XP-39 II - The Groundhog Day Thread

[P-39 Expert]

Hello All,

Attached is the text of the test between P-39D-1 and A6M2 that was captured in the Aleutians and repaired.
View attachment 596607
Acceleration Tests:
5000 feet 230 MPH indicated – 248 MPH TAS
10000 feet 220 MPH indicated – 256 MPH TAS
15000 feet 210 MPH indicated – 265 MPH TAS
20000 feet 200 MPH indicated – 288 MPH TAS
25000 feet 180 MPH indicated – 269 MPH TAS

- Ivan.

My point is that the early P-39s (D/F/K/L) were way to heavy and could have been lightened easily by removing the useless .30calMGs (200-400lbs depending on whether 300 or 1000 rounds per gun was loaded) and the useless nose armor plate (100lbs). This would have improved climb rate to exceed that of the Zero. P-39 was already 40mph faster than the Zero at all altitudes.

 

[Ivan1GFP]

My point is that the early P-39s (D/F/K/L) were way to heavy and could have been lightened easily by removing the useless .30calMGs (200-400lbs depending on whether 300 or 1000 rounds per gun was loaded) and the useless nose armor plate (100lbs). This would have improved climb rate to exceed that of the Zero. P-39 was already 40mph faster than the Zero at all altitudes.

Hello P-39 Expert,

We are in partial agreement here.
The early P-39 really was too heavy. The problem though was that there wasn't that much that could be done about it without a significant redesign as was tried in the P-39E.
Removing the Wing Guns probably wasn't a bad idea, but that didn't leave enough armament to suit American requirements. We'll get back to that in a minute.
Unless there was a radical shift in equipment, the nose armour has to stay to make the aeroplane flyable.

FWIW, the early P-39 WASN'T 40 MPH faster than the Zero at all altitudes. The Aleutian A6M2 wasn't a really good representative in terms of actual performance. Keep in mind that it was an aircraft that was crash landed in a bog, flipped and sat partially underwater for a significant amount of time.
One of the pilots commented that it was probably in about 95% condition. Basically, at best, it was a slightly bent bird. I will cover those details in a later post.

If the objective was to improve the P-39, I believe the following would be a reasonable solution:

1. Design new Outer Wing Panels. There is no good reason to use a symmetrical airfoil.
The Wing Tips do have a different airfoil and that actually does serve as "aerodynamic twist" instead of washout but the problem as shown in the NACA report was that the entire rest of the wing stalled at about the same time as shown by tufts attached to the wings.
The new wings should have a more common NACA 23000 series airfoil and a bit of a sweep to put the center of lift further aft to address the CoG problem. At THAT point, perhaps the 100 pound Gearbox armour could be removed.
This is not a new idea. This kind of thing was done on the IL-2 Sturmovik to address the CoG change caused by adding a rear gunner.
This is also a much easier process on the P-39 because the outer wing panels were detachable at the inboard edged of the main landing gear bay and would not greatly affect other aircraft structures.

This may also be an opportunity to increase the wing area slightly and also to improve aileron effectiveness.
Peak roll rate on the P-39 was only around 80 degrees / second which is rather mediocre performance.
Both the NACA report and British testing commented on excessive friction in the aileron linkages which suggest this may be common to the type.

With a redesign of the outer wing panels, it might also be a good idea to move the oil coolers to the roots of the outer panels and free up space in the belly for additional radiator capacity to handle more powerful engines.
Perhaps fuel capacity could also be improved.

2. Replace the 37 mm cannon with a 20 mm Belt Fed weapon. The drum fed HS-404 gun could not carry enough ammunition. Perhaps something similar to the installation in the P-38 might work.
Add back the .30 cal cowl mounted MG from the P-39C if room permits.
This should result in pretty good sustained firepower from guns with similar ballistics.

- Ivan.

 

[tomo pauk]

P-39 was lighter than P-40. What P-39 (and P-40) have had was the lesser engine than what powered current Spitfire/P-51B, Bf 109, P-38, let alone F4U, P-47 or a working Typhoon.

 

[P-39 Expert]

Hello P-39 Expert,

We are in partial agreement here.
The early P-39 really was too heavy. The problem though was that there wasn't that much that could be done about it without a significant redesign as was tried in the P-39E.
Removing the Wing Guns probably wasn't a bad idea, but that didn't leave enough armament to suit American requirements. We'll get back to that in a minute.
Unless there was a radical shift in equipment, the nose armour has to stay to make the aeroplane flyable.

The 37mm cannon and twin .50cal MGs provided more firepower than four .50s in the P-51A/B/C, FM2 and F8F with centerline fire and an exploding shell.

A small shift of moving the SCR-535 IFF radio from the tailcone to above the engine just behind the pilot would have compensated for removal of the nose armor plate. The nose armor weighed 100lbs and the IFF radio weighed 120lbs and was actually farther away from the COG than the nose armor. Or just remove the IFF radio altogether as it wasn't included in the early Thunderbolts or the P-40E. Save 100lbs deleting the nose armor, 200-400lbs deleting the wing guns and 120lbs deleting the IFF radio. All this could have been done immediately at forward combat bases by existing maintenance personnel.

FWIW, the early P-39 WASN'T 40 MPH faster than the Zero at all altitudes. The Aleutian A6M2 wasn't a really good representative in terms of actual performance. Keep in mind that it was an aircraft that was crash landed in a bog, flipped and sat partially underwater for a significant amount of time.
One of the pilots commented that it was probably in about 95% condition. Basically, at best, it was a slightly bent bird. I will cover those details in a later post.

Yes the P-39 was 40mph faster than the A6M2 Zero. Look at the performance graph for the P-39K in Mike Williams' site. 370mph at 16000'. 1942 Zero did 330mph at best. Biggest speed differential was 50mph at 10000' and maintained 40mph differential over 25000'. This for a standard 7650lb P-39 with the early 8.8 supercharger.

If the objective was to improve the P-39, I believe the following would be a reasonable solution:

1. Design new Outer Wing Panels. There is no good reason to use a symmetrical airfoil.
The Wing Tips do have a different airfoil and that actually does serve as "aerodynamic twist" instead of washout but the problem as shown in the NACA report was that the entire rest of the wing stalled at about the same time as shown by tufts attached to the wings.
The new wings should have a more common NACA 23000 series airfoil and a bit of a sweep to put the center of lift further aft to address the CoG problem. At THAT point, perhaps the 100 pound Gearbox armour could be removed.
This is not a new idea. This kind of thing was done on the IL-2 Sturmovik to address the CoG change caused by adding a rear gunner.
This is also a much easier process on the P-39 because the outer wing panels were detachable at the inboard edged of the main landing gear bay and would not greatly affect other aircraft structures.

This may also be an opportunity to increase the wing area slightly and also to improve aileron effectiveness.
Peak roll rate on the P-39 was only around 80 degrees / second which is rather mediocre performance.
Both the NACA report and British testing commented on excessive friction in the aileron linkages which suggest this may be common to the type.

With a redesign of the outer wing panels, it might also be a good idea to move the oil coolers to the roots of the outer panels and free up space in the belly for additional radiator capacity to handle more powerful engines.
Perhaps fuel capacity could also be improved.

How long will it take to design a new wing and get it into production? A year? P-39 wing was just fine. COG was adequate. Remove the useless equipment to increase the climb/ceiling. Could have been done in 2-3 hours at any combat base. Roll was adequate and the standard P-39 would out turn any plane not made in Japan or named Spitfire.

2. Replace the 37 mm cannon with a 20 mm Belt Fed weapon. The drum fed HS-404 gun could not carry enough ammunition. Perhaps something similar to the installation in the P-38 might work.
Add back the .30 cal cowl mounted MG from the P-39C if room permits.
This should result in pretty good sustained firepower from guns with similar ballistics.

Personally I'd like a 20mm cannon with 120 rounds just like a Spitfire. Save another 80lbs over the 37mm. Give the .30cal MGs to the infantry.

- Ivan.

 

[P-39 Expert]

P-39 was lighter than P-40. What P-39 (and P-40) have had was the lesser engine than what powered current Spitfire/P-51B, Bf 109, P-38, let alone F4U, P-47 or a working Typhoon.

Yes the P-39 had the Allison engine. 1942 P-39D/F/K/L equipped correctly at 7100lbs would have been a match for the 1942 Spitfire V and Bf109F at all altitudes. P-38F/G, F4U, F6F, Thunderbolt and a workable Typhoon didn't get into combat until very late 1942 or 1943. By that time the P-39N would substantially outclimb all of them at all altitudes at it's normal combat weight of 7650lbs, no weight reduction needed.

 

[buffnut453]

Yes the P-39 had the Allison engine. 1942 P-39D/F/K/L equipped correctly at 7100lbs would have been a match for the 1942 Spitfire V and Bf109F at all altitudes. P-38F/G, F4U, F6F, Thunderbolt and a workable Typhoon didn't get into combat until very late 1942 or 1943. By that time the P-39N would substantially outclimb all of them at all altitudes at it's normal combat weight of 7650lbs, no weight reduction needed.

The problem is that in mid-1942, the Spitfire MkIX was coming on-stream. By all means, compare the P-39D/F/K/L with the MkV but the MkV had been in operational service at least 18 months by 1942...and 18 months was a long time in aircraft development during WW2.

 

[tomo pauk]

Yes the P-39 had the Allison engine. 1942 P-39D/F/K/L equipped correctly at 7100lbs would have been a match for the 1942 Spitfire V and Bf109F at all altitudes. P-38F/G, F4U, F6F, Thunderbolt and a workable Typhoon didn't get into combat until very late 1942 or 1943. By that time the P-39N would substantially outclimb all of them at all altitudes at it's normal combat weight of 7650lbs, no weight reduction needed.

Nope.
1942: Bf 109F-4/G2 is both better climber and it is faster than any P-39, even the stripped-down versions. Spitfire V is better climber, and it is faster above 15000 ft. Spitfire IX - no contest.
1943: P-47 is faster above 17000 ft, so is the P-38, F4U, F6F. What is more important, they can either match or better the Axis fighters in speed, especially above 20000 ft (bar F6F vs. German opposition). They all carry better firepower and can offer either superior range or carrier suitability or both vs. what P-39 can offer.

In 1942-43, the non-turbo V-1710 was two years behind curve vs. Merlin, BMW 801 and DB 601/605; same vs. R-2800. Compared with US ww2 that lasted less than 4 years, half of that is huge. Even in 1941, the V-1710 was well behind the curve vs. Merlin or DB 601.

 

[P-39 Expert]

The problem is that in mid-1942, the Spitfire MkIX was coming on-stream. By all means, compare the P-39D/F/K/L with the MkV but the MkV had been in operational service at least 18 months by 1942...and 18 months was a long time in aircraft development during WW2.

P-39D was in production in early 1941 and we weren't at war until December 1941.

Spitfire IX was just coming into service in the last half of 1942 with a few operational squadrons and not really into full production until early 1943.

For a real performance boost don't bother with the extensively reworked P-39E, just install the mechanical two stage Allison V1710-47 into any P-39 when it became available in April 1942. At 7650lbs plus the 175lb auxiliary stage supercharger and another 75lbs for a four blade propeller, the two stage P-39 would have weighed 7900lbs instead of 8900lbs for the P-39E or later P-63. Move the carb from the auxiliary stage to the normal location on the engine and increase critical altitude from 21500' to 25000'. That plane would have been a rocket.

 

[Ivan1GFP]

Yes the P-39 was 40mph faster than the A6M2 Zero. Look at the performance graph for the P-39K in Mike Williams' site. 370mph at 16000'. 1942 Zero did 330mph at best. Biggest speed differential was 50mph at 10000' and maintained 40mph differential over 25000'. This for a standard 7650lb P-39 with the early 8.8 supercharger.

Hello P-39 Expert,

All I can say is that your information is simply incorrect. The Aleutian A6M2 in much less than perfect shape achieved 335 MPH WITHOUT Overboost. In the reports that were distributed, this was corrected down to 332 MPH.
Richard Dunn has a pretty good article supported also by recollections of Saburo Sakai that suggest a more likely maximum speed to be 345 MPH on Overboost. From the combat reports he quotes, the A6M2 seems to be a bit faster than one might expect.
Personally, I believe 340 MPH to be a bit more credible though.
Weight affects climb but really doesn't make that much of a difference in maximum speeds.

- Ivan.

 

[P-39 Expert]

Nope.
1942: Bf 109F-4/G2 is both better climber and it is faster than any P-39, even the stripped-down versions. Spitfire V is better climber, and it is faster above 15000 ft. Spitfire IX - no contest.
1943: P-47 is faster above 17000 ft, so is the P-38, F4U, F6F. What is more important, they can either match or better the Axis fighters in speed, especially above 20000 ft (bar F6F vs. German opposition). They all carry better firepower and can offer either superior range or carrier suitability or both vs. what P-39 can offer.

Beg to disagree. At 7100lbs the 1942 P-39 outclimbs the SpitfireV and 109F at all altitudes. Check the P-39C in Mike Williams' site but use 3000rpm for climb. I'm aware that the P-39C was not combat capable but it did weigh 7100lbs and had the same engine, propeller and drag as a P-39D/F at 7100lbs.

The 1943 P-39N would do over 380mph at 20000' but more importantly it would substantially outclimb the P-38F/G, F4U, F6F, Thunderbolt, Zero and FW190. By about 600feet per minute at 20000'. The ability to get above your opponent trumps speed since you are diving on them. Range with 120 gallons internal was about the same as a Thunderbolt, F4U or F6F under the same conditions and better than FW190. P-39 firepower was at least adequate.

In 1942-43, the non-turbo V-1710 was two years behind curve vs. Merlin, BMW 801 and DB 601/605; same vs. R-2800. Compared with US ww2 that lasted less than 4 years, half of that is huge. Even in 1941, the V-1710 was well behind the curve vs. Merlin or DB 601.

Comparing the Allison with the Merlin, BMW801 or DB601 didn't really matter. What was important was the performance of the planes that were powered by those engines. 1942 P-39s were grossly overweight (7650lbs vs 7100lbs) but that could have been easily corrected by removing excess useless armor, .30cal wing guns and in some cases IFF radios. Resulting performance was at least the equal of the Spitfire V and 109F.

Two stage Allison -47 was running in the P-39E in April 1942. Problem was the extensively revised E model weighed 8900lbs. Had six .50cal MGs AND a 37mm cannon. And didn't have a 4 blade propeller. Put the -47 into a regular P-39, add a 4 blade propeller and you had a 7900lb rocket.

 

[Ivan1GFP]

How long will it take to design a new wing and get it into production? A year? P-39 wing was just fine. COG was adequate. Remove the useless equipment to increase the climb/ceiling. Could have been done in 2-3 hours at any combat base. Roll was adequate and the standard P-39 would out turn any plane not made in Japan or named Spitfire.

Hello P-39 Expert,

However long it would have taken to design a new wing, this was a basic design flaw in the P-39 that was a serious limitation. The symmetrical airfoil had a pretty low coefficient of lift relative to more modern airfoils.
The wing retained good lateral control because the tips did not stall with the rest of the wing, but the rest of the wing tended to stall all at once. (from NACA Report)
If the aircraft had any directional misalignment during the stall, the stall was asymmetrical and the aircraft would flip.
This was described in NACA testing.
Even with equipment shifts to attempt CoG corrections, the problem appears to be a significant rear weight bias and there are not that many places equipment can be moved with such a small airframe.
The CoG may have been fine with the aircraft loaded but was not so fine when loads were expended. Both conditions need to be addressed.

As for taking a year to design a replacement wing, that is a pretty sad commentary. The Soviets managed to do this kind of a thing much quicker.

Roll at a MAXIMUM of 80 degrees per second was hardly adequate. Whether or not a new wing is involved, this should have been improved.

As for turn rates, the Hurricane typically also turns better than the Spitfire.....
I suspect some of the Soviet fighters did better as well.

- Ivan.

 

[P-39 Expert]

Hello P-39 Expert,

However long it would have taken to design a new wing, this was a basic design flaw in the P-39 that was a serious limitation. The symmetrical airfoil had a pretty low coefficient of lift relative to more modern airfoils.
The wing retained good lateral control because the tips did not stall with the rest of the wing, but the rest of the wing tended to stall all at once. (from NACA Report)
If the aircraft had any directional misalignment during the stall, the stall was asymmetrical and the aircraft would flip.
This was described in NACA testing.
Even with equipment shifts to attempt CoG corrections, the problem appears to be a significant rear weight bias and there are not that many places equipment can be moved with such a small airframe.
The CoG may have been fine with the aircraft loaded but was not so fine when loads were expended. Both conditions need to be addressed.

As for taking a year to design a replacement wing, that is a pretty sad commentary. The Soviets managed to do this kind of a thing much quicker.

Roll at a MAXIMUM of 80 degrees per second was hardly adequate. Whether or not a new wing is involved, this should have been improved.

As for turn rates, the Hurricane typically also turns better than the Spitfire.....
I suspect some of the Soviet fighters did better as well.

- Ivan.

Pretty much every source says the P-39 had good stall characteristics. No plane will fly with a significant rear weight bias. Expending the nose ammo had no effect in normal flight maneuvers or anytime during landing.

 

[tomo pauk]

Beg to disagree. At 7100lbs the 1942 P-39 outclimbs the SpitfireV and 109F at all altitudes. Check the P-39C in Mike Williams' site but use 3000rpm for climb. I'm aware that the P-39C was not combat capable but it did weigh 7100lbs and had the same engine, propeller and drag as a P-39D/F at 7100lbs.

The 1943 P-39N would do over 380mph at 20000' but more importantly it would substantially outclimb the P-38F/G, F4U, F6F, Thunderbolt, Zero and FW190. By about 600feet per minute at 20000'. The ability to get above your opponent trumps speed since you are diving on them. Range with 120 gallons internal was about the same as a Thunderbolt, F4U or F6F under the same conditions and better than FW190. P-39 firepower was at least adequate.


Comparing the Allison with the Merlin, BMW801 or DB601 didn't really matter. What was important was the performance of the planes that were powered by those engines. 1942 P-39s were grossly overweight (7650lbs vs 7100lbs) but that could have been easily corrected by removing excess useless armor, .30cal wing guns and in some cases IFF radios. Resulting performance was at least the equal of the Spitfire V and 109F.

Two stage Allison -47 was running in the P-39E in April 1942. Problem was the extensively revised E model weighed 8900lbs. Had six .50cal MGs AND a 37mm cannon. And didn't have a 4 blade propeller. Put the -47 into a regular P-39, add a 4 blade propeller and you had a 7900lb rocket.

I'm not sure there is point anymore to discuss P-39 with you here.

 

[P-39 Expert]

I'm not sure there is point anymore to discuss P-39 with you here.

You don't believe my numbers?

 

[fubar57]

Wiki but still...

Soon after entering service, pilots began to report that "during flights of the P-39 in certain maneuvers, it tumbled end over end." Most of these events happened after the aircraft was stalled in a nose high attitude with considerable power applied. Concerned, Bell initiated a test program. Bell pilots made 86 separate efforts to reproduce the reported tumbling characteristics. In no case were they able to tumble the aircraft. In his autobiography veteran test and airshow pilot R.A. "Bob" Hoover provides an account of tumbling a P-39. He goes on to say that in hindsight, he was actually performing a Lomcovak, a now-common airshow maneuver, which he was also able to do in a Curtiss P-40.[36] [N 6] An informal study of the P-39's spinning characteristics was conducted in the NASA Langley Research Center 20-foot Free-Spinning Tunnel during the 1970s. A study of old reports showed that during earlier spin testing in the facility, the aircraft had never tumbled. However, it was noted that all testing had been done with a simulated full ammunition load, which drew the aircraft's center of gravity forward. After finding the original spin test model of the P-39 in storage, the new study first replicated the earlier testing, with consistent results. Then, the model was re-ballasted to simulate a condition of no ammunition load, which moved the aircraft's center of gravity aft. Under these conditions, the model was found to often tumble when thrown into the tunnel

As soon as I can find it again, an article mentions that Soviet pilots showed an unbelieving U.S. official how the -39 tumbles after expending forward ammo

 

[fubar57]

From this site...The P-39 Airacobra - Warfare History Network

".....At the same time, the plane was considered underpowered with its 1,200 horsepower Allison V-1710 engine, although it could do 376 miles per hour at 15,000 feet. Also, it lacked a supercharger that limited its effectiveness above 17,000 feet. Worse, the P-39 had a reputation for tumbling out of control when operated by inexperienced pilots....

 

[tomo pauk]

From this site...The P-39 Airacobra - Warfare History Network

".....At the same time, the plane was considered underpowered with its 1,200 horsepower Allison V-1710 engine, although it could do 376 miles per hour at 15,000 feet. Also, it lacked a supercharger that limited its effectiveness above 17,000 feet. Worse, the P-39 had a reputation for tumbling out of control when operated by inexperienced pilots....

P-39 was always powered by a supercharged engine.

 

[Peter Gunn]

Wish I could remember the book/author but bear with me. When my oldest son was in the summer symphony program (violin) I used to walk across the street to the Sarasota library. They had an "old" section that was massive to say the least, you could spend days in there. Regardless, I found a book written I believe in the 1950's by a German test pilot. It was very interesting to get his take on "our" aircraft.

I remember him not overly liking the B-24 but was astounded by the performance of the P-51B with what he considered a rather low powered Merlin. Not sure why he used that term but I believe he had also just flown a captured Thunderbolt, so perhaps that had some bearing on his judgement. He couldn't believe the speed he got out of the Mustang.

When he mentions the P-39 well, he was impressed with the tricycle landing gear and I think he did compliment it on being an airplane he didn't mind flying but as a combat plane he pretty much gave it a zero. I believe he had stability problems, probably stemming from the lack of ammo/ballast issues discussed above, and was decidedly NOT impressed with the speed or rate of climb. He did say, if memory serves, that it was doubtful he was getting the full performance out of the plane but even making allowances for that he still wasn't to high on it as a combat aircraft.

With that last sentence in mind, I believe one should be careful about comparing ANY allied aircraft with the Aleutian A6M2.

 

[Ivan1GFP]

Pretty much every source says the P-39 had good stall characteristics. No plane will fly with a significant rear weight bias. Expending the nose ammo had no effect in normal flight maneuvers or anytime during landing.

Hello P-39 Expert,

I didn't say the P-39 had particularly bad stall characteristics. You really need to read NACA Report L602.
What I stated was that the maximum coefficient of lift was lower than for the more common 23000 series airfoil as used in the wing tips. Stalls came without warning and were abrupt.
As for "No plane will fly with a significant rear weight bias.", that statement is not entirely true but a detailed discussion is way beyond the scope of this thread.
Consider though that if a disposable load in the nose of the plane can compensate enough for the rear weight bias, then the aeroplane should behave normally.
When that disposable load is expended, things may get ugly.

Your statement that expending the nose ammunition had no effect in normal flight maneuvers is directly contradicted by many pilot accounts.

- Ivan.

 

[Ivan1GFP]

Wish I could remember the book/author but bear with me. When my oldest son was in the summer symphony program (violin) I used to walk across the street to the Sarasota library. They had an "old" section that was massive to say the least, you could spend days in there. Regardless, I found a book written I believe in the 1950's by a German test pilot. It was very interesting to get his take on "our" aircraft.

Hello Peter Gunn,

Could this be the book you are remembering?
It might not be the same edition or cover.

- Ivan.