XP-39 II - The Groundhog Day Thread

[GrauGeist]

With those bombers boxed in so tight and everyone shooting at the same enemy fighter, how many gunners claimed the same aircraft?

 

[Stig1207]

I have never heard that argument. I thought bomber crew attrition rate, especially 8AF, were high --- when not escorted.

You are correct, but you have misunderstood what I wrote.

 

[Stig1207]

With those bombers boxed in so tight and everyone shooting at the same enemy fighter, how many gunners claimed the same aircraft?

Yes, we all know that the bomber gunners overclaimed, that is an undisputed and accepted fact; but let's ignore that for the moment.

In 1943 the 8th AAF bomber gunners claimed 3253 ea destroyed, according to Caldwell's Day Fighters in Defence of the Reich, and this is when the big raids numbered 200-300 bombers. On the basis of these we could conclude that, if we accepted those claims as factual, that the daylight bomber campaign was succeeding and wearing down the enemy fighter defences rapidly. So instread of fighter escorts we just need more bombers to fly in bigger formations and shooting down even more enemy fighters and at the same time reducing own losses as the enemy is weakened.

Naturally, knowing what really happened the fallacy of above argument is clear, as the Germans weren't losing near as many fighters to the bomber gunners.

Back to the top scoring Soviet aces flying the P-39, how many Axis aircraft did they actually shoot down? After all, the arrival of the P-39 didn't change the overall exchange rate which favoured the Luftwaffe. We can conclude that the Soviet P-39 pilots in all likelihood did better than their American counterparts, we can perhaps also say that they comparatively did better than Soviet pilots flying other fighter types, but imo, we need more concrete numbers of enemy losses to P-39's on the Eastern Front to properly evaluate it's combat performance.

 

[P-39 Expert]

Hello P-39 Expert,



This really isn't saying very much. The Thunderbolt didn't climb particularly fast at any altitude.

1943 Thunderbolt climbed about the same at 20000' as the Corsair, Hellcat, Fw190 and Zero. Slower below 20000' but better above because of turbocharging. At 25000' Thunderbolt climb was pretty good.



Above the Airacobra's critical altitude generally wasn't where the battles were happening.

In any air battle the key was getting above your opponent. Normal combat formation for the Soviets was the "flying bookshelves" or "Kuban stairs" with four planes at 5000meters (16500') four more at 6000meters (20000') and the last four at 7000meters (23000') angled up toward the sun.



Do you have evidence that the P-39D-1 used in the test against A6M2 actually was loaded to 7850 pounds?
They didn't actually back off to 44 inches Hg. In one test, they backed off to 52 inches Hg. In another test they ran at 55 inches Hg. These are War Emergency settings or above for the D-1. The A6M2 also had an "Overboost" setting but that was never used.
One of the things that was discovered in these tests was that the A6M2 really wasn't a particularly fast climbing aeroplane. It just had a very good zoom climb and climbed at a very steep angle. Another problem was that the automatic mixture control apparently didn't work in the rebuilt carburetor. The carb originally was designed to prevent negative G flooding but that feature was not restored on this captured A6M2 either.
Note also that the P-39D-1 could not continue these tests past 25,000 feet because it was low on fuel (!)
That DOES say something about lack of range, doesn't it?

P-39D-1 Gross weight was 7850#. If it carried 300rounds per gun (instead of 1000) for the .30calMGs then it grossed 7650lbs. Still way too much.

I don't believe war emergency power was available quite yet if this test was conducted in October 1942. WEP was first available for the Allison in December.


The single 20 mm cannon and 2 MG does provide greater firepower but only for 6 seconds. After that, you have only two synchronized .50 cal HMG with a rather low ammunition load. One of the main advantages of the 4 gun Wildcats versus 6 gun versions was that the ammunition load and duration of fire was much greater.
For the 37 mm, it would be 12 seconds, but there you also have a cannon whose trajectory isn't remotely close to the .50 cal.
As for "newer" fighters, most of them went to MORE .50 cal guns. With the exception of the P-38, just about everything else used a uniform battery of guns. As for the F8F, although the gun count was lower, the gun used was the M3 which had a higher rate of fire.

The 20mm cannon did need a bigger ammo magazine. 6 seconds was too short, but about equal to that Zero's 20mm ammunition. Contemporary Spitfire V used a 120round belt which gave 12 seconds firing time. P-38 cannon held 150 rounds for a 15 second firing time. 12 seconds for the 37mm was somewhat comparable. .50cal firing time on the P-39 was about 25 seconds.

Trajectory for the 37mm cannon was actually flat out to 400 yards, which was about the limit for accuracy for any WWII gun. Beyond that distance just meant that the .50calMGs wouldn't be used at the same time.

Another interesting note regarding acceleration tests against Koga's A6M2 was that the majority of the tests were contrived to show that the US fighters had greater acceleration. This was done by picking a "cruising speed" to start that was usually only about 30-40 MPH below A6M2's maximum level speed. Aircraft don't accelerate quickly when they are near their maximum speed.
I wonder how these drag races would have gone had they started at 120-150 MPH?

- Ivan.

 

[Mad Dog]

.....Just when and where did the NA engineers study this captured 109?
The French had a captured one in the spring of 1940......

The story goes they studied one from the Spanish Civil War, though whether it was a B, C or D model is unclear. There were early Es in Spain in 1939 but I don't think any of those fell into French hands.

 

[FLYBOYJ]

The story goes they studied one from the Spanish Civil War, though whether it was a B, C or D model is unclear. There were early Es in Spain in 1939 but I don't think any of those fell into French hands.

AFAIK the only -109 captured during the Spanish Civil war was a Bf109 B1. It fell into Republican hands January 1938 and was evaluated by the Soviets and possibly the French. This was documented in the Book "L'Aviation Republicaine Espagnole" by Patrick Laureau. There are several photos of this aircraft in that book.

 

[Mad Dog]

Thanks, Tomo!

Here's the original XP-39 in 2 configurations, both as the XP-39 with turbocharger (right) and as the "modified" XP-39B.

View attachment 595814

In the top row, I have it mis-labeled. The pic on the right is the XP-39 and the pic on the left is the same airplane without the turbocharger, but still with the original canopy. All protrusions have been removed, including the exhaust stacks.

In the second row, this is the same airplane, but you can see the canopy has been lowered and streamlined a bit. In the second row on the right, I'm not too sure, but it looks like the same airplane with dummy exhaust stack (possibly no engine yet). The stacks look kind of "flat" to me.

Nice pics! I always liked shiny and polished prototypes in bare aluminium.

 

[Mad Dog]

......However, I can think of no other fighter in WW II that was modified to such an extent for an acceptance test, there may have been some?

All Bell did was game the acceptance criteria set by the BPC. If anyone thinks that doesn't happen every day, even today, then Shortround has a bridge he keeps going on about wanting to sell.

 

[tomo pauk]

The story goes they studied one from the Spanish Civil War, though whether it was a B, C or D model is unclear. There were early Es in Spain in 1939 but I don't think any of those fell into French hands.

Ah, the 'story goes' source. Those are the best.

All Bell did was game the acceptance criteria set by the BPC. If anyone thinks that doesn't happen every day, even today, then Shortround has a bridge he keeps going on about wanting to sell.

He does?

 

[glennasher]

I am inclined to believe that the reason the USSR used so many P-39s was because the USAAF didn't want them, and they were sent to the USSR because, while we wanted to help them, we didn't want to help them "too much". Remember that until 1941, the USSR and Germany had that pesky non-aggression pact...............................Yes, we sent them aircraft, but they were not first-line aircraft by any means. I cannot be persuaded that the P-39 was anything but motorized crap.

 

[GregP]

Comparative tests show that below about 16,000 feet, the P-39 was a formidable airplane.

 

[Mad Dog]

I am inclined to believe that the reason the USSR used so many P-39s was because the USAAF didn't want them, and they were sent to the USSR because, while we wanted to help them, we didn't want to help them "too much". Remember that until 1941, the USSR and Germany had that pesky non-aggression pact...............................Yes, we sent them aircraft, but they were not first-line aircraft by any means.....

True, we didn't send them Spitfires until later, but then we (stupidly) kept Spitfires only in the UK until 1942, sending old MkIs and MkIIs to training units and sending Hurricane MkIs to the Desert. We didn't send anything to the Soviets until after June 1941, by which time we had already decided to send Tomahawks to the Desert. When we did send Spitfires (Vs and IXs) to the Soviets, they said the P-39 was a better fit for their frontline units. And after Pearl Harbor, the worried Yanks grabbed everything they could from foreign orders, including P-400s and P-66s (<- yeah, that desperate!).

......I cannot be persuaded that the P-39 was anything but motorized crap.

The P-39 didn't work for the RAF in 1941 because the aircraft still needed development, didn't have the altitude performance demanded of the theatre, and because of Spitfire snobbery. It did (kind of) work for the USAAC in New Guinea, but didn't have the range required to be a real success (unlike the P-38F), and still had issues with altitude. But in Soviets' hands, the P-39 was a big success, because their theatre requirements were different to the West or Pacific, and their own fighter aircraft were - frankly - crap. When you consider that the preferred fighter of the Soviet pilots before the arrival of the P-39 was the I-16, you suddenly understand why they didn't ignore the P-39.

 

[wuzak]

Trajectory for the 37mm cannon was actually flat out to 400 yards, which was about the limit for accuracy for any WWII gun. Beyond that distance just meant that the .50calMGs wouldn't be used at the same time.

How does that work?

The 37mm had a significantly lower muzzle velocity than the 0.50" Browning HMG, so if you're hitting a target at 400 yards with the 0.50" HMGs then you would, most likely, be missing it with the 37mm. Not accounting for the trajectory difference, which was enough that the XP-54 had a mechanism and a targeting system designed to compensate.

 

[fubar57]

View attachment 596594​

 

[fubar57]

 

[GrauGeist]

And after Pearl Harbor, the worried Yanks grabbed everything they could from foreign orders, including P-400s and P-66s (<- yeah, that desperate!).

Because the week following 7 December '41, the U.S. was suddenly in a world war with Japan and Germany declaring war.
The U.S. military was still constrained by a cash-strapped budget due to the depression and needed everything they could get their hands on.
Much like Britain in 1940, the U.S. had no idea if there would be an invasion or not and had to build up their aircraft numbers asap.
The 50 P-66s retained by the USAAF were used to bolster numbers at key locations on the west coast until newer types could replace them.
The week following Pearl Harbor, a half-dozen Northrup A-17s arrived at the airfield by where I live (Benton - O85) and remained on patrol duty until they were replaced the following Spring.

 

[Shortround6]

In 1941 the US was not cash strapped. FDR had called for the 50,000 plane air force in the summer of 1940 and the funding to go with it.
However it takes time to turn that cash into actual factories that are producing much of anything.

Ford, Studebaker and Buick were all brought into the production plans in 1940 in addition to Packard. This was in addition to large expansions if P&W and Wright and Allison leaning heavily on GM for subcontracting, management and engineering help. Engine production just about tripled from 1940 to 1941 and more than tripled in 1942 over 1941. But the planning/factory building had started in 1940.

 

[Ivan1GFP]

Hello P-39 Expert,

1943 Thunderbolt climbed about the same at 20000' as the Corsair, Hellcat, Fw190 and Zero. Slower below 20000' but better above because of turbocharging. At 25000' Thunderbolt climb was pretty good.

You keep referring to the "1943 Thunderbolt". What particular model of the Thunderbolt is that?
As for comparisons to the FW 190, I presume you mean the FW 190A which was not noted for great performance at altitude. As for the Zero, if this is the same A6M2 as was tested against the P-39D-1, please note that ITS critical altitude is 13,800 feet or just a bit over a thousand feet higher than the P-39D. It had a pretty good ceiling but not that much altitude performance.

In any air battle the key was getting above your opponent. Normal combat formation for the Soviets was the "flying bookshelves" or "Kuban stairs" with four planes at 5000meters (16500') four more at 6000meters (20000') and the last four at 7000meters (23000') angled up toward the sun.

This is the ideal case when you have the tactical situation, time and FUEL to implement.
Now think about what happens when the fight starts? Where is it going to be?
Also think about what happened with the P-39D-1 in test against A6M2. Testing could not be conducted above 20,000 feet because the P-39D (with a fuel capacity of 120 gallons) was running out of fuel. (!)
Now think about what the Soviets requested with late model Airacobra; They asked that the fuel capacity be reduced to 87 gallons.
It sounds to me like they had entirely different tactics in mind.

P-39D-1 Gross weight was 7850#. If it carried 300rounds per gun (instead of 1000) for the .30calMGs then it grossed 7650lbs. Still way too much.

I don't believe war emergency power was available quite yet if this test was conducted in October 1942. WEP was first available for the Allison in December.

From the test against A6M2, I do not believe the folks setting up the P-39D-1 had any intention of making this a representative performance comparison. I don't believe 70 inches Hg was ever an approved Emergency Power setting for the P-39D. That factor was actually documented in the testing.
If someone is willing to pull tricks like that, there are plenty of other things that can be done to make the P-39D perform better than it would under combat conditions.

Trajectory for the 37mm cannon was actually flat out to 400 yards, which was about the limit for accuracy for any WWII gun. Beyond that distance just meant that the .50calMGs wouldn't be used at the same time.

This is not how projectiles fly.
If you play some games with a fairly large target and the concept of "Point Blank Range", you can get all the guns to line up to a fairly reasonable distance.
The problem with this idea is that it only works against a non-maneuvering target. As soon as there is any significant amount of deflection added to the shot, the difference in trajectory becomes pretty important.

By the way, I have already commented more than a few times that I believe the cannons on the early A6M did not have an adequate ammunition supply.

- Ivan.

 

[Ivan1GFP]

Hello All,

Attached is the text of the test between P-39D-1 and A6M2 that was captured in the Aleutians and repaired.

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.

 

[P-39 Expert]

How does that work?

The 37mm had a significantly lower muzzle velocity than the 0.50" Browning HMG, so if you're hitting a target at 400 yards with the 0.50" HMGs then you would, most likely, be missing it with the 37mm. Not accounting for the trajectory difference, which was enough that the XP-54 had a mechanism and a targeting system designed to compensate.

The 37mm trajectory was about the same as the .50calMGs out to 400 yards. The difference in trajectory was only 23 inches at 400 yards. Past 400 yards the drop in trajectory was greater. 400 yards was about the outside limit for accuracy of airborne guns. Most wing mounted machine guns were sighted to converge at about 300 yards.