XP-39 II - The Groundhog Day Thread

[windhund116]

Hey guys,

Thanks for the reply. The Japanese characters 零戦 do seem to mean 'zero fighter' via Google translate.

Yeah, contraction of Kanji (Chinese) characters.

 

[ThomasP]

Hey windhund116,

Thanks for the link.

 

[Ivan1GFP]

Hello P-39 Expert....

Why do I get the feeling I have been here before?

Attached please find a performance graph for the P-39K which had the same 8.8 supercharger gears as the P-39D. Also attached is a P-39D performance test. The numbers are virtually the same.

The test report you gave on the P-39D is actually quite interesting. You should actually read it for content instead of just picking the maximum speed figures out of it.
Please note the following:
These people were tweaking manual mixture controls to get the P-39D up to 368 MPH. This is not something that is done in combat. With Auto Rich, it was doing 365 MPH which is still pretty good though.
From other test reports (358 MPH), it is pretty obvious that maximum speed varied a bit as one might expect for production aircraft.

As you can see from the graph the top speed of the K model was 370mph up to 16000'. The A6M2 is commonly listed at 330mph at 15000' with speed falling off at higher altitudes at the same rate as the K. That's a 40mph difference at all altitudes.

If you are still going with the "commonly listed" number for the A6M2, you obviously are forgetting the IIS 85 test report you claim to have read or don't understand it.
If a crashed and repaired A6M2 can achieve 335 MPH in tests (corrected down to 332 MPH for dissemination) while NOT running War Emergency Power, then 330 MPH for a fighter in good condition in Japanese service is unlikely.

Here is a pretty good discussion along with anecdotes from combat encounters that supports a substantially higher number.
I just went through the article again and am more inclined to believe Mr. Dunn's conclusion is correct.
A6M2 Performance

Please note that the manifold pressure conversions are not quite correct. I contacted him years ago about this issue and he acknowledged the issue but never corrected the article.

The climb numbers on the chart and the report show that power was reduced from combat power (3000rpm) to normal power (max. continuous) when the 5 minute limit was reached. This limit was increased to 15 minutes in mid 1942, just after these tests. The red dots represent rate of climb at combat power 3000rpm. This closely approximates the A6M2 climb rate.

The IIS 85 report was based on testing after mid 1942 and shows P-39D climb rate as distinctly slower past about 12,500 feet or so.

- Ivan.

 

[Ivan1GFP]

Pardon me, I'm comparing the 1942 P-39s and A6M2. The A6M5 was later and should be compared to the P-39N of late 1942 early 1943.

If you don't like comparing climb rates at 15,000 feet between P-39D and A6M5, then you will probably like a comparison between P-39D and A6M2 even less.
15,000 feet is well below the critical altitude for the A6M5 but it is very close to the 4200 Meter critical altitude for the A6M2 and the A6M2 is quite a bit lighter.

 

[Stig1207]

Try reading P-39/P-400 vs A6M3 Zero New Guinea 1942 by Michael John Claringbould. It has some good combat accounts, some of which will say the P-39 and A6M were close in speed, some were the P-39 was faster, likely due to acceleration being close for the two aircraft. But the main reason to read the book is the in-depth look at combat records; the P-39 and A6M were even in the win-loss stats. That surprised me.

Surprised me too, not least the way the combat records are interpreted; 44 P-39's were lost in engagements with Zero's, of which 15 also were lost. All 15 Zeros were shot down by P-39's, and 15 P-39's were shot down by Zeros. Some P-39's were lost to AAA, running out of fuel, mechanical defects, etc; but in many cases no one actually saw what happened and there were some MIA with no probable cause given.

Perhaps it would be more right to say that it cost 44 P-39s too shoot down 15 Zeros.

 

[P-39 Expert]

Kanji 零戦 means Zero Fighter. "Reisen" is a contraction of Reishi Kikanjo Sentouki.

Reishi Kikanjo Sentouki

Shows the top speed of an A6M2 as 330mph at 15500'.

 

[P-39 Expert]

So your numbers mean nothing then, just fluff.

How do you account for the additional 1000fpm climb? The only difference in the two planes was their weight. Same engine, propeller and airframe. Same HP at same altitude. One weighed more because it had different internal components (self sealing tanks, armor plate and glass, more .30cal MGs.) None of those internal components had any effect on performance other than their weight.

 

[P-39 Expert]

Hello P-39 Expert....

Why do I get the feeling I have been here before?



The test report you gave on the P-39D is actually quite interesting. You should actually read it for content instead of just picking the maximum speed figures out of it.
Please note the following:
These people were tweaking manual mixture controls to get the P-39D up to 368 MPH. This is not something that is done in combat. With Auto Rich, it was doing 365 MPH which is still pretty good though. Just the pilot slightly varying the mixture control to get the best power based on feel and sound. Why wouldn't that be done in combat?
From other test reports (358 MPH), it is pretty obvious that maximum speed varied a bit as one might expect for production aircraft.




If you are still going with the "commonly listed" number for the A6M2, you obviously are forgetting the IIS 85 test report you claim to have read or don't understand it.
If a crashed and repaired A6M2 can achieve 335 MPH in tests (corrected down to 332 MPH for dissemination) while NOT running War Emergency Power, then 330 MPH for a fighter in good condition in Japanese service is unlikely. Highest speed listed in that report is 321mph at 18000'.

Here is a pretty good discussion along with anecdotes from combat encounters that supports a substantially higher number.
I just went through the article again and am more inclined to believe Mr. Dunn's conclusion is correct. See Windhund's post #1020, 330mph at 15500'.
A6M2 Performance

Please note that the manifold pressure conversions are not quite correct. I contacted him years ago about this issue and he acknowledged the issue but never corrected the article.



The IIS 85 report was based on testing after mid 1942 and shows P-39D climb rate as distinctly slower past about 12,500 feet or so. That was the D-1 model that weighed 7850lbs. Could have easily weighed 7250lbs after removing redundant or unnecessary equipment. Read the chart in my post #1006 and compare those climb numbers to IIS 85 test. That P-39 weighed 7650lbs.

- Ivan.

Please expand above.

 

[XBe02Drvr]

P39 Expert said:
Just the pilot slightly varying the mixture control to get the best power based on feel and sound. Why wouldn't that be done in combat?

Spoken like a true ground pounder, devoid of flight experience or knowledge thereof!! Getting accurate, safe (you don't want to burn up YOUR ONLY engine over the sea or the Owen Stanley mountains) lean mixture settings requires stabilized steady state flight and engine temperatures settled down from any climb or maneuvering demands, and time to tweak and analyze it. Fine for flight testing or long distance cruising; ain't happ'nin in combat, where you're already suffering from a situational awareness overload.
Best power comes just on the near side of the onset of detonation, and detonation under the demands of combat maneuvering will result in engine damage or destruction. Auto rich is your safety margin if you're going to be hammering on your engine. The risk is not worth the few (3??) extra MPH. Today's planes have both Cylinder Head Temperature and Exhaust Gas Temperature gauges, making it easier and safer to tweak mixture at altitude. Back then, they weren't equipped with the critical component, the EGT.

 

[XBe02Drvr]

How do you account for the additional 1000fpm climb?

Quite likely partly due to differences in test conditions buried in the small print. Achieving identical test conditions (or correction factors to compensate if not achieved) is more involved than it may seem to the uninitiated.
Who actually conducted each test? Where and when? Corrected to standard atmosphere values? A P39C prepped at the Bell plant and tested in wintertime in Niagara will certainly put up better performance differential numbers vs a P39D in combat trim randomly selected from the flight line at Langley in summertime than the actual difference in the aircrafts warrants. Larry Bell was not above "detailing" a plane about to be tested to maximize performance. How do you KNOW such shenanigans didn't happen?

 

[glennasher]

That depends. Is that Vespa from 1942? When the war had only been going for a year?

It was a postwar Vespa, made from the scrap metal of the P-39s crashed by the Italians after the USAAF gave them some.

 

[P-39 Expert]

Quite likely partly due to differences in test conditions buried in the small print. Achieving identical test conditions (or correction factors to compensate if not achieved) is more involved than it may seem to the uninitiated.
Who actually conducted each test? Where and when? Corrected to standard atmosphere values? A P39C prepped at the Bell plant and tested in wintertime in Niagara will certainly put up better performance differential numbers vs a P39D in combat trim randomly selected from the flight line at Langley in summertime than the actual difference in the aircrafts warrants. Larry Bell was not above "detailing" a plane about to be tested to maximize performance. How do you KNOW such shenanigans didn't happen?

Both were official performance tests conducted at Wright Field by the AAF on brand new airplanes. Larry Bell was back in Buffalo. No "shenanigans" would account for a 1000fpm difference in climb rate. One was 836lbs lighter than the other.

 

[wuzak]

How do you account for the additional 1000fpm climb? The only difference in the two planes was their weight. Same engine, propeller and airframe. Same HP at same altitude. One weighed more because it had different internal components (self sealing tanks, armor plate and glass, more .30cal MGs.) None of those internal components had any effect on performance other than their weight.

"Airplane equipped with Allison V-1710-35 engine and 3-bladed constant speed propeller, blade design No. 614CC1.5-21, blades are not equipped with cuffs."

"Oil and Prestone temperatures do not meet Air Corps requirements in climb"

http://www.wwiiaircraftperformance.org/P-39/P-39C_40-2988.pdf


The test on the P-39D did not evaluate cooling, and so whether it met the required standards was unknown.

"Airplane equipped with Allison V-1710-35 engine and 3-bladed constant speed propeller, blade design No. 614-1C1.5-21."

"Individual port backfire screens not installed in engine."

http://www.wwiiaircraftperformance.org/P-39/P-39D_41-6722_PHQ-M-19-1325-A.pdf


But, by all means remove self sealing tanks, armour plating, armoured screen, ditch the radios and IFF, lose the .30" mgs. Then you will have an aircraft with 40 extra gallons of fuel and that extra 1,000fpm climb. It should be able to take the Luftwaffe on over Berlin!

 

[wuzak]

Also, for the P-39C:

"The [cooling] flaps are spring loaded and are therefore not positive in action; with the controls in the wide open position the flaps are pushed up towards the flush position by the force of the air stream."

That was referring to the level speed runs, not sure if it applied to climb conditions too - maybe the springs were good enough for climb speeds.

 

[P-39 Expert]

"Airplane equipped with Allison V-1710-35 engine and 3-bladed constant speed propeller, blade design No. 614CC1.5-21, blades are not equipped with cuffs."

"Oil and Prestone temperatures do not meet Air Corps requirements in climb"

http://www.wwiiaircraftperformance.org/P-39/P-39C_40-2988.pdf


The test on the P-39D did not evaluate cooling, and so whether it met the required standards was unknown.

"Airplane equipped with Allison V-1710-35 engine and 3-bladed constant speed propeller, blade design No. 614-1C1.5-21."

"Individual port backfire screens not installed in engine."

http://www.wwiiaircraftperformance.org/P-39/P-39D_41-6722_PHQ-M-19-1325-A.pdf


But, by all means remove self sealing tanks, armour plating, armoured screen, ditch the radios and IFF, lose the .30" mgs. Then you will have an aircraft with 40 extra gallons of fuel and that extra 1,000fpm climb. It should be able to take the Luftwaffe on over Berlin!

Same propeller. Cooling was a problem with most AAF fighters. Backfire screens were eliminated in mid 1942 on Allison engines without turbochargers, and in September on turbocharged P-38s. If the backfire screens were installed in the P-39C then performance would have improved with their removal. Even better climb.

I've never advocated removing the self sealing tanks. Only remove the nose armor, no other planes had the reduction gear armored. Never advocated removing the voice radio. C and D models didn't have the IFF radio anyway. And yes, definitely lose the .30cal MGs on the early D/F/K/L. The 37mm cannon and two .50cal MGs were plenty of armament.

The P-39N available from late 1942 had the extra 1000fpm climb even with the items I've advocated for removal. It did successfully take on the Luftwaffe. But removal of the .30s and substituting additional fuel tanks would have greatly improved endurance.

 

[Peter Gunn]

300lbs lighter at 1.2fpm per pound would mean adding 360fpm to the values on the chart in my post #1006.

While an interesting and compelling argument, I think I read somewhere that it's not quite that easy.

 

[Milosh]

Same propeller.

A different 'dash' number means there was a difference in the part.
614-CC1.5-21
614-1C1.5-21

 

[DerAdlerIstGelandet]

How do you account for the additional 1000fpm climb? The only difference in the two planes was their weight. Same engine, propeller and airframe. Same HP at same altitude. One weighed more because it had different internal components (self sealing tanks, armor plate and glass, more .30cal MGs.) None of those internal components had any effect on performance other than their weight.

Where are you including atmospheric conditions in your calculations? What are the test conditions?

 

[buffnut453]

Oh good grief...this really is the Groundhog Day thread.

After 52 pages, we're still hitting the reset button and going back to the same discussions we were having right at the start.

I think it's time to take a jump to the left...

 

[DerAdlerIstGelandet]