P-39 Expert
Non-Expert
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.
XP-39 II - The Groundhog Day Thread
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No P-39 will ever be as light or lighter than an A6M2.
P-39 Expert
Non-Expert
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.
special ed
1st Sergeant
- 5,153
- May 13, 2018
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.
P-39 Expert
Non-Expert
One would think that having a speed, climb and dive advantage might be better than just outturning your opponent.
special ed
1st Sergeant
- 5,153
- May 13, 2018
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.
FLYBOYJ
"THE GREAT GAZOO"
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
special ed
1st Sergeant
- 5,153
- May 13, 2018
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.
SaparotRob
Unter Gemeine Geschwader Murmeltier XIII
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.
P-39 Expert
Non-Expert
Please expand above.
fubar57
General
I don't buy it
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.
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GregP
Major
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.
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.
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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 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.
GregP
Major
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
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
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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.
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.
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GregP
Major
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 ...
I THOUGHT I was mistaken once, but I was wrong ...
P-39 Expert
Non-Expert
This isn't addressed to me, hope I'm not intruding but made some comments above.
ThomasP
Senior Master Sergeant
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.
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.
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GregP
Major
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.
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.
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ThomasP
Senior Master Sergeant
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.
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.
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