Aerodynamic Drag Properties of the A6M

[Ivan1GFP]

I checked in Squadron/Signal Zero and the A6M3 -> A6M5 did change air intake slightly higher and wider on same cowl shape. The A6M7 cowl appears have an even higher upper cowl line and a bulge in lower cowl, but I had to look so hard my eyes hurt! Model 62/63 is a very confusing model, and I think engineers rather work on A6M8 or A7M!

Hello Taly01,
I also have quite a few of the Squadron In Action books but had not thought to look there.
Thanks for the reference.

Osprey Aces Ki-43 has pilots claims that the last Ki-43-II which came with wing racks was slower than earlier Ki-43-II (no racks) and state "some 15mph slower" (although japanese army would have used kph). It must have been with the bomb claws attached.

Found some new comprehensive Jp.wiki data

栄 (エンジン) - Wikipedia

Ha-115/Sakae.21 all used +200mm as standard boost and +300mm as T.O. boost, so the Anti-Detonant-Injection water only increased power by "evaporative cooling density change of compresed intake air". All 7.2:1 compression all peak T.O. at +300mm boost and 2750rpm

A6M6 Sakae 31 T.O. 1300hp ***failed engine***
A6M7 m.62/3 Sakae 31a/b T.O. 1210hp

Ki-43-IIa (early) Ha-115 T.O. 1130hp
1st stage peak 1100hp @ 2850m +200mm
Ki-43-III Ha-115-II T.O. 1300hp
1st stage peak 1230hp @ 2800m +200mm

So at 200mm boost Ki-43-IIa vs Ki-43-III is 1100 vs 1230 = 12%
So at 300mm boost Ki-43-IIa vs Ki-43-III is 1130 vs 1300 = 15%

Nice improvement!

It sounds like you are making the assumption that the speed loss was exactly the same with the wing bomb racks on -II and -III and the result did not actually come from test data.

Regarding power figures, the power differences are enough that I am wondering what else may have changed in these engines.
Do you happen to have any numbers for power output at the critical altitude for the second speed of the supercharger?
That would be the number that would make the most difference for maximum speeds.

- Ivan.

 

[taly01]

It sounds like you are making the assumption that the speed loss was exactly the same with the wing bomb racks on -II and -III and the result

There is very little data on late model Ki-43 (or A6M5c/7) and no defined speed effects of racks. I found actual tests on wwiiaircraftperformance.org where P-51 and F6F lost 11-13mph with racks so its a relevant baseline.

Regarding power figures, the power differences are enough that I am wondering what else may have changed in these engines.

The 12% change from the 1100-1230hp at low altitude first speed on same boost is plausibly due to the water-injection effects (~6%) and better exhaust stacks cylinder scavenging (~6%), hypothetically Its second speed power is harder to compare as heights are different due to different supercharger speed gearing.

Sakae 21/31b & Ha115
Two speed 980 hp / 2,700 rpm / boost + 200 mmhg (altitude 6,000 m)

Ha115-II
Two speed 950 hp / 2,700 rpm / boost + 200 mmhg (altitude 6,800 m)

 

[Ivan1GFP]

There is very little data on late model Ki-43 (or A6M5c/7) and no defined speed effects of racks. I found actual tests on wwiiaircraftperformance.org where P-51 and F6F lost 11-13mph with racks so its a relevant baseline.

Hello Taly01,
I am not convinced that those numbers from tests of P-51 and F6F are a useful baseline for determining the effects of wing racks on Hayabusa.

The 12% change from the 1100-1230hp at low altitude first speed on same boost is plausibly due to the water-injection effects (~6%) and better exhaust stacks cylinder scavenging (~6%), hypothetically Its second speed power is harder to compare as heights are different due to different supercharger speed gearing.

Sakae 21/31b & Ha115
Two speed 980 hp / 2,700 rpm / boost + 200 mmhg (altitude 6,000 m)

Ha115-II
Two speed 950 hp / 2,700 rpm / boost + 200 mmhg (altitude 6,800 m)

Modifications almost never add up as nicely as the numbers you are getting here.
I figure if you can't change the boost pressure or the RPM, then the Water injection is acting as an intercooler and its effects as an anti-detonant might be allowing more ignition advance.
I am certainly no expert on engines, especially aero engines, but I am finding it very hard to believe that water injection and ejector exhausts would give this level of power increase without some other changes.
I can see this level of power increase with a street car engine, but that presumes that it had silencers in place before and aero engines don't have that kind of restriction.

Your second speed critical altitude numbers between Sakae 21 and Ha-115-II don't seem significantly different.

- Ivan.

 

[Shortround6]

I am not at all sure that once you go to positive pressure in the intake manifold that the exhaust system back pressure is quite as important as on a naturally aspirated engine.
Not saying there is no effect but on an engine with no supercharger changing the exhaust flow may have a bigger percentage of impact,

I am also not sure about how much "scavenging" was going on. On cars (and motorcycles) you can sometimes get a high degree of scavenging by having the length of the exhaust pipes and the speed of sound in them coincide with certain rpm ranges the engine is running at. This would result in timing a low pressure wave traveling back up the pipe to hit the exhaust port at about the time the valve opened up.

With short pipes the useful pressure wave travels correspond to an RPM level unknown to aircraft engines. As in if works at 6000rpm on car engine x you need a pipe twice as long for it to work at 3000rpm. ANd then it doesn't work at 6000rpm or you are an order lower in effect.

I will note that once you get to the 7 outlet exhaust stack design you are getting some horsepower from the exhaust thrust. Perhaps 6-10% depending on speed and altitude.
I would also note that exhaust thrust is dependent on the weight of the exhaust gases/products and if you start injecting water and alcohol at the rate of 6-12 pounds a minute (over and above the weight of the fuel and air) and have the resulting steam and by products coming out the exhaust pipes at 1300-1800fpm (depending on altitude and other things) that might also increase the propulsive effect acting on the airplane. This propulsive effect is NOT measure in the power ratings at the propshaft but may explain changes in performace that don't seem to line up with changes in prop shaft HP.

Or it may not.

 

[taly01]

I am not at all sure that once you go to positive pressure in the intake manifold that the exhaust system back pressure is quite as important as on a naturally aspirated engine

Yes thats probably true,

, reducing back pressure and such are not going to reduce the heat load on the exhaust valves even if they reduce the heat of the exhaust manifold.

I also reply to an earlier question why lower exhaust back pressure may be kinder on exhaust valves, that is by a Pressure drop results in a temperature drop in a gas... basic gas laws. Oh course theory and practice are sometimes different.

I suspect the often quoted 351mph speed for A6M5 was a pre-production test machine.

Guessed Lucky and found this!
Mitsubishi Zero: Japan's Legendary Fighter By Peter C Smith p.159
A prototype was converted from a model 22 machine and made its debut in August 1943 achieving a maximum speed of 351.07mph (565kph).

 

[taly01]

On another link here I found a US intel manual of 1/1944 with translated docs of Japanese water injection tests. It states to use 50:50 water methanol (MW50) and more HP is generated under boost with MW50+92 than with high test gasoline alone!

The Kasei 2x series (~1800hp) engine gains ~50hp, so about a ~3% gain from MW50 alone.

WW2 aircraft Data Base: Japanese Aircraft Engines

check page 124 to see this

The page above appears to be a Sakae 2x series engine from hp shown and seems to be for 87 octane only and peaked at +270mm, The text says on MW50+87 octane it could reach +500mm boost?! Unfortunately that graph was not translated

 

[Ivan1GFP]

I also reply to an earlier question why lower exhaust back pressure may be kinder on exhaust valves, that is by a Pressure drop results in a temperature drop in a gas... basic gas laws. Oh course theory and practice are sometimes different.
).

Hello Taly01,
Just to put this in context, we are discussing the effects of water / water methanol injection increasing power without increasing RPM and boost and as a separate issue the idea that individual ejector stacks and their effect on reducing backpressure.
While I can see how water injection can act as a charge air cooler and increase charge density, I am not convinced that ejector stacks significantly reduce backpressure over the collector ring setup.

Even if it did, you are assuming that lower pressure in the exhaust manifold would lower the cylinder pressures and thus reduce the temperature according to Boyle's Law. I believe this not to be the case because the exhaust is only open AFTER the combustion has happened in the cylinder which is closed off to the exhaust. Also, if the scavenging is better, then the effect (assuming some valve overlap) is that there will be a larger charge going into the cylinder for the next power stroke and there will certainly be more gas going past the exhaust valve and it will also be at a higher temperature by that same Boyle's Law.

Guessed Lucky and found this!
Mitsubishi Zero: Japan's Legendary Fighter By Peter C Smith p.159
A prototype was converted from a model 22 machine and made its debut in August 1943 achieving a maximum speed of 351.07mph (565kph).

I will have to go look for that book, but even if that is one listing for that speed for a prototype A6M5, it does not necessarily mean that production versions were slower.

The page above appears to be a Sakae 2x series engine from hp shown and seems to be for 87 octane only and peaked at +270mm, The text says on MW50+87 octane it could reach +500mm boost?! Unfortunately that graph was not translated

I believe the notes / translation got scrambled a bit. From what I can see, the actual Kasei description seems to begin at about Page 121 and it is not entirely clear what is being done in the test.
I started looking at this but have not finished reading it or trying to figure it out yet.
There are several things to consider:
It is not clear what the Low Octane + MW actually is. Is it 87 octane or 91 octane?
What exactly is the "High Octane" fuel? 96 octane?
Is 96 octane sufficient for the engine to run without detonation?
If Water / Methanol is being used as a charge cooler and detonation happens eventually ANYWAY but is just delayed a bit, it sounds more like the charge cooler is reducing the heat buildup in the engine so that it takes longer for the engine to heat soak and that is really all we are seeing.

- Ivan.

 

[Shortround6]

If Water / Methanol is being used as a charge cooler and detonation happens eventually ANYWAY but is just delayed a bit, it sounds more like the charge cooler is reducing the heat buildup in the engine so that it takes longer for the engine to heat soak and that is really all we are seeing.

- Ivan.

Some radials (and few V-12s) used extra mixture as a both a charge cooler and an internal engine coolant. Any picture showing a piston plane taking off with black smoky exhaust probably means they are using excess fuel as a coolant. They sure aren't burning it.
A number of American engines, R-2800s in particular, used significantly less fuel when the water injection was activated. They had de-enrichment circuits added to the carb or injection unit.
The alcohol is mainly to keep the water from freezing. Water soaks up a huge amount of heat when it flashes to steam, much more than the same amount (weight) of gasoline.

So yes, there are a lot of things going on.

as far as exhaust scavenging goes on supercharged engines, look at the difference between the incoming air/mixture pressure and the outgoing.

On an unsupercharged engine the difference between the pressure in the exhaust ports/pipes and the intake manifold is slight, in fact the pressure before the exhaust valve opens might even be higher (most unsupercharged engines operating at under 14.7lbs in the intake manifold) and a lot of the movement of the gases in under momentum?

If you have a supercharged engine where the mixture coming through the intake valve is under pressure 33% to 80% (5lbs to `12lbs boost) higher than the air pressure in the exhaust port/manifold when the exhaust valve opens I think we can see that the partially burned mixture is not going to stay there very long.
Or that worrying about the difference of 1-2lbs difference in resistance in the exhaust manifold is not going to matter that much.

Not saying there is no effect, but behavior may be different than supercharged engines.

 

[Ivan1GFP]

Some radials (and few V-12s) used extra mixture as a both a charge cooler and an internal engine coolant. Any picture showing a piston plane taking off with black smoky exhaust probably means they are using excess fuel as a coolant. They sure aren't burning it.
A number of American engines, R-2800s in particular, used significantly less fuel when the water injection was activated. They had de-enrichment circuits added to the carb or injection unit.
The alcohol is mainly to keep the water from freezing. Water soaks up a huge amount of heat when it flashes to steam, much more than the same amount (weight) of gasoline.

Hello Shortround6,
Another great example of this is the C3-Einspritzung system that was installed in late model FW 190A/F/G aircraft.
It sprayed fuel directly into one side of the supercharger as an anti-detonant. Apparently the older MW50 system tended to eat spark plugs and this system made for simpler plumbing with a fuel tank replacing the water-methanol tank.
Unfortunately, it seriously raised fuel consumption, but such is the trade off.

as far as exhaust scavenging goes on supercharged engines, look at the difference between the incoming air/mixture pressure and the outgoing.

On an unsupercharged engine the difference between the pressure in the exhaust ports/pipes and the intake manifold is slight, in fact the pressure before the exhaust valve opens might even be higher (most unsupercharged engines operating at under 14.7lbs in the intake manifold) and a lot of the movement of the gases in under momentum?

I believe there is a lot more to the story that you are neglecting here.
I am pretty sure nothing I am describing is new to you but some others may not be so familiar, so apologies in advance.
As you point out: On an unsupercharged engine, the intake manifold is usually running UNDER ambient pressure while the exhaust manifold which gets input from the heated exhaust gases is running ABOVE ambient pressure. So what is keeping the flow moving in the right direction? Momentum is part of it, but something had to develop that momentum to begin with.
The answer is actually the mechanical pumping action of each piston / cylinder.
On the intake stroke, charge moves into the cylinder because the piston is moving down and the volume of the cylinder is increasing and ONLY (mostly true) the intake valve is open.
On the exhaust stroke, combustion gas moves out because the piston is moving up and reducing the space in the cylinder and the exhaust valve is open. Note that the combustion gas is typically quite hot, so the pressure inside the cylinder is likely to be a LOT higher than the exhaust manifold.
Note that although intake and exhaust valves flow the same mass of gas, the exhaust valves are a lot smaller because pressure is higher and they don't need to be as big to flow the same.

The characteristic that a lot of folks may not see is that the flow through each cylinder is not constant; It is a sequence of pulses spaced apart by 720 degrees of engine rotation (for a 4 stroke engine).
This is where momentum of the fuel-air mixture and exhaust gases come into play.
Although the flow in each intake runner / exhaust header is a series of pulses, the flow in the intake plenum / exhaust collector can be much more constant.
If sized properly, the stopping of a column of air as one intake valve closes can be timed to coincide with the opening of the next intake valve and the momentum of the charge air in the plenum "rams" additional charge into the next cylinder.
The same applies to the exhaust header primaries; The closing of one exhaust valve coincides (at the collector) with the opening of the next exhaust valve to produce a scavenging effect.
This momentum idea is often carried a step further with high performance engines.
The spent charge inside the cylinder also has a momentum as it leaves the cylinder.
If the Intake Valve is opened slightly before the Exhaust Valve is closed, this momentum can be used to pull more charge air into the cylinder.
This characteristic is called Valve Overlap. If designed properly, at high RPM, an engine is sometimes (not often) capable of actually pumping more air through than its swept volume would indicate. Volumetric efficiency can be above 100%.
The cost is that valve overlap reduces the efficiency of the mechanical pumping motion of the engine at low RPM.
This degrades the signal to the carburetor / MAP sensor and low RPM operation and off idle response is sacrificed.
This is why some high performance engines are often set to idle fairly high and miss a lot at low RPM
This is my understanding of some of the factors involved.

- Ivan.

 

[Ivan1GFP]

Hello All,

Attached is an image that I find to be rather interesting.
I believe it is the instrument panel of the NASM A6M5.

The interesting thing about this image is that with all the discussions about manifold pressures, I already had an image of the Boost Gauge of the A6M2 and expected to find a different gauge for the A6M5,
The maximum boost of the A6M2 is +250 mm which this gauge is able to display.
The maximum boost of the A6M5 is +300 mm which this gauge is NOT able to display and yet it is installed in this example in the Smithsonian which is one of the aircraft captured at Saipan.

A gauge with the proper range of readings would not have been difficult to acquire. Many Japanese Army fighters used gauges that had a higher range, so one has to wonder why such a thing was installed here.

- Ivan.

 

[taly01]

Perhaps the US test used the markings on the gauge as the "red zone", it would explain why they got ~15mph less than the book values for the A6M5 top speeds.

 

[Ivan1GFP]

Perhaps the US test used the markings on the gauge as the "red zone", it would explain why they got ~15mph less than the book values for the A6M5 top speeds.

Hello Taly01,
I don't believe that situation is very likely because by that time the Allies had already figured out that the Ki 43 and A6M and many other aircraft used the same basic engine.
Keep in mind also that they already had experience with the A6M2 with at least two flyable examples captured in 1942.
Note also that the manual that I have been using as a reference was also captured and translated at the time and I am using a translated and annotated copy.

The part that has me wondering is what exactly the "Overboost Knob" on the panel actually did.
Was this additional throttle movement similar to the "through the gate" on some engines?
It does sound like that in descriptions of the A6M2.
In this case, does that also mean that even at critical altitude there was still additional supercharger capacity?
If so, how much was this capacity?
Was this just a manifold pressure regulator setting in which case there was additional capacity only below critical altitude?

- Ivan.

 

[taly01]

Hi Ivan, found something interesting.

Referring to the US captured and test A6M5 that only went to 250mm Hg, I found that an A6M5 rebuilt in Japan goes to 500mm! and red zone 300mm per specification. I assume this is the historic gauge as part of restoration.

I think this is the plane.

 

[Ivan1GFP]

Hello Taly01,

I believe the panel image is actually from the Chino A6M5, perhaps when it was visiting Japan.
If you check the arrangements of where the instruments are located, they are not really correct for a typical A6M5.
The place normally taken by the artificial horizon is blanked off. The TBI occupies a space normally empty or containing a EGT gauge.
Another point worth noting is that although the colouring is red and black like a typical Japanese gauge, the markings are more appropriate for inches Hg. The Red section shows positive boost up to about where the maximum boost would be for a Sakae 21 if it were marked in inches Hg.
Note there is no negative section as there would be on a Japanese gauge.

- Ivan.

 

[Ivan1GFP]

Perhaps the US test used the markings on the gauge as the "red zone", it would explain why they got ~15mph less than the book values for the A6M5 top speeds.

Hello Taly01,

I had been meaning to get back to you on this issue.
The attached image is a combination of what I believe to be significant excerpts from the testing of captured A6M5 model 52 against US Fighters. It achieved only 335 MPH which is what I believe you were referring to in your comment about 15 MPH less.

In reading through this report, they emphasize qualitative rather than quantitative results (probably for a reason).
As I see it, they had a pretty seriously bent and probably broken but still flyable bird.
There is no reason why the tail should stall first on these aircraft
There is no reason why there should be excessive vibration at a diving speed of only 250 Knots.
Note the conclusion states clearly that performance was not representative of a fully functional A6M5.

I believe however that the markings on the gauges DID mislead the people at Eagle Farm when they tested their rebuilt A6M3 model 32. Note that 40 inches MP is approximately +250 mm boost and 36 inches MP is approximately +150 mm boost.
Note also that RPM and critical altitude are both a bit less than one would expect for the Sakae 21.

 

[taly01]

It does seem the later Zeros the allieds tested, A6M3 had a tired engine/supercharger, and the A6M5 was a bit twisted somewhere in the airframe. The data from the captured Japanese summary on "Saipan 1944" you earlier posted (i also seen in Mikesh Zero: combat & development) must be accepted on face value, it also reinforces that the normal speed of the A6M2 was 275kt(318mph) as Sakai etc said. So then ~336mph is the normal speed of the A6M3->5, which is about what the allied test also got for them also.

Model Speed Altitude
A6M2-21 275 Kts 4400 m
A6M3-32 290 Kts 6150 m
A6M3-22 292 Kts 5900 m
A6M5-52 294 Kts 5900 m

I have seen people say that the widely quoted 351mph for A6M5 meant it could go even faster "as Japanese tested at military power", however its clear 336mph is the actual military power speed.

There is no reason why there should be excessive vibration at a diving speed of only 250 Knots.

A few months ago I found part of a memoir on the web from a Japanese pilot where he got to test the late A6M5 at the test centre in Japan, and he said it could be dived at 400kt, but he said he wouldn't try that in actual combat! I assume he meant it was basically uncontrollable at that speed.

 

[Ivan1GFP]

It does seem the later Zeros the allieds tested, A6M3 had a tired engine/supercharger.....

Hello Taly01,

Maybe the A6M3 had a tired engine, maybe it didn't, but they certainly ran it as if they were using guidance from the gauge face.
Their engine output listed for 2nd speed supercharger is about 100 HP too low and 4000 feet too low which is VERY close to what one might have gotten from a Sakae 11. The did raise the engine speed above 2550 RPM though.

I have seen people say that the widely quoted 351mph for A6M5 meant it could go even faster "as Japanese tested at military power", however its clear 336mph is the actual military power speed.

Model Speed Altitude
A6M2-21 275 Kts 4400 m -- 316 MPH @ 14,436 feet
A6M3-32 290 Kts 6150 m -- 333 MPH @ 20,177 feet
A6M3-22 292 Kts 5900 m -- 336 MPH @ 19,357 feet
A6M5-52 294 Kts 5900 m -- 338 MPH @ 19,357 feet

These are not speeds at Military Power. They are speeds at "Normal" Power.
For the Sakae 11, this was +50 mm boost 2350 RPM
For the Sakae 21, this was +75 mm boost 2500 RPM

This is well below what the manual calls "Rated Power" which we have been calling "Military Power". That was....
For the Sakae 11, this was +150 mm boost 2500 RPM
For the Sakae 21, this was +200 mm boost 2700 RPM

A few months ago I found part of a memoir on the web from a Japanese pilot where he got to test the late A6M5 at the test centre in Japan, and he said it could be dived at 400kt, but he said he wouldn't try that in actual combat! I assume he meant it was basically uncontrollable at that speed.

I believe you are correct about the aircraft being "uncontrollable". The control forces were very high to the point where direction changes of any kind were near impossible. It didn't necessarily mean that the aircraft would vibrate. The earlier versions with less structural strength did not. In reading the test report on the Aleutian A6M2, it is pretty clear that the US pilots had no idea that the maximum diving speed of the J-fighter was so low when they were comparing diving speed and acceleration against US fighters.

- Ivan.