choice of engine for what became the A6M Zero fighter.
He considered the Kinsei engine and the Zuisei engine. The Kinsei would have resulted in a larger and heavier fighter overall, so his choice was the Zuisei engine with a two blade propeller. The propeller was replaced with a three blade version and the engine was replaced with the Nakajima Sakae before the definitive A6M2 production model.

- Ivan.

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The Ha-115-II was the army version of the Sakae engine. The army version of the late model Kinsei was Ha-112-II.

The Zusei was basically a short stroke version of the Kinsei, though its development seems to have ended earlier. Later versions did get a two-speed supercharger and were rated at 50-80 hp less than the Sakae 21.

taly01 said:

The wing loading on even the "small wing" A6M5 was very low compared to late war fighters, even the A6M5 looks like a stick with wings! Could small wings ala USSR Yak-9 have given A6M5 ~380mph?

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Some time ago, I saw an A6M in a museum and was surprised at the area and thickness of the wing. I don't doubt that the preoccupation with turning ability resulted in an undesirably large wing.

I imagine that there were no design resources available to do anything beyond improvisations, Perhaps this is why the engine exhaust system was not modified earlier to provide exhaust augmentation.

rinkol said:

Some time ago, I saw an A6M in a museum and was surprised at the area and thickness of the wing. I don't doubt that the preoccupation with turning ability resulted in an undesirably large wing.

I imagine that there were no design resources available to do anything beyond improvisations, Perhaps this is why the engine exhaust system was not modified earlier to provide exhaust augmentation.

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Hello Rinkol,

I am a bit surprised that no one has commented as yet.
The wing on the A6M is actually about average length and area when compared to the P-40, P-51, or Spitfire.
If the museum you visited was the NASM in Washington, DC, then the appearance of a very large wing on their A6M5 is mostly due to the angle from which it is viewed. I don't think a P-40 is in the same building but a Spitfire Mk.VII and a P-51D are both in the same room.

- Ivan.

At the Planes of Fame, we restored our A6M5 Model 52 with the help of the original designer. He said to all present that the A6M was designed for 6g with a 100% safety factor. That is, you could damage it by pulling more than 6g, but it should not come apart until 12g or more, if undamaged. US fighter had an 8g limit with a 50% safety factor, and so were ultimately exactly as strong when the failure was supposed to happen. It was NOT a weak airframe.

That being said, the A6M could sustain many fewer hits than a US fighter before critical damage was done due to lighter sheet metal as well as lighter bulkheads and formers/stringers/etc. The A6M had good, but not overall excellent workmanship. It was as strong as was required, and it served quite well until the new crop of US fighters (Hellcat, Corsair, P-51) showed up. When that happened, it was outclassed as was the F4F in US service, but no replacement was forthcoming for the A6M as has been noted. Our own FM-2 saw service as a mop-up fighter on small carriers because it could operate from them, not because it was superior in any particular way. It succeeded quite well due to the low caliber of opposition it found in mop-up operations against bypassed targets.

GregP said:

...A6M could sustain many fewer hits than a US fighter before critical damage was done due to lighter sheet metal as well as lighter bulkheads and formers/stringers/etc

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No.
As the skin thicknesses were in the class of under one tenth of an inch (of aluminum alloy) in every aircraft of the period (including US ones), that is obviously not an explanation.
Any caliber of bullet (even 0.30 AP) won't even notice going through a sheet(s) that thin.
A6M simply did not have any armor, nor any self sealing tanks.

Hello Timppa,

I believe GregP is correct.
The ability to sustain hits would not be because the aluminum skin can resist being punctured but more a matter of what happens when it is damaged.
With stressed-skin construction, the skin of the aircraft represents a substantial portion of the structural strength and as integrity is lost, the strength is reduced. With lighter structure, it would take less damage before the structure fails under what would be a normal load. There simply wasn't the margin of strength designed in to the A6M as with many other designs.

- Ivan.

Armour doesn't hold aircraft together, or even stop aircraft from being damaged(maybe with one or two exceptions like the A-10) . Thickness of the material, in the context of the outer skin can affect the structural integrity in high G manouvres, or if dive limits are exceeded. And this certainly happened to both US and Japanese a/c.


But the principal determinant relating to structural strength of an aircraft is the subframe, and linked to this is the numbers of fasteners used in the construction of the frame. Compared to a hellcat the zeke had roughly half the numbers of frame fasteners, This was its main reason for its structural weakness. horikoshi attempted to compensate for that by making the spar and the frame a one piece assembly (at least that's what ive read....), and this went some way towards solving the problem. The new alloy used in the skin was revolutionary and much stronger than the old wood/canvas combinations that were still in vogue at the time the A6M alloys were being invented. I'm led to believe that on a weight for weight basis these new alloys for the Zeke were still ahead of the game at the time the hellcat was under development, but I cant confirm that.

Fact is the A6M just didn't have the mass to make it strong.

Hello Parsifal,
Armour may not hold an aircraft together but it would certainly reduce structural damage.
Consider what happens if armour is present and stops the bullet. There is no further damage.
If there is no armour, the bullet continues on to damage the next part and so on until the energy is expended.

The other thing to observe is to look at photographs of wrecks of the Japanese Type Zero Fighter.
From the ones I have seen, there tends to be a lot of distortion of structure and skin, much more so than for most other aircraft. Even on aircraft that are not "damaged", often one can see the frames and stringers underneath the skin presumably because the gauge of the skin is that thin.
If you check the maximum diving speeds of the A6M series, they were generally pretty low except for the very last models which indicated a lack of strength for resisting aerodynamic forces. The improvement was described as increasing the gauge of the skin on the wing.
If you have an aircraft just sitting on the ground and shoot a hole through it, perhaps nothing really happens until you damage enough structure so that it can't support its own weight, but if it is flying, there are plenty of aerodynamic forces to increase damage beyond what the projectile caused. Damaged pieces don't tend to be nearly as aerodynamic as designed and if you happen to be near the load limit, things could get really bad with increased loads and skin ripping off to further weaken the structure.

- Ivan.

Timpa,

The skin thickness of most WWII U,S, fighters was generally 0.040". The skin thickness of the A6M was generally 0.032" with some 0.025" (more then a little) and some 0.020". When everything is connected, they are about as stiff as one another. When some rivets go missing, the 0.040" skin is MUCH stiffer. Doesn't seem as if it should make much difference unless you put them together with rivets and try it out, but it IS.

I have been working on WWII fighters for about 11.5 years and have helped on the A6M5 Model 52 (admittedly not much, but enough to have stripped the fabric off the ailerons and elevators and help with some small, insignificant sheet metal repairs. In other words, I have held the structure in my hands and have seen how stiff it is when some rivets, including ALL have been removed (we replaced one gear well floor during overhaul, and I got to watch close up - like 2 feet - the "we" is not me). I made a small structure with the same metal (0.025") for fun, and it is stiff when together but weak when even 3 or 4 rivets are missing. U.S. planes are not at all.

I am no expert but I DO have 11+ years of working on warbird sheet metal. The A6M5 restoration was mostly done by Cory O'brian at Fighter Rebuilders (Steve Hinton's shop). He does GREAT work. The gear well floor was done by Alex Gonzales and I was right next to him when he did most of it. The metal was 2024-T3 and I still have some of it at my workbench.

No, it won't stop a bullet in one thickness (but we DID find a bullet in the structure). However, 0.040" 2024-T3 is MUCH stronger then 0.020" or 0.025" or 0.032" is.

GregP said:

A lot of folks thought the P-40 was a dog.

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I found these recently in some US archive material - it appears McArthur preferred the P-40 over the P-38 so that means he at least did not consider it a dog

It looks like he was more concerned with maintenance and increased vulnerability than with performance, which was not mentioned other than the P-38 being of better high-altitude performance.

P40Q was a non-starter. In 1942 we has the 425 MPH P-47 in production, the 400 MPH P-38, and were exploring adding a Merlin engine to the 390 MPH P-51. We were building out large production capacity for the P-47 and P-51. In addition, the Navy had the 405 MPH F4U in production, with the F6F hot on its heels.