XP-39 II - The Groundhog Day Thread

[Peter Gunn]

Hello P-39 Expert....

*SNIP*

I just went through the article again and am more inclined to believe Mr. Dunn's conclusion is correct.
A6M2 Performance

*SNIP*

- Ivan.

Finally had time to read this, I see that stateside testing wasn't getting the same results that combat pilots were facing in the SWPA re Zero performance. I see mention of the P-39 being 40MPH faster than the test Zero's but over Guadalcanal and New Guinea Airacobras were decidedly NOT outrunning Japanese fighters at any altitude. One pilot even mentioned a Zero staying with him in a power dive up to 450MPH.

I must confess I don't know where this "40MPH faster than a Zero at any altitude" mantra is coming from in reality. Testing was one thing but it is glaringly apparent that in combat those test results pretty much went out the window.

 

[P-39 Expert]

The Spitfire doubled in weight in its life, it also doubled its initial rate of climb. This is conclusive proof that increasing weight increases rate of climb, any discussion of thrust, residual thrust and lift are the babblings of the deranged.

Doesn't prove anything. Spitfire also doubled its horsepower in its life.

 

[P-39 Expert]

What's interesting to me is how the A6M2 holds its speed at all altitudes (almost), the deviation from 12k to 20k doesn't fluctuate more than 15mph.

From 12000' to 22500' the P-39K only lost 4mph.

 

[XBe02Drvr]

P39 Expert said:
Was dividing the increase in climb rate by the increase in weight to find the climb rate increase per pound.

Do you understand the meaning of the terms "linear" and "non-linear"? Have you noticed that almost all lines on performance charts are curved, not straight? Aerodynamics is full of non-linear relationships because the compressibility of air (or any other gas) is exponential, not linear.
Consequently, putting two points on a graph anchored by data points of ROC/weight for two aircraft and connecting them with a straight line and expecting all the points between to be correct is a fallacious assumption. Only a person with zero understanding of practical aeronautics would fall for it.
Add to that, your assertion that the C and D were identical except for weight ignores the fact that they had similar but not identical propellers, so while rated horsepower may have been identical, it doesn't automatically mean effective thrust is the same at all altitudes, speeds, and power settings.

 

[P-39 Expert]

Finally had time to read this, I see that stateside testing wasn't getting the same results that combat pilots were facing in the SWPA re Zero performance. I see mention of the P-39 being 40MPH faster than the test Zero's but over Guadalcanal and New Guinea Airacobras were decidedly NOT outrunning Japanese fighters at any altitude. One pilot even mentioned a Zero staying with him in a power dive up to 450MPH.

I must confess I don't know where this "40MPH faster than a Zero at any altitude" mantra is coming from in reality. Testing was one thing but it is glaringly apparent that in combat those test results pretty much went out the window.

Testing was factual and comparable. Pilot reports were hearsay and opinion.

 

[P-39 Expert]

P39 Expert said:
Was dividing the increase in climb rate by the increase in weight to find the climb rate increase per pound.

Do you understand the meaning of the terms "linear" and "non-linear"? Have you noticed that almost all lines on performance charts are curved, not straight? Aerodynamics is full of non-linear relationships because the compressibility of air (or any other gas) is exponential, not linear.
Consequently, putting two points on a graph anchored by data points of ROC/weight for two aircraft and connecting them with a straight line and expecting all the points between to be correct is a fallacious assumption. Only a person with zero understanding of practical aeronautics would fall for it.
Add to that, your assertion that the C and D were identical except for weight ignores the fact that they had similar but not identical propellers, so while rated horsepower may have been identical, it doesn't automatically mean effective thrust is the same at all altitudes, speeds, and power settings.

Do you understand that all rate of climb lines ARE linear above the critical altitude (except the Me109G)? Above critical altitude every single Lightning, Warhawk, Cobra, Thunderbolt, Mustang, Wildcat, Hellcat, Corsair, Fw190A, Zero, Oscar Spitfire, Hurricane and Typhoon has a rate of climb that declines in a straight line as altitude is gained above their critical altitude? Every single one.

And please stop worrying about what I understand and don't understand. You're not talking above my head.

 

[fubar57]

Never believe the people who were there.

 

[FLYBOYJ]

Was dividing the increase in climb rate by the increase in weight to find the climb rate increase per pound

I mentioned this on October 5, 6 and 19th. THIS WILL DEPEND WHERE YOU ARE REMOVING THE WEIGHT FROM.

NONLINEAR

non·lin·e·ar
/nänˈlinēər/

adjective

1. 1.
   not denoting, involving, or arranged in a straight line.
2. 2.
   not linear, sequential, or straightforward; random.

From Chris...

"Do you really think that only weight influences climb, and that all factors are equal? That when removing weight at one location, and adding weight at another won't have differing effects. How is your new CG going to effect performance. Oh wait who cares about CG right?"

Does anyone want a ground horse meat burger?

 

[Ivan1GFP]

Testing was factual and comparable. Pilot reports were hearsay and opinion.

Testing may have been factual, but hardly comparable. From what I have been able to find, there never actually was a test of a A6M of ANY version that was in good running order.
Many of the reports also don't list critical details. When those details are considered, all that is certain is that the performance of the actual aircraft in Japanese service was better than the test results.

- Ivan.

 

[XBe02Drvr]

Without the automatic mixture control working, the pilot would need to compensate for any altitude changes immediately.

Not just altitude changes! If you've manually fine tuned your mixture for best power, any change in stress on the engine (WEP, high power zoom climb, etc) can push you over the line into detonation unless you're compensating your mixture second by second. That's a distraction you don't need in a dogfight. They put that aneroid altitude compensator in your carburetor for a reason. Use it!

 

[Ivan1GFP]

Finally had time to read this, I see that stateside testing wasn't getting the same results that combat pilots were facing in the SWPA re Zero performance. I see mention of the P-39 being 40MPH faster than the test Zero's but over Guadalcanal and New Guinea Airacobras were decidedly NOT outrunning Japanese fighters at any altitude. One pilot even mentioned a Zero staying with him in a power dive up to 450MPH.

I must confess I don't know where this "40MPH faster than a Zero at any altitude" mantra is coming from in reality. Testing was one thing but it is glaringly apparent that in combat those test results pretty much went out the window.

Hello Peter Gunn,

I believe that "40 MPH faster" is pure garbage. I suspect it is from a book written by a P-39 Fanboy but haven't had the time to go hunting for the book to go look for the quote. Even Koga's A6M2 in less than perfect shape and not running full power was able to do better than this.

Keep in mind though that 450 MPH indicated is actually beyond the limitations (340 Knots) listed in the manual for the A6M2.
The Aleutian A6M2 was actually pushed past its official dive speed limits as well because the US pilots didn't know any better and apparently didn't suffer for it. Maybe the Japanese were a little conservative?

This is my OPINION, but is fairly well supported by anecdotes: The A6M series was actually quite strong structurally. Horikoshi was a very good engineer and didn't build anything stronger or heavier than it absolutely needed to be. The problem with that idea was that it had no redundancy and didn't tolerate battle damage.
(Strong ENOUGH, but I still think it was flimsy.)
The Sakae engine also seemed to tolerate maximum or emergency settings pretty well without failures. I am guessing that this might be because it wasn't highly boosted and also it was one of the better designed powerplants.
Note that even later in the war, when more was known about the Sakae, TAIC made an odd note about "Flash performance" in the Hayabusa that used the same engine and comment that the Japanese pilots apparently didn't take the official limitations very seriously and were apparently getting away with it.
Note also Saburo Sakai's encounter with multiple Hellcats in which he overboosted his engine without any real consequence.

- Ivan.

P.S. Another indication that the Sakae might have been able to tolerate a bit more than the typical engine is the maximum RPM limits.
The Sakae 12 allowed a maximum of 2550 RPM at its Take-Off rating and 2500 RPM at its Military rating.
The manual stated that it would tolerate up to 3100 RPM.
The Sakae 21 allowed a maximum of 2750 RPM at its Take-Off rating and 2700 RPM at its Military rating.
The manual stated that it would tolerate up to 3250 RPM.

I believe these numbers are a bit higher than typical for a big radial engine.

 

[P-39 Expert]

Hello P-39 Expert,

This is looking a lot like the movie Groundhog Day.
Hopefully you might learn something like Bill Murray did.



You obviously don't know how the carburetors work in these aircraft. Obviously.
The Auto Rich setting gives a slight margin for detonation under maximum throttle settings and the automatic mixture controls also compensate for altitude (air density) changes with varying flight conditions.
To achieve the extra 25 HP and 3 extra MPH, the pilot had to switch to manual mixture control and lean out the mixture.
This might work just fine if you are flying along straight and level with nothing else is going on, but in COMBAT, this is just plain STUPID. Without the automatic mixture control working, the pilot would need to compensate for any altitude changes immediately. Losing a few thousand feet of altitude and finding that your engine is now detonating or burning valves is a pretty lousy way to find you need to walk or swim home.

As in the test all the pilot had to do was adjust the mixture control slightly. It was a lever on the throttle quadrant. Again we're talking about 3mph.

You are correct. IIS 85 is not the correct reference.
You are also not reading the report correctly. It actually says 326 MPH. I was off by one line. 326mph is not 335mph.
It is interesting though that the actual information that was eventually distributed more widely was 332 MPH. 332mph is not 335mph.

This is a better reference. Glad you finally found your 335mph reference. Most other references say 330mph. You're fighting over 5mph like it won or lost the war.
Memorandum 23 Oct 1942

Note that maximum speed in this report is 335 MPH @ 16,000 feet and the test aircraft is still achieving 331 MPH at 20,000 feet. P-39K did 370mph at 16000' and was still achieving 364mph at 20000'.

Do you remember my comment that if you didn't like a comparison of climb rate with the A6M5 at 15,000 feet, you would like the comparison with the A6M2 even less?
Note that the climb rate is 2480 feet/minute at 15,000 feet. How does that compare with the P-39D???? Look at the P-39K graph in post #1006. I think you'll find the climb rates very comparable.

The most interesting information listed in this memorandum report is the note at the bottom which states that the manifold pressure was limited to 35 inches Hg. This is only +129 mm boost which isn't even Military Power.
Military Power would be +150 mm or 35.83 inches Hg
Emergency Power would be +250 mm or 39.76 inches Hg
I had missed this note in my earlier readings.......

This is why I am tending to believe now that Mr. Dunn's argument is probably correct considering that a rebuilt wreck can achieve 335 MPH without even using MILITARY power and with a carburetor that wasn't working correctly. Probably?

Correct or not, you're talking 5mph.

First of all, the source of information that is listed in that table is a book from 1981.
Second, your conversion isn't entirely correct. Speed should be 331.5 MPH (288 Knots). 331mph is not 335mph.
Third, you should read the footnote to that entry. The information that is listed there suggests that the author is a bit confused:

"The speed was increased after thickening the outer wing panel"? How does THAT improve speed?
Sounds like he was confusing level speed with maximum diving speed..... There is much more detail to this discussion about the authors errors that is beyond the scope of this discussion.
Note also that earlier speed was specified as 275 Knots. This was NEVER the maximum speed as we understand the term here.
275 Knots (316 MPH) was the maximum speed under "Normal Power" settings of +50 mm boost at 2350 RPM. 316mph is not 335mph.

One report shows 335mph, most others show 330mph. Or less. Take your pick.

If you have information that can confirm that the P-39D-1 in the test was loaded to 7850 pounds please post it. 7850lbs was the listed weight of the P-39D-1.
Considering that the pilot in the test was willing to try 70 inches Hg at Take-Off to prove he could beat the Zero off the line suggests that they were not above a few tricks. More tricks? The pilot overboosted the engine at takeoff indicating he either was new in the P-39 or was a lousy pilot.

In any case, 7250 pounds was not the loaded weight of operational P-39D-1 in service and your hypotheses are not proven and irrelevant. I have always stated that this weight was easily achievable at forward bases and this was hypothetical. You have not proven anything either.

- Ivan.

Expand above.

 

[DerAdlerIstGelandet]

A simple adjustment of a mixture control is probably not so simple in combat, as others have kindly pointed out already...

 

[P-39 Expert]

View attachment 599915

A simple adjustment of a mixture control is probably not so simple in combat, as others have kindly pointed out already...

Probably. Maybe. Maybe not.

 

[pbehn]

From 12000' to 22500' the P-39K only lost 4mph.

And in the same altitude range a Spitfire Mk XIV gains around 50 MPH.

 

[XBe02Drvr]

P39 Expert said:

As in the test all the pilot had to do was adjust the mixture control slightly. It was a lever on the throttle quadrant. Again we're talking about 3mph.
Clearly you've got no comprehension of what manual mixture leaning entails in maneuvering flight. It requires very fine adjustment of the mixture control while feeling for the first increment of increased vibration to indicate rough running, in the absence of an EGT gauge. Since none of these vintage planes had perfect intake manifold mixture distribution, you'll always have leaner running and richer running cylinders, even in your beloved V1710. Rough running while leaning at altitude means your leaner cylinders are starting to detonate, which is not conducive to health and long life.
Considering how attention demanding this process is, is it worth the risk while you're maneuvering violently and trying to keep track of what several Zeroes and several P39s are doing and trying to stay out of everyone's gunsights and avoid midair collisions, while in a massive adrenaline rush and frequent G loads? If you think you're that cool a customer, more power to you. I think not. Somebody needs to take you out for a session of bank and yank. Take your motion sickness pills, you're going to need them.

 

[P-39 Expert]

And in the same altitude range a Spitfire Mk XIV gains around 50 MPH.

Seriously? You're comparing a 1944 two stage airplane to a 1942 single stage airplane.

 

[DerAdlerIstGelandet]

Probably. Maybe. Maybe not.

Please elaborate. Do you have any first hand knowledge?

While I have never flown a warbird in combat, I do regularly adjust mixture when I fly. I have to make very fine adjustments, listening to my engine, and seeing how it responds. Not terribly hard to do while straight and level in a lil slow private plane, not getting shot at and flying combat maneuvers. I, however, can imagine it being pretty difficult to adjust mixture, while flying combat maneuvers, keeping an eye on your enemy, and also trying to shoot your opponent down, while possibly yanking and banking to avoid getting shot down yourself.

In my combat flying experience (656 hours of combat time, as a crew member not pilot), I do know that a lot is going on, and that was in an aircraft with 4 crew members working together, not a cramped high performance single pilot aircraft.

 

[DerAdlerIstGelandet]

Seriously? You're comparing a 1944 two stage airplane to a 1942 single stage airplane.

Why not? You think you are the only one who can move the goal posts?

 

[pbehn]

Seriously? You're comparing a 1944 two stage airplane to a 1942 single stage airplane.

I was just pointing out that it was actually normal for aeroplanes to get faster not slower between that altitude range. The Spitfire Mk V gained about 27 MPH between 10,000 and 25,000 with its single stage supercharger, basically what I am saying is it isn't simple, as you are obviously aware since you mentioned the two stage engine of the Mk XIV straight away.