XP-39 II - The Groundhog Day Thread

[Peter Gunn]

A few (dozen?) pages back I believe Wes brought up that the difference on one P-39 test where the propeller had a different model number, I believe that some here poo pooed that a bit as not seeming to be relevant.

I was looking into P-40 data this morning (an aircraft I consider superior to the P-39 for several reasons) and found this, it's been a while since I did much Excel work so I just did a couple of screen shots. Hopefully they are the same size. I call attention to the RoC FPM columns on this table and what an influence just a different prop can have on performance.

And here's one further down with a V-1650-1 Packard - Merlin

 

[SaparotRob]

If nothing else, this thread has been good for garnering positive ratings such as BACON - WINNER - AGREE and of course plain old LIKEs.

Does a thread starter get an award for opening a thread that gets over a thousand posts or something? I know nothing about aeronautical engineering and even I'm recognizing terms and concepts I had no idea about.

 

[P-39 Expert]

A few (dozen?) pages back I believe Wes brought up that the difference on one P-39 test where the propeller had a different model number, I believe that some here poo pooed that a bit as not seeming to be relevant.

I was looking into P-40 data this morning (an aircraft I consider superior to the P-39 for several reasons) and found this, it's been a while since I did much Excel work so I just did a couple of screen shots. Hopefully they are the same size. I call attention to the RoC FPM columns on this table and what an influence just a different prop can have on performance.

View attachment 600932

And here's one further down with a V-1650-1 Packard - Merlin

View attachment 600933

Yes, these are gigantic differences in performance. 4.5mph in speed from the slowest to the fastest, and 80fpm in climb at 15000'. That's going to make up for a 1000fpm climb rate difference.

Not too concerned about the P-40F tests since neither the P-39C nor D had a Merlin engine. But the standard propeller had within 1mph of the best speed and 40fpm of the best climb.

Please note that even with the Merlin engine the P-40F was still slower than the P-39D/F/K/L. Climb was about the same at combat power.

 

[fubar57]

Never go near the whales

​

 

[XBe02Drvr]

... because the atmospherics are different in different parts of the world. That is why you see "adjusted to standard atmosphere" of data sheets.

Ever hear of "density altitude"? The warmer and more humid the air, the more your performance suffers.
Standard atmosphere is 69°F/15°C at MSL, 29.92" Hg, 65% RH. This is what aircraft performance numbers are corrected to. Now fly your Supercobra from a South Pacific island where the ground temp is pushing 90°F, the RH is 90+%, and the lapse rate through the altitudes behaves accordingly, and your single speed, single stage Allison, WAY above its critical altitude, isn't going to give your inefficient symmetrical airfoil Supercobra anywhere near the observed performance your "standardized" test reports would lead you to expect.
I used to see 1,000 FPM climb performance differences between a July afternoon and predawn on a January morning on the same routes at the same altitudes with the same planes. BUF -> BOS, we'd be level at FL 250 before passing ROC in January, and nearly to SYR before we got up there in July. We would also see significant climb rate deterioration on individual aircraft as the engines aged.

 

[Peter Gunn]

Yes, these are gigantic differences in performance. 4.5mph in speed from the slowest to the fastest, and 80fpm in climb at 15000'. That's going to make up for a 1000fpm climb rate difference.
And over 300' at 28k - I can cherry pick stats too.

Not too concerned about the P-40F tests since neither the P-39C nor D had a Merlin engine. But the standard propeller had within 1mph of the best speed and 40fpm of the best climb.
Merlin Engine Irrelevant, look at the climb differences, just the "different part number" prop can mean 400' of climb loss/gain

Please note that even with the Merlin engine the P-40F was still slower than the P-39D/F/K/L. Climb was about the same at combat power.
Please note that is irrelevant to the discussion of climb performance using the EXACT same aircraft loaded EXACTLY the same way but with the ONLY difference being the propeller.

As they say, please expand.

 

[P-39 Expert]

As they say, please expand.

Okay, how are you going to make up the other 700fpm? Phantom CG problems? A tail fin fillet? The extra inch of fuselage length?

Think it was maybe the 836lbs of extra weight?

 

[P-39 Expert]

Ever hear of "density altitude"? The warmer and more humid the air, the more your performance suffers.
Standard atmosphere is 69°F/15°C at MSL, 29.92" Hg, 65% RH. This is what aircraft performance numbers are corrected to. Now fly your Supercobra from a South Pacific island where the ground temp is pushing 90°F, the RH is 90+%, and the lapse rate through the altitudes behaves accordingly, and your single speed, single stage Allison, WAY above its critical altitude, isn't going to give your inefficient symmetrical airfoil Supercobra anywhere near the observed performance your "standardized" test reports would lead you to expect.
I used to see 1,000 FPM climb performance differences between a July afternoon and predawn on a January morning on the same routes at the same altitudes with the same planes. BUF -> BOS, we'd be level at FL 250 before passing ROC in January, and nearly to SYR before we got up there in July. We would also see significant climb rate deterioration on individual aircraft as the engines aged.

And the A6M2 would be flying through the same atmosphere, right? With it's single stage engine? WAY above it's critical altitude?

And isn't the air temperature the same all over the world at 25000'?

 

[GregP]

I can't believe I'm still trying, but the P-39D test must have been done at normal power (2600 rpm), not military power (3000 rpm).

That is one out of several possible explanations, and they DID say they got the power from a tech order unrelated to the test, not a torque meter in the airplane.

It wouldn't be the first flight test to be misrecorded and likely won't be the last.

 

[P-39 Expert]

I can't believe I'm still trying, but the P-39D test must have been done at normal power (2600 rpm), not military power (3000 rpm).

That is one out of several possible explanations, and they DID say they got the power from a tech order unrelated to the test, not a torque meter in the airplane.

It wouldn't be the first flight test to be misrecorded and likely won't be the last.

The P-39C test was at normal power 2600rpm for climb after the 5 minute limit, just like the D model.

Standard performance test at Wright Field where most all the fighters were tested. Please don't blame the test, that's all we really have to compare the planes.

It may have been the 836lbs extra weight in the D model.

 

[XBe02Drvr]

And the A6M2 would be flying through the same atmosphere, right?

Righto, old chap, and better able to handle it for the aerodynamic reasons mentioned in my previous post (#1371). Any way you slice it, combat between single stage single speed fighters in the upper twenties is going to be a sort of slow motion minuet, rather than high energy rock n' roll, as neither plane has much stall margin or excess power, and there's not much G available for maneuvering. The P39 will bleed energy faster in any maneuver due to its less efficient wing in that regime.

 

[XBe02Drvr]

And isn't the air temperature the same all over the world at 25000'?

If it were, do you think we'd have the jetstream?

 

[GrauGeist]

And isn't the air temperature the same all over the world at 25000'?

Simple answer: No.

 

[FLYBOYJ]

And the A6M2 would be flying through the same atmosphere, right? With it's single stage engine? WAY above it's critical altitude?

And isn't the air temperature the same all over the world at 25000'?

Today at 24,000'

 

[Milosh]

Not too concerned about the P-40F tests since neither the P-39C nor D had a Merlin engine. But the standard propeller had within 1mph of the best speed and 40fpm of the best climb.

It is NOT about the engine being used BUT the difference ROC with different props.

 

[Ivan1GFP]

As has been explained before, a standard P-39D/F/K/L at normal weight before any weight reduction had a service ceiling of over 30000' as defined by still climbing at 100fpm. The term "service ceiling" was obviously being used incorrectly in the AAF South Pacific report. Why would the General be pleased that a P-39K that had been lightened by 650lbs have a "service ceiling" below that of a standard P-39K? The meaning of "service ceiling" in the report obviously referred to an altitude at which the plane could fight.

Perhaps there were other conditions in theater or about the aircraft that made the normal expected performance unachievable.
I find it very unlikely that this many professionals would let a mistake like this pass without comment.

My SUSPICION is that the P-39K was carrying a drop tank.

- Ivan.

 

[P-39 Expert]

Today at 24,000'

View attachment 600936

Total variance less than 20 degrees? About 17 to about 37?

 

[P-39 Expert]

Righto, old chap, and better able to handle it for the aerodynamic reasons mentioned in my previous post (#1371). Any way you slice it, combat between single stage single speed fighters in the upper twenties is going to be a sort of slow motion minuet, rather than high energy rock n' roll, as neither plane has much stall margin or excess power, and there's not much G available for maneuvering. The P39 will bleed energy faster in any maneuver due to its less efficient wing in that regime.

I'm not buying the less efficient wing. I put that in there with the fin fillet, one inch extra length fuselage and phantom CG issues.

Think it might be the 836lbs?

Come on guys, its the weight. We all know it. And the more weight the less climb. The AAF South Pacific report pretty much proved it.

 

[FLYBOYJ]

Total variance less than 20 degrees? About 17 to about 37?

Yes - 20 degrees C (36F) and when computing performance calculations, it will make a difference.

 

[P-39 Expert]

Perhaps there were other conditions in theater or about the aircraft that made the normal expected performance unachievable.
I find it very unlikely that this many professionals would let a mistake like this pass without comment.

My SUSPICION is that the P-39K was carrying a drop tank.

- Ivan.

The chart says that even with a drop tank the "service ceiling" was over 27000'. Wish the report had been more specific.

I think most would be inclined to believe that the P-39K could handle itself at 27000' with the weight reduction. General was pleased with the results.

Reducing the weight increased the climb rate and ceiling just like I have been telling you.