XP-39 II - The Groundhog Day Thread

GregP · Oct 30, 2020

Hey guys, there is something screwy about the two tests in wwiaircraftperformance for the P-39C and P-39D.

If we go to the P-39D at 7,525 pounds test and use the rate of climb formula from Aerodynamics for Naval Aviators, we get the power required for level flight at 5,000 feet to be 529.76 hp, no problem. But, if we go to 20,000 feet and solve for it, we get 413.56 hp! No way! Something is fishy here, and I think it is because they got the engine power from a tech order instead of using a torque meter reading in the airplane. Could be wrong here, but I don't think they were getting the full power at lower altitudes.

If I look at the P-39C at 6,689 pounds, the power required for level flight at 5,000 feet is 395.97 hp and at 20,000 feet is 401.36 hp - which sounds much closer to normal. I'd expect more power to be required at 20,000 feet, not less!

I think the P-39D test had a sour engine or something that they never documented or seem to have noticed. Perhaps it never got to full rated MAP.

Maybe Bill Marshall should look at the tests, especially the P-39D climb tests. Something just looks wrong to me and I can't put a solid finger on it.

Just saying ... cheers! Sorry to drop a bomb at THIS point!

jmcalli2 · Oct 31, 2020

Ivan1GFP said:
Hello jmcalli2,

The most effective Japanese Naval Fighter Unit that was fighting from Lae was the Tainain Air Group.
The unit was formed in October 1941, so it was hardly a veteran unit.
Although they had some experienced pilots from the war in China, that war was hardly comparable to a modern war.

Here are a couple questions for you regarding inexperience with the V-1710:
How much more experience would the AVG have had with their P-40s in China?
Did it make a difference that all the USAAC personnel in New Guinea were at least US Army instead of a mix of USN, Marines, US Army and civilians? Would these other people have had better experience working on V-1710 engines?
Would their supply chain have been any better than a base in New Guinea that was within shipping distance of Australia?
How well did P-40s do in New Guinea?
Did the Australians have the same problems with Allison equipped aircraft?
They could not possibly have had any more knowledge than the service that gave them the aircraft or could they?

- Ivan.

Hi, Ivan;
ANY combat experience was more than the P-39 pilots had. And that goes for logistics too.
My impression, and I could be wrong, was that New Guinea was not at the top of the "supply first" list.
AVG had their problems with logistics and maintenance too, but they and the Australians had better early warning of incoming attacks than New Guinea did.
Again, that's my impression. Could be wrong.

Ivan1GFP · Oct 31, 2020

Hello GregP,

There seems to be a bit of weirdness in the engine output versus altitude for the P-39N as well.
I didn't find anything quite as radical as you did but it does strike me as a bit curious.
Note that the T.O. specifies engine critical altitude for WEP as 10,900 feet.
Maximum speed however is achieved at 9,700 feet and is noted as being 1.5 MPH SLOWER at 10,900 feet.
One would normally expect some ram effect.

I was thinking that perhaps the propeller tips might be going supersonic but that is pretty unlikely.
I can't tell the propeller diameter from the blade part number given but the largest diameter propeller installed in the N was 11 feet 7 inches which would give a tip speed of 639.5 feet / second.

- Ivan.

jmcalli2 · Oct 31, 2020

FLYBOYJ said:
I've heard similar, somewhere I think it was calculated a 1 to 1.45 kill/ loss ratio, Zero to P-39

If you want to include all causes

Some points made, the P-39, despite it's reputation in the SWP did hold it's own but at the same time look at some of the P-39 units that transitioned into the P-38 in the later part of 1942 and the leap in combat victories. Those same early P-38 units operated under dismal conditions as well and had the P-38 been available earlier it would have made a huge difference. Research the 39th and 80th fighter squadrons and some of the aces that emerged from those units and how things turned around into 1943, all occurring when they got the P-38.

Definitely agree on the improvement when the P-38s got there, but one caveat to that: were the pilots now veterans? They certainly benefited from having a 5,000 ft + altitude advantage instead of a 5,00 ft disadvantage. That's the turbos.
In reading "Twelve to One V Fighter Command Aces of the Pacific" by Tony Holmes, I noticed that the aces all said to keep above the enemy, keep your speed up and get close before firing. I didn't get the impression from the P-39 guys that they'd figured that out.
There's no argument that the P-38 was a better fighter than the P-39.

Ivan1GFP · Oct 31, 2020

jmcalli2 said:
Hi, Ivan;
ANY combat experience was more than the P-39 pilots had. And that goes for logistics too.
My impression, and I could be wrong, was that New Guinea was not at the top of the "supply first" list.
AVG had their problems with logistics and maintenance too, but they and the Australians had better early warning of incoming attacks than New Guinea did.
Again, that's my impression. Could be wrong.

Hello jmcalli2,

I figure that if it was experience working with the Allison engine, the air service from the country manufacturing the engine would have the most experience especially since they had operated it in the P-40 for over a year.
In any case one would expect Army mechanics to have more experience with a piece of Army equipment than the assortment of mechanics that were rounded up from various places for the AVG.
I believe AVG logistics and support was wherever they could get it and usually they did not. If they did, it was coming through the port of Rangoon, Burma or from India. Technically they were not a part of any major air service. They were just a bunch of mercenaries fighting for China.

Saburo Sakai's books make pretty good reading if you are curious as to the issues faced by the other side and that was from the unit with the greatest success. Other naval units were doing rather poorly in general. The Japanese pilots were not going to places like Australia every so often. They only way they were leaving was if they were injured too badly to fight or if they died.

- Ivan.

Stig1207 · Oct 31, 2020

jmcalli2 said:
Hi, Ivan;
ANY combat experience was more than the P-39 pilots had. And that goes for logistics too.
My impression, and I could be wrong, was that New Guinea was not at the top of the "supply first" list.
AVG had their problems with logistics and maintenance too, but they and the Australians had better early warning of incoming attacks than New Guinea did.
Again, that's my impression. Could be wrong.

The P-39 units had better early warning (Coast Watchers) of incoming attacks than their opponents, who had no early warning at all.

XBe02Drvr · Oct 31, 2020

Stig1207 said:
The P-39 units had better early warning (Coast Watchers) of incoming attacks than their opponents, who had no early warning at all.

If you habitually abuse the natives, they're more likely to eat your coast watchers than help them.
"Oomm gahwa, yellow long pig tonight!"

GregP · Oct 31, 2020

Hi Ivan1GFP,

If I have it right, the prop diameter is 124.5 inches or 10.375 feet. Suppose we are at 13,800 feet and the forward speed is 368 mph.

The Allison V-1710-35 had a prop reduction gear ratio of 1.8 : 1. All that being the case, the radius is 5.1875 feet, prop rpm is 1,666.667 rpm, so rotational speed is 174.5329 radians per second, and tangential speed is 905.39 feet / sec or 617.34 mph. Forward speed is 368 mph or 539.71 ft/sec. Helical tip speed would be the square root of tangential speed squared (905.39^2) plus forward speed squared (539.71^2), or 1054.05 ft/sec, which is 718.7 mph. At 13,800 feet, the standard speed of sound is 722.953 mph.

So, I calculate the helical tip speed at 718.7 mph and the Speed of Sound is 722.953 mph. Mach Number is 0.99412.

An efficient prop will have a propeller efficiency of 0.85 to 0.90 at best, and we definitely have an issue with prop tip speed since we don't really want the prop tip going any faster than M.84 to M.88 or so before the prop efficiency starts to drop off. I'd say the airplane is going slower than it should because the prop tip speed is too fast. In fact, the tip speed will be over M.88 at any airspeed above 154 mph! To lower the tip speed to M.88, we'd need to reduce the rpm to 2,525 at 368 mph forward speed at 13,800 feet on a standard day.

Most later Allisons had a propeller reduction gear ration of 2 : 1, instead of 1.8 : 1. If we had been running an Allison with a 2 : 1 ratio, the Mach number would be M.92 at the conditions above and we'd still have to reduce rpm to 2,803 to get M.88.

These things figure rather prominently at Reno and in world airspeed record attempts. This stuff is well known now, but wasn't exactly precisely known before WWII. Maybe the XP-39 would have gone faster than it did if the prop tip Mach number wasn't above what is now accepted as best efficiency velocity.

If I missed anything, let me know.

FLYBOYJ · Oct 31, 2020

jmcalli2 said:
Definitely agree on the improvement when the P-38s got there, but one caveat to that: were the pilots now veterans? They certainly benefited from having a 5,000 ft + altitude advantage instead of a 5,00 ft disadvantage. That's the turbos.
In reading "Twelve to One V Fighter Command Aces of the Pacific" by Tony Holmes, I noticed that the aces all said to keep above the enemy, keep your speed up and get close before firing. I didn't get the impression from the P-39 guys that they'd figured that out.
There's no argument that the P-38 was a better fighter than the P-39.

Some were, some weren't. There were leaders there like Tom Lynch who tutored new arrivals, Ken Sparks, Tom McGuire and of course Dick Bong who first flew with the 39th FS. On one of his first missions he shot down 2 aircraft, late Dec., 1942.

P-39 Expert · Oct 31, 2020

Ivan1GFP said:
First of all, you are looking at the wrong detail. The important thing is NOT the 3 MPH difference in maximum speed because in a maneuvering fight with changing altitudes you will never see the difference in maximum speed.
The objective here is the extra 25 HP that gives that extra 3 MPH maximum speed.
The fuel mixture requirements of the engine will be constantly changing as the power (torque) demands and altitude are changing. In combat, the pilot has many other things demanding constant attention. Assessing the instantaneous fuel needs of the engine is not a smart distraction in that situation. I never said the pilot would be adjusting the mixture control after initially going to full power.

If you are asking an honest question, I will give you an explanation because you obviously think there is more there than there actually is.
Only the first five entries for the Zero are for the A6M2.
There are five entries, but only two aircraft were tested.
One was a A6M2 that fell into the hands of the AVG in China. It was tested against a P-40K and a P-43.
Its Propeller was never in spec and it could not make more than about 2075 RPM, thus its performance is hardly indicative of a properly functioning A6M2.

The second aircraft tested was the crashed and rebuilt Aleutian A6M2 and I won't rehash what was wrong with that testing which I have covered before. Its testing and summaries for different audiences covers about 4 of the 5 reports.

As I have stated before, the only service that had good operational A6M2 and knew how to operate them properly (the only service that had the opportunity) never chose to test for maximum speed in a manner comparable to the Allies. So how do we determine the top speed of the A6M2? British trials of the Mk. II Hap with the more powerful -21 engine operating at 2600rpm showed only 328mph.

We have been here before. We got here several times over the last couple days.

And we'll stay here until you acknowledge that the C and D were the same plane except the D had more weight from different internal components. Nothing about those two planes could account for a 1000fpm (37%) different climb rate except the 836lbs of weight. Reduce the weight and increase the climb. Simple math.

The testing of the Aleutian A6M2 was meant to show the superiority of American fighters as much as possible. Your opinion. More likely just a means of evaluating the performance of the A6M2 vs US planes.
I have already explained the evidence (I believe to the satisfaction of others here). Maybe it should not have been a contest, but the participating pilots were most likely informed of the intended results. I have seen government "tests" in other areas that were conducted that way.

- Ivan.

Expand above.

Ivan1GFP · Oct 31, 2020

GregP said:
Hi Ivan1GFP,

If I have it right, the prop diameter is 124.5 inches or 10.375 feet. Suppose we are at 13,800 feet and the forward speed is 368 mph.

The Allison V-1710-35 had a prop reduction gear ratio of 1.8 : 1. All that being the case, the radius is 5.1875 feet, prop rpm is 1,666.667 rpm, so rotational speed is 174.5329 radians per second, and tangential speed is 905.39 feet / sec or 617.34 mph. Forward speed is 368 mph or 539.71 ft/sec. Helical tip speed would be the square root of tangential speed squared (905.39^2) plus forward speed squared (539.71^2), or 1054.05 ft/sec, which is 718.7 mph. At 13,800 feet, the standard speed of sound is 722.953 mph.

So, I calculate the helical tip speed at 718.7 mph and the Speed of Sound is 722.953 mph. Mach Number is 0.99412.

An efficient prop will have a propeller efficiency of 0.85 to 0.90 at best, and we definitely have an issue with prop tip speed since we don't really want the prop tip going any faster than M.84 to M.88 or so before the prop efficiency starts to drop off. I'd say the airplane is going slower than it should because the prop tip speed is too fast. In fact, the tip speed will be over M.88 at any airspeed above 154 mph! To lower the tip speed to M.88, we'd need to reduce the rpm to 2,525 at 368 mph forward speed at 13,800 feet on a standard day.

Most later Allisons had a propeller reduction gear ration of 2 : 1, instead of 1.8 : 1. If we had been running an Allison with a 2 : 1 ratio, the Mach number would be M.92 at the conditions above and we'd still have to reduce rpm to 2,803 to get M.88.

These things figure rather prominently at Reno and in world airspeed record attempts. This stuff is well known now, but wasn't exactly precisely known before WWII. Maybe the XP-39 would have gone faster than it did if the prop tip Mach number wasn't above what is now accepted as best efficiency velocity.

If I missed anything, let me know.

Hello GregP,

Some of MY calculations in the spreadsheet last night were simply wrong. I left out a little number called Pi.....
Some of the details you have for the P-39 are not for the correct version.

The P-39N has a propeller diameter of either 11 feet 4 inch or 11 feet 7 inch
Propeller reduction is 2.23:1 as stated in the test report.
Altitude is around 10,000 feet.
Forward speed is 398.5 MPH @ 9,700 feet.

What I am getting WITH Pi... (embarrassing mistake) is 1003.65 fps and Mach 1 at 10,000 feet from a table is 1077.43 fps.
Mach = 0.93. Much closer.....

- Ivan.

Ivan1GFP · Oct 31, 2020

P-39 Expert said:
never said the pilot would be adjusting the mixture control after initially going to full power.

So your idea is that the pilot goes to full throttle, sets maximum RPM, and then spends a couple minutes twiddling with the mixture to get something that SOUNDS like full power.......
THEN doesn't change altitude, engine loads or any other conditions IN COMBAT which would completely invalidate those results?
You have an interesting view of what happens in combat flying.

P-39 Expert said:
So how do we determine the top speed of the A6M2? British trials of the Mk. II Hap with the more powerful -21 engine operating at 2600rpm showed only 328mph.

You determine the maximum speed of the A6M2 any way you want. I have presented the information to you. You choose not to read it. I can't do any more. You can go back and re-read what has already been presented. Some of the posts were not directed to you.

The British trials of Mk.II HAP are a garbage test as well. Those folks got even less right when they ran the test.
The Mk.II "Hap" is an entirely different aeroplane with a different engine, different engine, different wing span, wing area, etc.
Test has no real relationship to performance of A6M2.

If someone wants to discuss what was wrong with the testing of the A6M3 Model 32 "Hap", we can go there but it isn't relevant to A6M2.

P-39 Expert said:
And we'll stay here until you acknowledge that the C and D were the same plane except the D had more weight from different internal components. Nothing about those two planes could account for a 1000fpm (37%) different climb rate except the 836lbs of weight. Reduce the weight and increase the climb. Simple math.

First of all, if you are really serious, you should observe that the propeller blades are NOT the same part number between the two aircraft. Also note that the testers did not believe the engines were quite the same. Note that the source for the engine output information was from DIFFERENT DOCUMENTS.
Please read the recent posts from GregP and try to understand their significance.
As I see it, the engine output or SOMETHING does not seem to be quite right in the P-39D testing.
Since there wasn't an actual torque meter installed to show actual power output (which will never be EXACTLY the same between any two engines or even the SAME engine at different times or conditions) so we don't know what power output each aircraft was actually making.

As everyone has been trying to tell you, there are way too many variables to assume everything is the same between two aircraft.
This is just comments about engine power. The way the aircraft were loaded and the consequences of differences in CoG and balance have already been explained.

If you are going to base an argument on comparing a couple test reports, you should probably read the entire reports and UNDERSTAND them instead of just picking out a couple numbers.

P-39 Expert said:
Your opinion. More likely just a means of evaluating the performance of the A6M2 vs US planes.

Can you explain why the acceleration tests were started at different speeds when flying against each US fighter?
What are the significance of those speeds? I have already given an explanation.

- Ivan.

Ivan1GFP · Oct 31, 2020

Hello GregP,

Since we are already on this track, and I already know you are pretty good with spreadsheets,
you might get some amusement with calculating the same values for the Lockheed P-3 Orion.
I had do this for a Flight Simulator discussion with some friends a few months ago and the result was quite revealing.

- Ivan.

P-39 Expert · Oct 31, 2020

Ivan1GFP said:
So your idea is that the pilot goes to full throttle, sets maximum RPM, and then spends a couple minutes twiddling with the mixture to get something that SOUNDS like full power....... More like a second or two. One time. Let's move on from this, we're talking about 3mph.
THEN doesn't change altitude, engine loads or any other conditions IN COMBAT which would completely invalidate those results?
You have an interesting view of what happens in combat flying.

You determine the maximum speed of the A6M2 any way you want. I have presented the information to you. You choose not to read it. I can't do any more. You can go back and re-read what has already been presented. Some of the posts were not directed to you.

The British trials of Mk.II HAP are a garbage test as well. Those folks got even less right when they ran the test. Now the British don't know how to run a test?
The Mk.II "Hap" is an entirely different aeroplane with a different engine, different engine, different wing span, wing area, etc.
Test has no real relationship to performance of A6M2. More powerful engine, smaller wing etc, all things that should make the MkII faster, but still only 328mph. Let's move on from this, the A6M2 was slower than the P-39.

If someone wants to discuss what was wrong with the testing of the A6M3 Model 32 "Hap", we can go there but it isn't relevant to A6M2.

First of all, if you are really serious, you should observe that the propeller blades are NOT the same part number between the two aircraft. Also note that the testers did not believe the engines were quite the same. Note that the source for the engine output information was from DIFFERENT DOCUMENTS.
Please read the recent posts from GregP and try to understand their significance.
As I see it, the engine output or SOMETHING does not seem to be quite right in the P-39D testing.
Since there wasn't an actual torque meter installed to show actual power output (which will never be EXACTLY the same between any two engines or even the SAME engine at different times or conditions) so we don't know what power output each aircraft was actually making.

As everyone has been trying to tell you, there are way too many variables to assume everything is the same between two aircraft.
This is just comments about engine power. The way the aircraft were loaded and the consequences of differences in CoG and balance have already been explained.

If you are going to base an argument on comparing a couple test reports, you should probably read the entire reports and UNDERSTAND them instead of just picking out a couple numbers.

No amount of slightly different propeller blades of the same diameter, slight differences of CG (if there were any) or slight differences of HP (if there were any) will account for 1000fpm of climb. They were the same airplane, same contract, same engine, same airframe, same plane, simply different internal equipment. If you take any airplane and reduce the weight the climb rate will improve. The only thing that will account for 1000fpm of climb is almost half a ton of weight.

By the way, the C model didn't have the nose armor, how did it keep from falling out of the sky?

Can you explain why the acceleration tests were started at different speeds when flying against each US fighter? Splitting hairs again. I'd like to move on from this and concentrate on weight and climb.
What are the significance of those speeds? I have already given an explanation.

- Ivan.

Please expand above.

Peter Gunn · Oct 31, 2020

Hello P-39 Expert,

"Let's move on from this, we're talking about 3mph."
No we are NOT, re read what Ivan said, we're talking about the extra 25 horsepower it takes to get that 3 miles per hour.

"Now the British don't know how to run a test?"
Convenient comeback but that's NOT what he's alluding to.

"Let's move on from this, the A6M2 was slower than the P-39."
Boyd Wagner clearly didn't think so.

"By the way, the C model didn't have the nose armor, how did it keep from falling out of the sky?"
Pixie Dust.

"Splitting hairs again. I'd like to move on from this and concentrate on weight and climb."
Not possible unless you start to understand what Ivan et. al. have been saying.

P-39 Expert · Oct 31, 2020

Peter Gunn said:
Hello P-39 Expert,

"Let's move on from this, we're talking about 3mph."
No we are NOT, re read what Ivan said, we're talking about the extra 25 horsepower it takes to get that 3 miles per hour.

"Now the British don't know how to run a test?"
Convenient comeback but that's NOT what he's alluding to.

"Let's move on from this, the A6M2 was slower than the P-39."
Boyd Wagner clearly didn't think so.

"By the way, the C model didn't have the nose armor, how did it keep from falling out of the sky?"
Pixie Dust.

"Splitting hairs again. I'd like to move on from this and concentrate on weight and climb."
Not possible unless you start to understand what Ivan et. al. have been saying.

Could it possibly be that you and Ivan need to start understanding what I am saying? Both of you have a very condescending attitude, getting old.

XBe02Drvr · Oct 31, 2020

P39 Expert said:
More like a second or two. One time. Let's move on from this, we're talking about 3mph.
3 MPH is insignificant in combat. The 25 HP it represents at top speed IS, because of its effect on maneuvering performance: acceleration in a dive, energy retention in a zoom, airspeed retention in a high G turn. The problem is: 1) tweaking to get that extra 25 HP as you go into combat is likely to get you shot down, as it's a major distraction, and 2) you've just shaved your detonation margin to the hairy edge, and combat maneuvering IS GOING to push you over. Your throttle setting is not the only factor affecting stress on your engine. Full throttle at enforced lower airspeed (sustained high G turns, high angle climbs, etc) will quickly raise engine temps into the detonation range. That's what AUTO RICH is there for.
Now the British don't know how to run a test?
As previously pointed out, nobody on the allied side had a correctly set up and correctly operated, fully performing Zero until much later when it was no longer relevant.

P-39 Expert said:
Could it possibly be that you and Ivan need to start understanding what I am saying? Both of you have a very condescending attitude, getting old.

We all thoroughly understand what you're saying. How could we help it? Sounds like a broken record, skipping at the same drum riff, repeating over and over, and immune to rest of the symphony. The recording is Walt Disney's Fantasia.

XBe02Drvr · Oct 31, 2020

This thread sounds like a reflection of the current national hysteria level. I'm taking a vacation until after Tuesday is over with.
See you on the other side.
Wes

P-39 Expert · Oct 31, 2020

XBe02Drvr said:
P39 Expert said:
More like a second or two. One time. Let's move on from this, we're talking about 3mph.
3 MPH is insignificant in combat. The 25 HP it represents at top speed IS, because of its effect on maneuvering performance: acceleration in a dive, energy retention in a zoom, airspeed retention in a high G turn. The problem is: 1) tweaking to get that extra 25 HP as you go into combat is likely to get you shot down, as it's a major distraction, and 2) you've just shaved your detonation margin to the hairy edge, and combat maneuvering IS GOING to push you over. Your throttle setting is not the only factor affecting stress on your engine. Full throttle at enforced lower airspeed (sustained high G turns, high angle climbs, etc) will quickly raise engine temps into the detonation range. That's what AUTO RICH is there for.
Now the British don't know how to run a test?
As previously pointed out, nobody on the allied side had a correctly set up and correctly operated, fully performing Zero until much later when it was no longer relevant.

We all thoroughly understand what you're saying. How could we help it? Sounds like a broken record, skipping at the same drum riff, repeating over and over, and immune to rest of the symphony. The recording is Walt Disney's Fantasia.

And I thoroughly understand what you are saying. Thoroughly. That doesn't mean that I agree.

If Ivan is correct, then lets just put that propeller from the C back onto the D, adjust the CG (if needed) and recover that lost 1000fpm of climb.

Why can't you just admit that the extra weight is the reason the C climbed better than the D, instead of acting like 836lbs of weight had no effect?

buffnut453 · Oct 31, 2020

P-39 Expert said:
Why can't you just admit that the extra weight is the reason the C climbed better than the D, instead of acting like 836lbs of weight had no effect?

For the simple reason that nothing, and I mean NOTHING, in an aircraft's performance is a straightforward binary association. Certainly lower weight will contribute to improved climb performance but lots of other factors come into play, as others on the thread have repeatedly tried to point out. Unfortunately, that doesn't seem to align with your viewpoint so you discount or ignore their comments.

Speaking of ignoring comments, have you found any USAAF pilots who said they wanted the 30 cals removed from the wings of their P-39s yet?

XP-39 II - The Groundhog Day Thread

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"THE GREAT GAZOO"

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