Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained)

[CORSNING]

Even the hottest early N models with the reduced fuel would have been a nonstarter at ETO altitudes against escorts with two speed or two stage superchargers or turbos.
Cheers,
Wes

OK,....In all my posts on this forum about the later P-39s with the V-1710-83 engine
I am pretty sure I never said it was a world beater even when it was first delivered in
November 1942. Although at that time until late 1943 it could hold its own against
most first line Axis fighters up to 5,000 m. provided it was flown to its strengths.

 

[special ed]

In the 1950s I was a Civil Air Patrol cadet as a teenager. We met at the New Orleans Lakefront airport. There were some old hands working there who remembered P-39s coming through. They were on their way to some place to go to Africa. I was told the ground run time in the heat was 20 minutes. If you weren't on your takeoff roll you were to shut down. The problem was, the temp was still in the green so if you were next the pilot thought he was good to go, however on climb out the engine overheated and quit. I was told the Cajuns who lived in the swamp knew where every crash was and made government money for body recovery.

 

[Shortround6]

OK,....In all my posts on this forum about the later P-39s with the V-1710-83 engine
I am pretty sure I never said it was a world beater even when it was first delivered in
November 1942. Although at that time until late 1943 it could hold its own against
most first line Axis fighters up to 5,000 m. provided it was flown to its strengths.

And would the strength of the P-39 at 5000meters (16,400ft) be to dive down to 3000meters upon sighting the German fighters?
Being a bit sarcastic, I will agree that the P-39 could display good performance at 3000 meters and below but 5000 meters is really pushing things.

From Mike Williams website Spitfire performance.
The P-39M-N-Q all used the same power section or had the power curves. they varied in reduction gear used and propellers used.
Unless the planes are flying level and at high speed much of the WEP power advantage goes away above 3000 meters. Leaving high level speed and diving as the only real strengths of the P-39.

 

[CORSNING]

And would the strength of the P-39 at 5000meters (16,400ft) be to dive down to 3000meters upon sighting the German fighters?
Being a bit sarcastic, I will agree that the P-39 could display good performance at 3000 meters and below but 5000 meters is really pushing things.
View attachment 512061

From Mike Williams website Spitfire performance.
The P-39M-N-Q all used the same power section or had the power curves. they varied in reduction gear used and propellers used.
Unless the planes are flying level and at high speed much of the WEP power advantage goes away above 3000 meters. Leaving high level speed and diving as the only real strengths of the P-39.

SR6, it looks like the WEP advantage lasts up to about 5,000 m. on the graph
you posted to me. I am just saying that I agree that the P-39 variants were
never a high or medium high altitude first class fighter, agreed. But down in
the lower regions the later versions held there own.

 

[Shortround6]

The WEP advantage lasts (barely) for straight and level flight at full speed.
Take another look at the power chart in the middle of the graph. The solid line is the WEP power available at climb speed around 170-175mph at sea level and just under 200mph at 15,000ft. The dashed lines show the power available at higher speeds, the extra boost from the RAM air at 330-370mph.

Now say your P-39 has just climbed hard up to 12,500ft and is doing about 200mph and you level out. You don't have a bit over 1300hp, you have a bit over 1100hp due to lack of ram. AS you accelerate you pick up RAM effect and if you accelerate long enough you will get to that 1300hp level and the just about 400mph speed. Now how long does it take you to accelerate from 200mph to 375mph or above? If you bank you loose lift and have to use a slightly higher angle of attack crating drag, If you really bank and turn you will bleed off speed and are never going to reach that high area of the WEP.
At 12,500ft the WEP is worth about 150fps in climb vs the 750fpm or so it was worth at lower altitudes. At 5000 meters it is worth 3-5mph in level flight, if that. Not something to pin your chances of either victory or survival on.

 

[XBe02Drvr]

AS you accelerate you pick up RAM effect and if you accelerate long enough you will get to that 1300hp level and the just about 400mph speed.

Good trainer for transition to a 1st generation jet?

 

[P-39 Expert]

Some of those numbers look suspiciously high. Higher than the numbers shown in the tests at Spitfire performance?
And those number were done with planes that weighed less than they should have. The only way you get a P-39N down to 7300lbs is to fly with less than full internal tanks. For some weird reason even the test of the N seems to show abnormally high climb rates compared to the M and the Q, They changed reduction gears and props between the M and N and that might account for it? But then they mounted a single 50 cal under each wing and yanked the .30s and the climb fell back down to just about the level of the M. We are talking about a 400 fpm change in both directions at some altitudes.

Once again, ALL the planes tested in official performance tests (not propeller comparison tests etc.) were lighter than their published weights reflecting an average fuel weight during the test flight. The British used 95% of takeoff weight as their "corrected" weight to reflect fuel burned. The P-39N test was a standard test and the figures should be respected.

 

[XBe02Drvr]

Now say your P-39 has just climbed hard up to 12,500ft and is doing about 200mph and you level out. You don't have a bit over 1300hp, you have a bit over 1100hp due to lack of ram.

So if you have time and are not immediately bounced, get "on the step" by continuing your climb to 13,000, then accelerating in a shallow descent back down to 12.5. I've done that in 99s and 1900s, as PT6 engines are somewhat sensitive to ram effect.
Cheers,
Wes

 

[XBe02Drvr]

The P-39N test was a standard test and the figures should be respected.

Yes, and the good guys wear white hats and always win!
Cheers,
Wes

 

[Shortround6]

So if you have time and are not immediately bounced, get "on the step" by continuing your climb to 13,000, then accelerating in a shallow descent back down to 12.5. I've done that in 99s and 1900s, as PT6 engines are somewhat sensitive to ram effect.
Cheers,
Wes

You could do that. However the British figured the Spitfire V, not usually noted for being the fastest accelerating fighter plane, took almost two minutes to go from just over 200mph to max speed. The P-39 was better, it was often about the middle of the pack among US fighters for acceleration but even a full minute is an eternity in combat.

I will fully admit that the WEP changes things a lot for the P-39 at lower altitudes where it is available regardless of the speed of the aircraft. But once above 3000 meters it becomes much less of a factor in anything except straight line flight. The claim of it being effective at 5000 meters is rather dubious.

 

[Shortround6]

The P-39N test was a standard test and the figures should be respected.

I would respect it a lot more if the tests for the aircraft preceding it, which used the same engine but a different prop and the planes that followed it which used the same engines but different props on some, were anywhere near it. of these five airplanes/tests/charts the weight of the aircraft varies by 400lbs form lightest to heaviest. From lightest to heaviest the weight difference is 4.1% and yet the lightest plane (the N) shows a best climb of 380fpm (10% over the next best and even more over the rest. The other four aircraft show weight variation of about 200lbs and yet the climb difference is barely over 100fpm. The two late model Qs have no guns in or under the wings and would have less drag than the N even if a few hundred pounds heavier. Even a test of the P-39Q-5 at 7871lbs (6th test) with no wing guns shows only a 200-300fpm fall off in climb even with an engine not quite making full power (55in) at 3600-3700fpm.

So if you have 5 out 6 planes closely clustered in weight and using the same power engine and closely clustered in climb and the 6th plane using the same engine and only slightly less weight showing a major increase in climb rate what are you to believe?
What guarantee do you have that a plane 5 or 10 serial numbers different than the test plane is going to act like the test plane or act like the other 5 planes tested?

 

[CORSNING]

So if you have 5 out 6 planes closely clustered in weight and using the same power engine and closely clustered in climb and the 6th plane using the same engine and only slightly less weight showing a major increase in climb rate what are you to believe?

That it was a good idea to take the four paint chippers out of
the wings like the Russians did?...

 

[Shortround6]

Problem is that three of the P-39Qs tested didn't have any guns in or under the wings and the plane that did post those wonderful numbers did have the paint chippers installed.

Perhaps the Russians goofed and the wing guns actually increase the rate of climb

the four paint chippers and their ammo (normal load, not full) weighed less than 175lbs.

 

[Greg Boeser]

So if you have time and are not immediately bounced, get "on the step" by continuing your climb to 13,000, then accelerating in a shallow descent back down to 12.5. I've done that in 99s and 1900s, as PT6 engines are somewhat sensitive to ram effect.
Cheers,
Wes

I seem to recall reading this instruction in a pilot's manual to increase cruise efficiency.

 

[CORSNING]

Perhaps the Russians goofed and the wing guns actually increase the rate of climb

Wow SR6, you might be on to something there. I'm going to have to
go into deep research now. ...

Actually, we are on the same page here. If you look at all the
P-39 tests they all mention the overheating problem when boosted.
At no time is it mentioned that the engine was not able to take
the punishment. It just says the USAAF didn't approve of it.

 

[Glider]

Perhaps the Russians goofed and the wing guns actually increase the rate of climb

Wow SR6, you might be on to something there. I'm going to have to
go into deep research now. ...

Actually, we are on the same page here. If you look at all the
P-39 tests they all mention the overheating problem when boosted.
At no time is it mentioned that the engine was not able to take
the punishment. It just says the USAAF didn't approve of it.

An obvious observation.
If the engine could take the punishment, then it wouldn't be a problem. If the USAAF considered it to be a problem, then overheating the engine must have had consequences. I don't pretend to know what those consequences were, but I think it would be unwise to pretend that there were no consequences.

 

[Shortround6]

The USAAC had certain standards for safety reasons. IN many cases to build a cushion or reserve. If you are running into the "red zone" in Nov over Buffalo NY you might only be slightly in the red zone and there would be no damage. One test says the coolant was at 126 degrees C with the out side air at -12 degrees C at 12,000ft. Now maybe the engine (and the oil) could survive running at 126 degrees C for several minutes (or even more) but the USAAC figured that 126C would wind up being 152 degrees C on a "standard" hot day (100 degrees F at ground level) or for those of us Celsius challenged individuals that is about 305 degrees Fahrenheit.

So you have a plane that will climb like a bandit in the Russian winter and needs to level off periodically to let the coolant get down to just over boiling when operating in the tropics or warm climates. Good luck getting the German and Japanese pilots to give you "cool down breaks" in combat.

 

[XBe02Drvr]

If the USAAF considered it to be a problem, then overheating the engine must have had consequences. I don't pretend to know what those consequences were, but I think it would be unwise to pretend that there were no consequences.

The test reports don't tell you whether the engine on that hot performing P-39N1 whose coolant temp "didn't conform to USAAF standards" suffered an engine failure in some subsequent flight or had to have its engine pulled early for overhaul. If the test was done in Niagara NY or Moscow RFSSR in winter, then the engine could probably take that abuse with limited hidden damage. With MSL OAT 60°-70°F below ISA, your density altitude is going to be only about 3/4 of your barometric altitude, and your engine's going to have more power and your radiators more efficiency, and the risk of thermal runaway much reduced. In fact the plane's going to think it really is at 3,000 meters when it's actually at 4,000.
Now all it takes is for the conversion of the raw measurements to ISA conditions to get lost in the shuffle, and there's your anomalous "hot Cobra".
Cheers,
Wes

 

[XBe02Drvr]

But once above 3000 meters it becomes much less of a factor in anything except straight line flight.

Under those conditions you gain total energy faster by climbing than by trying to accelerate from slow speed in level flight. The extra 500 feet can't take more than 20-30 seconds at the outside, and then gravity is helping you accelerate and enhancing the "bootstrap effect" of your increasing ram as your speed builds.
All of this is dependent on how sensitive your induction system is to ram effect. Flow losses due to filters, screens, and awkward ducting can of course impact gains from ram effect. I've been told this was part of the Spitfire V's problem.
Cheers,
Wes

 

[Shortround6]

The test reports don't tell you whether the engine on that hot performing P-39N1 whose coolant temp "didn't conform to USAAF standards" suffered an engine failure in some subsequent flight or had to have its engine pulled early for overhaul. If the test was done in Niagara NY or Moscow RFSSR in winter, then the engine could probably take that abuse with limited hidden damage. With MSL OAT 60°-70°F below ISA, your density altitude is going to be only about 3/4 of your barometric altitude, and your engine's going to have more power and your radiators more efficiency, and the risk of thermal runaway much reduced. In fact the plane's going to think it really is at 3,000 meters when it's actually at 4,000.
Now all it takes is for the conversion of the raw measurements to ISA conditions to get lost in the shuffle, and there's your anomalous "hot Cobra".
Cheers,
Wes

My conversions may be a bit on the sketchy side as I am using on old book for the temperature chart but at 12,000ft on a standard day (59 degrees F at sea level) the temperature at 12,000ft was supposed to be 16.21 degrees F or about -8.8 degrees C vs the -12 degrees C in the test.

I do take your point though,