XP-39 II - The Groundhog Day Thread

GrauGeist said:

The P-39D is lighter?

With the A6M2 at 3,700 pounds empty (6,160 pounds gross) and the P-39D at 5,450 pounds empty (7,500 pounds gross) I am thinking you have that backwards?

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By "lighter P-39" I meant lighter than the heavier P-39. No P-39 will ever be as light or lighter than an A6M2.

special ed said:

Indeed. He is a combat pilot, there to fly combat. Why would he dive away or climb away when he thought his modifications gave him a superior P-39

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Those were his two choices, since he knows not to dogfight an A6M2. Simply climb away then use the altitude advantage to dictate combat. Or dive away depending on the situation since the A6M2 could not follow that either.

Is he a combat pilot or not? Is he up there to run away? What is the purpose of the mods to the P-39 if not to give it a better advantage? If it is still inferior after the mods, stay home or get another type of fighter.

special ed said:

Is he a combat pilot or not? Is he up there to run away? What is the purpose of the mods to the P-39 if not to give it a better advantage? If it is still inferior after the mods, stay home or get another type of fighter.

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One would think that having a speed, climb and dive advantage might be better than just outturning your opponent.

If you run away, what good is your climb and dive advantage, except to save your life? You must get guns on in order to eliminate the enemy, or he will be back.

special ed said:

If you run away, what good is your climb and dive advantage, except to save your life? You must get guns on in order to eliminate the enemy, or he will be back.

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If you don't have a tactical advantage, you run away (disengage), you could be in a P-39, P-51 or F-22. Fighter tactics, 101

Unfortunately, there are times when you "run what you brung" and do the best you can with what you have. A P-39 example from May 1942 N. Guinea is on the front cover of P-39 aces, who low on fuel chose to engage. He did have an initial altitude advantage which he used for first contact but the remaining part of the battle was at the same altitude and he did pick up holes in his P-39D but still made it home. Of course, he could have used his height advantage to dive away and go to base, but that was not why he was in New Guniea.

special ed said:

Unfortunately, there are times when you "run what you brung" and do the best you can with what you have. A P-39 example from May 1942 N. Guinea is on the front cover of P-39 aces, who low on fuel chose to engage. He did have an initial altitude advantage which he used for first contact but the remaining part of the battle was at the same altitude and he did pick up holes in his P-39D but still made it home. Of course, he could have used his height advantage to dive away and go to base, but that was not why he was in New Guniea.

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I haven't heard "run what you brung" in decades. A friend of mine raced his clapped out '63(?) Dodge Dart with a push button transmission against a new '87 Monte Carlo and won. He got "blowed away" by a guy in a pickup.

XBe02Drvr said:

Now let's take a little walk back to Peter Gunn's post #1381 on page 70 for an apples-to-apples comparison of three different blade sets on the same model hub, same engine, same airplane (in this case, a P40F, same s/n for all tests), and same test conditions for all trials. The parameter being tested is rate of climb at 18,000 feet. The hub is a C-532D, which, in a version with a cannon channel, is also used on some P39s.
Blade #89306-19S climbs at 1460 ft/min.
Blade #89301-03 climbs at 1970 ft/min. This is the standard production propeller, why are you comparing it with the lower climbing propeller? Same standard production propeller as in the P-40E Allison test in the same post. And why wouldn't you use the Allison comparison since the P-39 also used the same Allison engine? Unless you can prove that the lower climbing propeller was used on the P-39D this is an invalid comparison.
Blade #89303-24W climbs at 2010 ft/min.
I'm no math whiz, but that looks like a 550 ft/min difference to me between best and worst. The fact that the engine is a Merlin, not an Allison is irrelevant, as this is merely a test of different prop blades, with all other factors kept equal. Admittedly the Allison will have a little less horsepower available at this altitude, but not enough to erase that large a difference between prop blades. This, plus the weight difference, plus added drag from additional antennas that weren't on the C and other small incremental drag increases (wing guns for example) can easily add up to the performance deficit of the P39D. The best thing you could do for the P39D, IMHO, would be to get the carburetor air intake out of that dead zone behind the canopy so it could get some ram effect. Seriously? The P-39 carb intake had no ram effect? Look at the P-30D test in wwiiaircraftperformance.org. About 120hp from ram effect between the speed test and the climb test at the same altitude.
836 lbs alone ain't gonna do it. The 836lbs is the main factor, as you well know. No negligible items like antenna drag, wing gun drag, fin fillet, phantom CG differences, symmetrical airfoil or the weather will account for a 1000fpm difference in climb. You have two official tests in wwiiaircraftperformance.org of the same plane with different weights. The one with 836lbs more weight climbed 1000fpm slower than the other.

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Please expand above.

I don't buy it

Peter Gunn said:

Hello Ivan,

Any info as to why they didn't go with the two .30's in the nose like the photo you posted? Looks like a rather lethal setup ala P-38 to me. Also, could they have ditched the .30's for 4 .50's? Or even 5 by replacing the 37mm hub cannon?

Thanks

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Hello Peter Gunn,

The P-39C also had the option of two additional .30 cal MG in the wings that could be installed if needed.
I believe the Nose Gun setup on the C was the best that was installed in any Airacobra which is why I was recommending going back to it. Those .30 cals had 300 rounds per gun. I don't think they could have replaced them with .50 cals for lack of room.

It sounds like General Harmon's crew did try replacing the 37 mm with an additional .50 cal and the higher rate of fire wasn't enough of a compensation for the hitting power of the cannon.

If I had to work on the Airacobra with factory resources and starting with a P-39D, I would have:
1. Put back the .30 cal Nose Guns. That and the ammunition supply should be a pretty close replacement for the Gear Box armour for balance but probably not quite.
2. Replace the 37 mm motor cannon with a 20 mm Hispano but with a 250 round belt feed instead of the 60 round drum.
3. Try to move the Oil Tank from the tail up to the nose. There might be room where the belt for the 37 mm cannon used to be.
The oil lines might be a bit longer, but the oil coolers are under the cockpit and about equal distance from the nose or tail anyway. The shape of the oil tank does not have to remain the same and without the oil tank so far back, the armour plate that protected it may not be needed or could be moved further forward to protect the coolant tank.
The original oil tank had a heating element inside and changing the shape might mean deleting that feature. Hopefully the Soviets won't get too upset.

Edit: And standardize on 270 rounds per .50 cal gun in the nose instead of 200 rounds. There was room for the extra ammunition and some US test aircraft flew with up to 250 rounds per gun.

The basic idea is to move the CoG forward so the normal loaded condition isn't sitting at 28.5% but without any permanent ballast.

- Ivan.

Hi Ivan,

I found an online reference that said the P-39C guns were over and under the nose (and the P-39D guns had 2 more 30s that werre all four wing-mounted) and used it. Very nice pic you found that disproves that one. Appreciate it.

Just curious ... where did you find those pictures? They are very good.

Edit: Nevermind, found the pics. Not labeled as a P-39C, but HAS to be a YP-39 or a P-39C because none of the other P-39s had four nose MG.

Cheers.

Hello GregP,

I just tried out the same formula that you were working with from Naval Aviators but with just one change.
I threw in a propeller efficiency factor.
The conclusion is.... It doesn't seem to make a difference other than change the Power Required.
I haven't tried it for different altitudes though.

Assuming that the P-39C and P-39D REALLY were identical except for weight, the only explanation for the difference in performance would be that the P-39D was running 133.79 HP less --- 1016 HP instead of 1150 HP at 10,000 feet.

- Ivan.

I used the actual data points at 10,000 and 20,00 feet to get Pr at those two altitudes. Then I changed the weight. Since I was always using data obtained at the altitude of the data point and I only changed the weight after gettinf Pr, the results for rate of climb should be pretty accurate.

My conclusion was the same as yours ... the power used in the P-39D test at 10,000 feet was not quite military power. However, it is tough to state it categorically since we can't talk with the people involved and the recorded test doesn't have all the data we want to look at. I'm thinking that whoever looked up the power in the tech order just went to the Military power line, but teh actual test was at normal rated power and 2,600 rpm.

Since I can't ask anyone, I'm left with the fact that the climb data for the P-39D shows and airplane that needs 1/3 more power in level flight at 10,000 feet than the P-39C did, and that doesn't sit nearly as well as the alternate conclusion, at least to me.

Cheers.

Edit: I wanted to do a reasonability check on it, so I went to the P-39M test. They don't have a data point at 10,000 feet, but they DO have one at 9,900 feet. Close enough. Short version of what I found: Something is off kilter ...

At 9,900 feet, the P-39M did 3,320 fpm with power at 1,250 / 3000 rpm, and the aircraft at 7,430 pounds. Low and behold, the power required for level flight is 502 hp, which is not all that far off from the P-39D number.

And, they have a data point at 20,000 feet. Since we are "ballparking" it, let's use the same weight. Pa = 880 hp, weight = 7,430 lbs, RC = 2,000 fpm, and Pr = 430 hp.

So, the P-39M matches more closely with the P-39D numbers than the P-39C does.

So far, the only conclusion I can come to is that the P-39C is not quite nearly as identical to the P-39D as we might suppose from the available reading. The report says the aircraft was at 28.6% MAC at takeoff, wheels up, propeller is the same as the P-39D, and the armament is the same as the P-39D (doesn't say which cannon was installed).

This might get interesting, from an analysis standpoint! That assumes the interest is there to keep going. I will, to a popint, but not just now since some "honeydos" are creeping into the picture.

Again, Cheers

Hello GregP,

The big difference between what I am doing and what you are doing is that I am throwing in an assumed propeller efficiency.
The idea that there needs to be a propeller efficiency is explained at the end of Page 154 and beginning of Page 156 (After the rather lousy picture of the F11F Tigers from the Blue Angels).
With the propeller efficiencies, the Power Required I am getting isn't the same as what you are getting.
The only problem is that I have to make a lot of assumptions to go any further and the data isn't that good to begin with and once a few assumptions are thrown in, all we can be sure of is that it is an aeroplane and not a pterodactyl.

- Ivan.

I was thinking ... pterodactyl myself with low-drag skin and automotive style doors. But, hey, I could be mistaken.

I THOUGHT I was mistaken once, but I was wrong ...

Ivan1GFP said:

Hello Peter Gunn,

The P-39C also had the option of two additional .30 cal MG in the wings that could be installed if needed.
I believe the Nose Gun setup on the C was the best that was installed in any Airacobra which is why I was recommending going back to it. Those .30 cals had 300 rounds per gun. I don't think they could have replaced them with .50 cals for lack of room.

It sounds like General Harmon's crew did try replacing the 37 mm with an additional .50 cal and the higher rate of fire wasn't enough of a compensation for the hitting power of the cannon.

If I had to work on the Airacobra with factory resources and starting with a P-39D, I would have:
1. Put back the .30 cal Nose Guns. That and the ammunition supply should be a pretty close replacement for the Gear Box armour for balance but probably not quite. Problem was space, with the .30s there was not enough room for the full 30 rounds of 37mm cannon ammo. Not much punch from the .30s and even less with prop synchronization.
2. Replace the 37 mm motor cannon with a 20 mm Hispano but with a 250 round belt feed instead of the 60 round drum. Excellent idea, I think you could have made do with 120-150 rounds. Spitfire had 120 rounds and P-38 had 150rounds. Save about 80-100lbs weight vs the .37mm. Some early Allison P-51s had 20mms with belt feed, so it was doable.
3. Try to move the Oil Tank from the tail up to the nose. There might be room where the belt for the 37 mm cannon used to be. Again, space was the problem but may be possible, especially with different shape. I always thought the oil tank and coolant tank could have been redesigned so that they occupied the space that the coolant tank occupied, just side by side. That would move the CG forward a little, certainly not as much as moving the oil tank to the nose.
The oil lines might be a bit longer, but the oil coolers are under the cockpit and about equal distance from the nose or tail anyway. The shape of the oil tank does not have to remain the same and without the oil tank so far back, the armour plate that protected it may not be needed or could be moved further forward to protect the coolant tank.
The original oil tank had a heating element inside and changing the shape might mean deleting that feature. Hopefully the Soviets won't get too upset.

Edit: And standardize on 270 rounds per .50 cal gun in the nose instead of 200 rounds. There was room for the extra ammunition and some US test aircraft flew with up to 250 rounds per gun. Excellent idea, especially if the .30s are deleted. Only added about 45lbs but increased firing time about 9 seconds. A 20mm w/120rds and 2x.50s w/270rds each is pretty potent.

The basic idea is to move the CoG forward so the normal loaded condition isn't sitting at 28.5% but without any permanent ballast.

- Ivan.

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This isn't addressed to me, hope I'm not intruding but made some comments above.

Hey GregP,

The relative end result discrepancies you are running into are more or less the same ones I ran into. When I run the numbers for:

the P-39C tests in the US
the P-39C/Airacobra I trials in the UK
the P-39D-1/Airacobra IA trials in the UK
the P-39D/D-1/M-3/N-1/Q-5 test tests in the US
and the P-39N-1 drag test done by NACA

the only signifiant outlier I get is the original P-39C ROC test. Hence I am forced to conclude that the original P-39C climb test value of 3750 ft/min sustained ROC is either:

incorrect and should be ~3270 ft/min, or
the power used was somewhere 1300 BHP+ for the first ~10,000 ft, or
the power used was 1150 BHP and a zoom climb was used (from Vmax of ~290 mph at SL) to get the additional ROC claimed.

Interesting stuff.

Hi ThomasP.

I pretty much came to the same conclusion. There is something hokey about the P-39C test, but I don't know what.

If I assume you are correct and the P-39C ROC should be 3270 fpm, then the Pr rises to 478 hp, which about in line with the P-39D and M. Then, if I vary the hp to get 3720 fpm, the hp turns out to be 1241 hp, not 1150. Here's the thing, we do NOT have the MAP readings for climb. When I was working at an Allison shop for a few years, we had retired General Davey Allison come by the shop for a visit. He told us they used to fight the P-40s in the AVG at 70" MAP. They had early Allisons, similar to the P-39C engine, but it had a prop shaft on the front in the P-40 and was probably a C-series engine ... likely a V-1710-33 in it. He did not know the horsepower, but it was definitely above what the stock MAP reading gave.

The early C-series nose cases were not too strong, but the -35 in the P-39C/D didn't HAVE the early nose case; they were early E-series engines and connected to a driveshaft, so they didn't need a nose case. They were strong enough for a LOT more popwer than stock.

If the P-39C pilot for the flight test we are looking at ran higher than normal MAP, he could well have been climbing at more than 1,150 hp, and may well have seen 3,720 fpm at the elevated hp and MAP. The thing is, we don't have any way to find out. But it is definitely different from the P-39D and M and other tests on the P-39.

As you said, interesting. Isn't precise analysis fun?

We know what we don't know, but we aren't sure what we know ... there's a joke in there somewhere. Either they ran the P-39D test at lower-than-reported hp or they ran the P-39C test at higher-than-reported hp ... or somebody misreported something ... or they held their tongue wrong during flight ... or ... a pretty girl walked past when they were recording the numbers.

At this point, I am leaning toward the P-39C test being run at higher-than-reported MAP and hp, combined maybe with a propeller that was better for climbing. But, the weight difference alone won't give you 1,000 fpm difference in climb. Some other factor was at work here. It may be forever a mystery unless we can find other P-39C tests. They only made 20 of them. How many tests were conducted? I surely don't know.

Poor toilet training as a kid on the part of the report writer might be in there somewhere, too.

Hey GregP,

Thanks for taking the time to look at the numbers again. I was pretty sure I had it ~right, but aerodynamics is not my specialty, and there is always a chance of missing something important in either the math or the data used. And yes, I do find the analysis fun, as do I find the discussion of such things with you and others on this forum enjoyable.