SHOULD the P39 have been able to handle the Zero? Was it training or performance?

DarrenW said:

Bell P-39 Airacobra - Wikipedia

A total of 149 P-39s would be used: the P-39N for training, while newer Qs were used in the front line....

You see, even the Italians realized how inadequate the P-39N was and relegated it to non-combat units!

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Not only, but in the strafing missions in the Balkans, knowing that P-39s, of any Mk, were easy meat for G-6s and G-10s, Regia Aeronautica had to escort them with the few worn-out Macchi 205 remaining. Strange, for a fighter escort...

Shortround6 said:

Not really, plenty of planes fought at altitudes well above WEP or it's equivalent. Think Spitfire Is and 12lbs boost in the BoB,
However the later P-39s were over 7000lb airplanes with around 770-780hp at 25,000ft (including RAM). And in 1943 that puts you behind the curve.

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Surely using maximum rpm and wide open throttle counts as WEP, even when above FTH?

Before the Spitfire Is were allowed +12psi boost, WEP was at +9psi boost and FTH was higher. So, surely that is still WEP?

Terms may differ between nations.
But basically, for the US, critical altitude is the altitude at which the throttle is fully open and the supercharger is supplying the rated amount of pressure (mass of air). In the case of the Allison it is making 1150-1125hp, that was the rated military power. Above that altitude, as the air gets thinner, the power falls off as the supercharger is maxed out, it can't supply any more air.
Below critical altitude you have the choice/ability to open the throttle and allow the supercharger to work closer to it's maximum ability in the denser air. This subject to detonation limits of the fuel and to the strength of the engine.
Here is a chart for the Merlin XX


At a given point in time the Air Ministry could decide that 9lbs boost was all the engine could stand on a regular basis. And the max height at which the engine/supercharger could supply 9lbs of boost is the critical altitude.
But they might decide that 12bs was Allowable for short periods and with increased maintenance (pulling and inspecting spark plugs, checking oil for metal particles)
That would be WEP or whatever a particular country chose to call it. But it doesn't work above the original critical height.
They may decide to say that the altitude at which they can get 12lbs is the new critical height and you do get a bit of boost between where you get 12lbs and the old 9lb critical height.
AS they improved fuel, strengthened parts and gained experience in how long the engines would last they would sometimes bump the max pressure up again to 15 and then 18lbs and the altitudes at which those pressure were available dropped the critical altitude.
Please note that had they opened the throttle on the original engine they could have had that power at those altitudes at any time (subject to fuel and breaking the engine).
For the Merlin 2850rpm was, except for the very early years, the max continuous power rpm and the allowable pressure varied a bit. 3000rpm might be considered military power or emergency power or some other term, But 3000rpm and 4lbs of boost at 24,000ft from a Merlin XX would not be considered as WEP. It would NOT require notations in log books or notification of the engineering officer that such a power setting was used.

I hope I have explained that properly?

pbehn said:

It is not only Drgondog's posts but others too. For example, Bill Runnels longest mission he said was 11 1/2 hours. The P 51 was considered a miracle because it could complete 6 hr missions. That huge difference in time plus the huge difference in speed of a fighter and bomber meant wave after wave of escorts handing over to each other, all of which needs fuel and all of which needs a contingency to avoid the FUBAR of a bomb group over Germany with no escort. Oh and did I mention the Bombers didn't have a "target" (sometimes listed by Stona), they had a list in case the priority target was obscured. No escort ever set course for the target, on many missions only about 20% even went there.

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Agreed, I did not mean to leave out/insult the many contributors that make far more informative posts than I, I merely used that as a prime example of the caliber of knowledge on this board. I've read and studied aviation history since the 1960's ( basically all my life ) but do not feel I have much to contribute here, I check my ego at the door and have learned a lot, which is daunting considering what I thought I knew when I first got here. Many of you have been extremely patient with my many dumb questions and for that you have my thanks.

OK, I'm off the soapbox now.

I too have learned a lot. Much from other contributors and some just from doing extra research. The scope of WW II aviation is such that one person (or even a small group) can not know everything and many here have areas in which they are quite knowledgeable and other areas where they are less so. But by sharing we all become more knowledgeable and the group as a whole is quite a large resource.

varsity07840 said:

So, you have a VERY short range aircraft with armament that includes useless .30 cal and 37mm gun(s), an altitude performance that in no way comes close to the P-47 and P-38, not to mention the P-51. It can't land on or take off from aircraft carriers. What's the upside of the P-39N?

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The upside of the P-39 in general is that it was available from the start of WWII along with the P-40 and F4F Wildcat. That was all we had for combat until the P-38 Lightning got into combat very late that year. As the P-38 was entering combat in the Pacific and Mediterranean the much improved P-39N was beginning production and around 2000 were completed before the line switched to the Q model in April '43. N and Q were almost exactly alike differing only in wing armament. Since the Russians removed the wing armament on the P-39s the N and Q were basically the same plane. 7000+ planes built to this standard between 12/42 and 8/44. Seems to me that a little weight reduction by the AAF would have improved these models like the Russians did. I'm comparing the P-39N to other planes in combat in 1943, like the P-38, P-40, P-47 (May '43), Hellcat (Aug '43), Corsair (Feb '43), FW190, Me109G, Zero and Oscar. The two stage Merlin P-51 only saw combat from Dec '43. The Spitfire IX had the same Merlin engine as the P-51B but weighed 2500# less than a P-51D so its climb rate was absolutely amazing and significantly higher than any of these planes. Great plane if you wanted to go to Belgium and back.

wuzak said:

Surely using maximum rpm and wide open throttle counts as WEP, even when above FTH?

Before the Spitfire Is were allowed +12psi boost, WEP was at +9psi boost and FTH was higher. So, surely that is still WEP?

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Talking about the Allison, no sir, WEP (in the US) was combat emergency power and extremely hard on the engine requiring the crew chief to do extensive maintenance prior to the next flight. There was a little wire or seal across the throttle and if you went past that into WEP your friend the crew chief had a lot of extra work to do and usually some candid comments for the pilot. Engines were recommended to be CHANGED after only 10 hours at WEP (only good for 5 minutes at a time). Takeoff and Military power were limited to 5 minutes (15 minutes after mid '42) and extra maintenance was not required.

WEP was not available above the critical altitude (FTH) of the engine since at that altitude the engine was already at max RPM and max boost. The only way to get more power would be to increase RPM but 3000RPM was the absolute maximum for the Allison. So no WEP above the critical altitude. WEP began in late '42.

P-39 Expert said:

The upside of the P-39 in general is that it was available from the start of WWII along with the P-40 and F4F Wildcat. .

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Some concepts are complex and difficult to absorb at first. It is essential to proceed step-by-step.

Step 1.

WWII started on 3rd Sept. 1939. as far as Western nations were concerned, July 7 1937 as far as Japan and China are concerned although Japan had been expanding throughout the 1930s.

Shortround6 said:

I would note that short ranged as the P-47C was it had roughly double the range of an 87 gallon Aircobra at similar low speeds.
In fact an 87 gallon Aircobra was roughly equal in range to the P-47C when using a 75 gallon drop tank.
Trouble is the drop tank knocked about 30mph indicated off the airspeed.

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Can we please use the normal 120 gallon internal capacity? PLEASE. The 87 gallon capacity was at the request of the Russians and only used on some N and Qs. A SPITFIRE had more range than a P-39 with 87 gallons.

The 110 gallon drop tank was in extensive use with the 8th AF in England and in the Pacific.

wuzak said:

No WEP at the altitude at which you want to fight?

Wouldn't that mean you are pretty muck fucked?

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See post #707. WEP was almost dangerously hard on the engine, military power was available at the fighting altitudes.

Just asking, but you can say fucked, but I can't say J*p? Is cursing permitted here? If so........

P-39 Expert said:

Can we please use the normal 120 gallon internal capacity? PLEASE. The 87 gallon capacity was at the request of the Russians and only used on some N and Qs. A SPITFIRE had more range than a P-39 with 87 gallons.

The 110 gallon drop tank was in extensive use with the 8th AF in England and in the Pacific.

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By all means and compare it to the Normal internal capacity of the P 51A of 180 gals, and the P51B of 268 galls. Did I mention that the Mustang Mk1 was the first allied single engine fighter to enter German airspace, in 1942.

Shortround6 said:

But basically, for the US, critical altitude is the altitude at which the throttle is fully open and the supercharger is supplying the rated amount of pressure (mass of air).

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Great explanation Shortround6, it helps me to further understand engine ratings. I've noticed that critical altitude with the application of WEP can be significantly lower than it would be without it's use. For example, let's say you are able to safely develop 60" Hg of manifold pressure at 18,500 feet, which is the critical altitude in high blower. But without WEP you are limited to 52" Hg and with this setting you reach critical altitude in high blower at 22,500 feet. Now would I be correct to say that, although the critical altitude while in WEP is 4,000 feet lower, the engine could still produce more horsepower at 22,500 feet than if I just kept manifold pressure at the 52" Hg setting at that altitude? Or in this case does the use of WEP serve no useful purpose above the 18,500 foot altitude?

I hope my question isn't too wordy or hard to understand.....

P-39 Expert said:

Can we please use the normal 120 gallon internal capacity? PLEASE. The 87 gallon capacity was at the request of the Russians and only used on some N and Qs. A SPITFIRE had more range than a P-39 with 87 gallons.

The 110 gallon drop tank was in extensive use with the 8th AF in England and in the Pacific.

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But you keep wanting to use the performance of the 87 gal versions.
And according to some sources quite a few of the Ns and Qs had the smaller capacity. Like out of the first 500 P-40Ns after no 166.
The 900 N-1s had the smaller tanks but kits to bring them back up, likewise the 695 N-5s had the small tanks plus kits. P-39Q-1 had the small tanks, P-39Q-5s had small tanks. P-39Q-10s went back to the 120 gallon tanks from the factory.
How they were used in the field may be different.

You keep saying the weight of the P-39N in the test was because of averaging fuel capacity and then you want to use those numbers as justification for using it as an escort fighter.
Guess what, when you drop the external tank you are near full fuel, minus only the amount used for take-off that was not replaced by the fuel return from the carb while flying on the drop tank.

A 110 gallon tank was used in both Europe and in the Pacific, now the question is was it used by P-39s of which the 8th Air Force had few, if any.
Does the use of a 110 gallon tank push the plane into a restricted weight catagory? P-39Q with the wing .50s went 8106lbs, you want an extra 68 gallons of fuel. Basically about 500lbs more with the weight of the tanks less the weight difference for the .50 cal guns (about 160lbs for the guns and ammo over the .30 cal installation)
And a few more pounds for the extra oil. P-39Q needed an extra 41lbs of oil when carrying a 75 gal drop tank.

DarrenW said:

Great explanation Shortround6, it helps me to further understand engine ratings. I've noticed that critical altitude with the application of WEP can be significantly lower than it would be without it's use. For example, let's say you are able to safely develop 60" Hg of manifold pressure at 18,500 feet, which is the critical altitude in high blower. But without WEP you are limited to 52" Hg and with this setting you reach critical altitude in high blower at 22,500 feet. Now would I be correct to say that, although the critical altitude while in WEP is 4,000 feet lower, the engine could still produce more horsepower at 22,500 feet than if I just kept manifold pressure at the 52" Hg setting at that altitude? Or in this case does the use of WEP serve no useful purpose above the 18,500 foot altitude?

I hope my question isn't too wordy or hard to understand.....

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It would be my understanding that there would be a decreasing level of power from 18,500ft to 22,500ft as the MAP fell from 60" to 52" in this case.
No extra power at 22,500ft but perhaps 56" of pressure at 20,500ft giving about 1/2 the power difference of the WEP at 18.5 and the military power at 22.5.

Hope that helps.

In the case of water injection it sometimes helps above the critical altitude of the blower as it does make the charge denser by helping cool it.

P-39 Expert said:

I'm comparing the P-39N to other planes in combat in 1943, like the P-38, P-40, P-47 (May '43), Hellcat (Aug '43), Corsair (Feb '43), FW190, Me109G, Zero and Oscar.

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Why do you continue to compare a lightweight army fighter to a shipboard naval aircraft? You do realize that if one decided to navalise the Airacobra the additional weight it would gain in such a venture would be problematic. I thought you wanted to remove equipment not add more? Or are you claiming that the P-39N could perform carrier duty as good as or better than these two Navy types without any modifications whatsoever?

And if you were unsure about what this would entail, here's some things to think about:

Navalised aircraft - Wikipedia

A navalised aircraft typically differs from its land-based equivalent by:

• The airframe, engine and avionics are marinised against salt water corrosion.
• It is designed to be used on a flight deck. For a fixed wing aircraft this typically means catapult attachment points, a tailhook and strengthened undercarriage.
• It is designed to occupy minimum hangar space – for example the wings may fold.
• There is enhanced protection against water ingress (including that from hosing down with fresh water to get rid of salt water).
• Equipment such as sensors and weapons are optimised for naval roles.
• The avionics is compatible with the complex electronic equipment of a warship, and that there is no electromagnetic interference between the two.
• There is provision for ditching at sea.

So in your expert opinion, how much would the total weight of the scrawny P-39N increase with these added features?

Also, yesterday I asked a question regarding the viability of the Airacobra as a front-line fighter in 1944 and beyond but you haven't commented on it yet. I sure hope that you're not avoiding the topic just because it may put your pet aircraft in a bad light.....

Shortround6 said:

It would be my understanding that there would be a decreasing level of power from 18,500ft to 22,500ft as the MAP fell from 60" to 52" in this case.
No extra power at 22,500ft but perhaps 56" of pressure at 20,500ft giving about 1/2 the power difference of the WEP at 18.5 and the military power at 22.5.

Hope that helps.

In the case of water injection it sometimes helps above the critical altitude of the blower as it does make the charge denser by helping cool it.

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Thank you for the explanation it was most helpful.

DarrenW said:

Well it looks like the climb rate of the P-39Q loaded to 7,200lbs can be equaled or even bested by the F6F-3 under similar WEP conditions. But seeing that it's not the N model, does this even count?

F6F-3:
View attachment 487513

P-39Q:
View attachment 487514

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Back at you dude. And no WEP. And your chart is from a test exploring larger water metering jets for WEP between Feb '44 and Feb '45 so highly doubtful if these measures could have been implemented in combat planes before the Japanese flyers were beaten in mid '44. The second climb chart is from a normal production F6F3.

Shortround6 said:

Changing the goal posts?

Original statement.

"Earlier D, F, K and L had the 12000' engines, the M, N and Q had the 15000' engines that were about 100hp better at all altitudes."

Now we have " just about 100hp difference at every altitude."

I don't know what college or degree you have but in most of the world being 100hp below, while being a difference, is in no way, shape or form
better.

the chart is also for WEP power.

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Shortstop, I really appreciate you challenging EVERY SINGLE STATEMENT that I make. Thanks.

Did you read this chart that I sent you? It shows the 9.6 engines developing over 100 more HP at every altitude above 8000' and WEP is available only below 4500' for the 8.8 engines and 9500' for the 9.6 engines. I stand by my statement.

pbehn said:

Of course it was there was a war on, everything goes quicker in war time. Now tell me a time between 1939 and 45 when the Spitfire didn't need a better engine, a better field of view, better wings, better armament and better cooling. Of the thousands produced I doubt the maximum of any type that was completely identical apart from paintwork (squadron markings) was only a few hundred.

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A digression I know but of all the 747's that were built only

pbehn said:

By all means and compare it to the Normal internal capacity of the P 51A of 180 gals, and the P51B of 268 galls. Did I mention that the Mustang Mk1 was the first allied single engine fighter to enter German airspace, in 1942.

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A little picky I know but PR Spits flew over germany more or less from the start

P-39 Expert said:

Back at you dude.

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LOL OK surfer boy!

So was the penciled in climb rate of the P-39N available like that from the website? Also, those climb rates given in the un-doctored chart that I provided were possible at normal safe power levels when using ADI so I'm am quite certain that an F6F-3 in service was capable of those numbers at those specific ratings. So again I ask you, do you have more than one official government document showing the remarkable performance of the P-39N that you are so proud of?

And there are many climb charts on the website for the F6F-3. Why only post this particular one? Oh I know, because it's the one displaying the poorest climb performance for the machine...