Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained) (1 Viewer)

[Shortround6]

The P-400's were heavier because they had more armour and what did they do with them, they shot up some invasion barges. Maybe that's what we needed them for in the first place, but no invasion, no P-400's required.

They had maybe, repeat, maybe 30-40 pounds more armor than the P-39D did. Which strangely disappeared when one was weighed for US service? (AHT Dean Page 193)

Most new squadrons, regardless of what they intended to use them for tended to get sent on barge, coastal shipping or coastal target strikes for their first few missions. Over two years later the first P-51B mission with the 354th fighter group was a fighter sweep over Belgium and France. Meeting no German Aircraft they did some ground strafing.
It was a way to ease a unit into combat by only keeping them in a dangerous area for a few minutes.

By the summer of 1941 the British had little or no need of the P-39 as a barge buster. Let alone in Oct/Nov of 1941. The P-400 mounted a single 20mm with a drum feed gun for barge busting plus the two .50 cal guns. Spitfires were getting two 20mm cannon, the unloved Whirlwind had four. Beaufighters had four and they were starting to be belt feed. Four 20mm cannon for Hurricanes were in the works. (showing up the fall of 1941?).
Until the British actually got some in England to test to they didn't realize what a dog it was at altitude. There was no quick and easy fix. They were flying a P-40 with a Merlin engine
on June 30th 1941 about the time the P-39Cs show up in England, and even the first of these lightweight machines only manages 359mph around July 6th. You can't put a Merlin in a P-39 (not without months and months of work and there aren't enough Merlins anyway)

This is a big part of this particular argument about the P-39, who knew what and when. Bell knew all along that it wasn't a 400mph fighter at 20,000ft or even 390mph at 15,000ft.
They have hoped it was, they may prayed that some miracle would make it so. But they never had a test airplane in 1939-40 come close to those numbers (at least one even remotely equipped for service use.) We know that the American army didn't expect that speed and climb performance from it either or they wouldn't have signed/agreed to the contracts for the P39D with lower performance.
So the question is when did the British know, or again, who knew when ( British test pilot and purchasing delegation in the US, British air ministry officials in England, or British pilots/squadron commanders who would have to use it) The P-39 was being touted as a 400mph airplane in ads and articles in British publications like Flight magazine during 1940.
Granted you don't put full details (especially short comings) in a public magazine in war time but the average Briton interested in aviation in 1940 expected the P-39/P-400 to be a 400mph airplane.

 

[Shortround6]

Maybe they got better later but 400-500 RPM territory was where the British tested them.

 

[Ivan1GFP]

Until the British actually got some in England to test to they didn't realize what a dog it was at altitude. There was no quick and easy fix. They were flying a P-40 with a Merlin engine
on June 30th 1941 about the time the P-39Cs show up in England, and even the first of these lightweight machines only manages 359mph around July 6th. You can't put a Merlin in a P-39 (not without months and months of work and there aren't enough Merlins anyway)

Hello Shortround6,
I suspect the British had a pretty good idea what kind of altitude performance to expect from the Airacobra before they every got one. Note that although they may have ended up with a few P-40s with Merlin engines, they had been flying early Tomahawks and Kittyhawks before the Merlin versions came along. The engine in the P-39D, D-1, and F would have been almost the same as that in a P-40E.

In one of the Airacobra books (I forget which one), there was a mention of the British requirement for snow filters on the intake which caused a significant loss of performance. In addition, it was pretty common practice for US Army operators to remove the anti-backfire screens from their engines which gave a noticeable (about 1000 feet) increase in critical altitude.

- Ivan.

 

[P-39 Expert]

Hello Shortround6,
I suspect the British had a pretty good idea what kind of altitude performance to expect from the Airacobra before they every got one. Note that although they may have ended up with a few P-40s with Merlin engines, they had been flying early Tomahawks and Kittyhawks before the Merlin versions came along. The engine in the P-39D, D-1, and F would have been almost the same as that in a P-40E.

In one of the Airacobra books (I forget which one), there was a mention of the British requirement for snow filters on the intake which caused a significant loss of performance. In addition, it was pretty common practice for US Army operators to remove the anti-backfire screens from their engines which gave a noticeable (about 1000 feet) increase in critical altitude.

- Ivan.

The useless backfire screens were finally discarded around the same time as the five minute rating was increased to 15 minutes, mid 1942. The 15 minute rating was for most of the combat engines including Allison and P&W.

 

[P-39 Expert]

Except you don't.

Four .30 cal guns = 99lbs
1200 rounds of 30 cal ammo= 72lbs

50 gallons of fuel weighs 300lbs
Weight of the self sealing tanks to put the fuel in???????

Wonder what the Mustang could do if you magicly took out 180lbs and added 400lbs of "stuff" with no penalty?

For the thousandth time, delete the wing guns AND THE USELESS 100# NOSE ARMOR PLATE and substitute 50 gallons of fuel at 300#, the weight of the self sealing tanks would be mostly offset by removing the gun mounts, chargers, heaters and ammunition boxes. Very minor weight gain if any.

 

[P-39 Expert]

They had maybe, repeat, maybe 30-40 pounds more armor than the P-39D did. Which strangely disappeared when one was weighed for US service? (AHT Dean Page 193) Armor plate/glass weighed 292# on the P-400 including the nose gearbox armor, armor for the oxygen bottles and armor just outside the rear armor glass. Delete these items (not used on other comtrmporary aircraft) and the weight of armor plate and glass is reduced to around 140#.

Most new squadrons, regardless of what they intended to use them for tended to get sent on barge, coastal shipping or coastal target strikes for their first few missions. Over two years later the first P-51B mission with the 354th fighter group was a fighter sweep over Belgium and France. Meeting no German Aircraft they did some ground strafing.
It was a way to ease a unit into combat by only keeping them in a dangerous area for a few minutes.

By the summer of 1941 the British had little or no need of the P-39 as a barge buster. Let alone in Oct/Nov of 1941. The P-400 mounted a single 20mm with a drum feed gun for barge busting plus the two .50 cal guns. Spitfires were getting two 20mm cannon, the unloved Whirlwind had four. Beaufighters had four and they were starting to be belt feed. Four 20mm cannon for Hurricanes were in the works. (showing up the fall of 1941?). The 20mm cannon in the P-400 was a non-starter with only a 60 round magazine good for 5-6 seconds firing time. It also jammed a lot. Give it the British belt fed 120 round magazine like the SpitfireV (just the logical development) and it would be a very useful gun.
Until the British actually got some in England to test to they didn't realize what a dog it was at altitude. There was no quick and easy fix. They were flying a P-40 with a Merlin engine
on June 30th 1941 about the time the P-39Cs show up in England, and even the first of these lightweight machines only manages 359mph around July 6th. You can't put a Merlin in a P-39 (not without months and months of work and there aren't enough Merlins anyway) Due to rediculously heavy weight as specified by the British. Removing the weight was a very quick and easy fix. British are comparing their 6450# Spitfire with their 7850# P-400 creation and wondering why performance is down. They knew all along. Again, please see the graph in post #388.

This is a big part of this particular argument about the P-39, who knew what and when. Bell knew all along that it wasn't a 400mph fighter at 20,000ft or even 390mph at 15,000ft.
They have hoped it was, they may prayed that some miracle would make it so. But they never had a test airplane in 1939-40 come close to those numbers (at least one even remotely equipped for service use.) We know that the American army didn't expect that speed and climb performance from it either or they wouldn't have signed/agreed to the contracts for the P39D with lower performance.
So the question is when did the British know, or again, who knew when ( British test pilot and purchasing delegation in the US, British air ministry officials in England, or British pilots/squadron commanders who would have to use it) The P-39 was being touted as a 400mph airplane in ads and articles in British publications like Flight magazine during 1940.
Granted you don't put full details (especially short comings) in a public magazine in war time but the average Briton interested in aviation in 1940 expected the P-39/P-400 to be a 400mph airplane. Bell promoted the P-400 as a 400mph airplane in '39 and '40 when it weighed 5000#, not after that. There were six weight increases under this contract between Feb 1940 and June 1941 increasing the weight from 5849# to 7635# of which the British were certainly aware since this was a direct purchase contract and they had to sign off on all changes. The big test (reflecting the decreased performance) was in April '41 and the final revision to the contract increasing the order from 170 to 675 planes was signed in June. That the British were somehow tricked or fooled by Bell is rediculous. The British had the best intelligence service in WWII and had already contracted for and built a large quantity of very high performance airplanes themselves. They knew exactly what they were doing.

Please see above.

 

[Shortround6]

The Backfire screens were there in late 1940-41 because Allison had a considerable amount of trouble with backfires during this time period. Several P-40s were lost due to engine fires (in flight, not starting up) and others suffered damage to the intake manifolds and on certain occasions parts of the cowling were loosened up enough that the airstream tore them loose. The early engines had a lightweight magnesium intake manifold that would sometimes catchfire and/or rupture with a severe backfire.
These were replaced with a heavier, stronger aluminum manifold that went a long way towards solving some of the problems although a severe backfire could still split these.
Changing the intake valve clearance also helped considerably (they went from 0.010 to 0.015in.) this delayed the opening of the valve slightly (very slightly but it did make a measurable difference). The British ran their Tomahawks in the dessert with the 0.015in valve clearance.
As late as Sept/Oct 1941 the US Army materiel command was still issuing directives concerning this problem and Allison was still working on the problem.
Another thing that helped solve the problem was using sodium cooled intake valves (most engines used the sodium filled valves on the exhaust only).
On Sept 11th Wright Field ordered Allison to provide 50,400 new intake valves to retrofit the 2100 engines that had been delivered or were expected to be delivered up to Oct 1st 1941.
These valves were to be provided at no cost under the "latent defect" clause in the contract.

The British did not get the new valves for engines that they had already purchased, all new engines provided from this point forward under lend lease got the new valves.

Another problem with prolonged use was that the valve clearance would change. the Intake valves would pound their seats and drive them further into head shorting the valve seat to crocker arm distance. This was also thought to be caused by the engines occasional over revving due the Curtiss electric props. The E and F engines got steel inserts in the cylinder heads for valve seats to help cure this problem.
There was also a change in the type of spark plugs used.

A much longer description of this is in "Vee's for Victory" pages 114 to 118.

Nobody liked the backfire screens at the time, they knew the problems they were causing with manifold pressure (which got much worse when they got clogged with dye form the fuel) but until they KNEW they had the backfire problem under control they were viewed as a necessary evil.

Part of KNOWING was the accumulation of thousands of hours of operation with the new manifolds, new valves, new valve settings and several other measures (better venting of the engine compartment to avoid flammable vapor build up was one).

Thinking that the early Allisons were up to the standards of the 1942/43 Allisons and only bureaucratic foolishness kept them from much greater performance is using a very rose shaded retrospectroscope.

Things do have to viewed in perspective. The Early Allison was initially rated at 150 hours before overhaul, Perhaps not quite as good as the equivalent Merlin but certainly better than the Hispano V-12s and their Russian cousins. May well have been better than the DB601? On Feb 2 1941 Allison recommended to Wright Field that 10 engines be allowed to got to 200 hours as a test to see if all the engines could be uprated. Allison V-12s, through a series of steps (and a bunch of new parts) would reach a 400 hour time limit between overhauls (which was often exceeded on training aircraft).

Now please note that Allison and Wright Field were still struggling to get the C-15 engine (the long nose) to where they wanted it to be as far as performance and reliability during the spring, summer and fall of 1941 when the Airacobra and P-40D/E Warhawks were starting to show up with their E and F engines. Much of what was being learned and applied to the C series engines could be applied (and was) to the later models.

Perhaps the Army did hold onto the backfire screens too long, but until they were sure the problem was fixed (thousands of hours of opertainal flights) yanking them out might well have increased operational losses.

 

[Shortround6]

For the thousandth time, delete the wing guns AND THE USELESS 100# NOSE ARMOR PLATE and substitute 50 gallons of fuel at 300#, the weight of the self sealing tanks would be mostly offset by removing the gun mounts, chargers, heaters and ammunition boxes. Very minor weight gain if any.

Please provide the weight/s for the gun mounts, chargers, heaters and ammunition boxes.

large flat fuel tanks weigh a lot more than short deep ones for the same fuel capacity.

The bullet proofing material weighs about the same per sq ft of tank to be covered, the more sq ft you need per gallon the heavier the tank.
A rough estimate of tank weight is from page 380 of "Aircraft Power Plants" by Arthur P Fraas ( a Packard engineer) who says that plain aluminum tanks can go from 3/4 lb per gallon for small tanks to 3/8lb per gallon for large tanks of 100 gal or more.
Bullet sealing tanks weigh between 0.7 and 1.5 lbs per gallon.

Tying to use the space where the .30 cal guns and ammo were for fuel tanks is going to result in about the worst possible shape of a tank from the standpoint of wall area to volume.

How much weight did Bell save by getting rid of the 33 gallons in the outer fuel cells?
And you want to add 50 gallons in an even thinner part of the wing?

 

[P-39 Expert]

Please provide the weight/s for the gun mounts, chargers, heaters and ammunition boxes. No idea, but I do know that they would need to be removed if fuel tanks were replacing them.

large flat fuel tanks weigh a lot more than short deep ones for the same fuel capacity.

The bullet proofing material weighs about the same per sq ft of tank to be covered, the more sq ft you need per gallon the heavier the tank.
A rough estimate of tank weight is from page 380 of "Aircraft Power Plants" by Arthur P Fraas ( a Packard engineer) who says that plain aluminum tanks can go from 3/4 lb per gallon for small tanks to 3/8lb per gallon for large tanks of 100 gal or more.
Bullet sealing tanks weigh between 0.7 and 1.5 lbs per gallon. So, at one pound per gallon the new tanks would weigh 25 pounds each (for two)? The normal tanks in the P-39 weighed about 1pound per gallon. The weight would have increased by 300# of fuel and 50# of new tanks for a total of a 350# increase. Weight would have been reduced by the .30 cal guns 99#, ammo 77#, the unneeded nose and selected other armor 140#, and the gun mounts, chargers, heaters and ammo boxes. If these last items weighed 34# (8.4# per gun) then the weight penalty for the additional fuel and tanks would have been exactly zero.

Tying to use the space where the .30 cal guns and ammo were for fuel tanks is going to result in about the worst possible shape of a tank from the standpoint of wall area to volume.

How much weight did Bell save by getting rid of the 33 gallons in the outer fuel cells? 200# plus the tanks.
And you want to add 50 gallons in an even thinner part of the wing? I have never been in favor of less fuel for the P-39. With more than the standard 120gal the P-39's effectiveness would have been increased. The standard 120 gallons carried by the P-39 was more than was carried internally on Spitfires, 109s, (and the 190 if it's much larger engine is considered) and most Russian fighters.

 

[XBe02Drvr]

delete the wing guns AND THE USELESS 100# NOSE ARMOR PLATE

And how are you going to compensate for the resulting rearward CG shift in a plane that's already borderline tail heavy? Move the battery and the radio boxes forward? Where you going to find room for them to gain back the moment arm you lost by deleting the armor? The farther the battery gets from the starter and generator, the fatter (and heavier) your cables are going to get and the more starting problems you're going to have. The farther from the CG your battery and radios are, the more G abuse they'll suffer in Air Combat Maneuvering. Tube radios located in the nose next to a 37 (or a 20) and two 50s are going to be subject to intense high frequency vibration stresses whenever you squeeze the trigger. There's more to it than just yanking the armor.
Cheers,
Wes

 

[pinsog]

And how are you going to compensate for the resulting rearward CG shift in a plane that's already borderline tail heavy? Move the battery and the radio boxes forward? Where you going to find room for them to gain back the moment arm you lost by deleting the armor? The farther the battery gets from the starter and generator, the fatter (and heavier) your cables are going to get and the more starting problems you're going to have. The farther from the CG your battery and radios are, the more G abuse they'll suffer in Air Combat Maneuvering. Tube radios located in the nose next to a 37 (or a 20) and two 50s are going to be subject to intense high frequency vibration stresses whenever you squeeze the trigger. There's more to it than just yanking the armor.
Cheers,
Wes

This is the kind of stuff that most of us would never even consider that also explains why aircraft are built in certain ways. It would have never occurred to me when redesigning a WW2 fighter that they wanted the radio close to the center of the plane due to g forces (vibration from guns might have occurred to me). I've learned so much even when topic go way off course.

 

[fubar57]

https://www.defensemedianetwork.com...39-airacobra-in-the-pacific-posed-challenges/

 

[Shortround6]

.
Armor plate/glass weighed 292# on the P-400 including the nose gearbox armor, armor for the oxygen bottles and armor just outside the rear armor glass. Delete these items (not used on other comtrmporary aircraft) and the weight of armor plate and glass is reduced to around 140#. Armor plate/glass weighed 292# on the P-400 including the nose gearbox armor, armor for the oxygen bottles and armor just outside the rear armor glass. Delete these items (not used on other comtrmporary aircraft) and the weight of armor plate and glass is reduced to around 140#.

Source for the 292lbs? about 30lbs seems to have disappeared when the planes were in US service. Armor for the oxygen perhaps?

The P-39 armor never got down to 140lbs. On a P-39Q-1 it was still 193lbs.

The 20mm cannon in the P-400 was a non-starter with only a 60 round magazine good for 5-6 seconds firing time. It also jammed a lot. Give it the British belt fed 120 round magazine like the SpitfireV (just the logical development) and it would be a very useful gun.

Pretty much irrelevant to the discussion at hand. Another member had suggested that Britain wanted the Airabora Is for invasion barge busting. Given the ordering of the planes before the BoB even started that seems unlikely and given the armament of the AIracobras as ordered (drum fed 20mm) they were not any better than a number of any other aircraft for this job.

Due to rediculously heavy weight as specified by the British. Removing the weight was a very quick and easy fix. British are comparing their 6450# Spitfire with their 7850# P-400 creation and wondering why performance is down. They knew all along. Again, please see the graph in post #388.

You have yet to show a reference or source that says the British specified this weight. Please do so.

You can put up all the graphs you want of planes that don't really apply to the discussion at hand. A Spitfire running at less than full throttle, The P-39K with its 1325hp engine won't show up until the fall of 1942, as for the P-39C, even the USAAC wasn't dumb enough to order more than 20 of them and that was before France fell. They changed the order for the last 60 to the P-39D which weighed, surprise 7697lbs, with 104 US gallons of fuel. add another 96lbs if the internal tanks are full.

Bell promoted the P-400 as a 400mph airplane in '39 and '40 when it weighed 5000#, not after that. There were six weight increases under this contract between Feb 1940 and June 1941 increasing the weight from 5849# to 7635# of which the British were certainly aware since this was a direct purchase contract and they had to sign off on all changes.

The P-39 was never a 5000lb airplane, ever. It was 'designed' for gross weight of 5500lbs, The XP-39 was changed to 5,855lbs before it was finished. when it showed up at Wright Field it weighed 6,104lbs, This was in April of 1939. It was already 10% overweight and had no guns.

Please give list of these changes to the British contracts and source/s

The big test (reflecting the decreased performance) was in April '41 and the final revision to the contract increasing the order from 170 to 675 planes was signed in June. That the British were somehow tricked or fooled by Bell is rediculous. The British had the best intelligence service in WWII and had already contracted for and built a large quantity of very high performance airplanes themselves. They knew exactly what they were doing.

You have the dates wrong or are confusing them, the British added 505 planes to the contract in May or June of 1940, not 1941. Now this may have been a letter of intent, or provisional contract or some other temporary agreement that was not finalized until June of 1941 but all parties (Bell, Allison, the USAAC, and the British) were moving forward under the assumption that the British were ordering 675 planes, that is making allocation of materials, long lead items and whose aircraft (British or American ) would be delivered (hopefully) in which months. Hundreds of extra P-39s were allocated to the British in the Spring of 1941 under lend lease and many of those were the P-39D-1s and D-2s.

Can you give us a list of British agents ( or at least a memoir from one) doing industrial espionage in the US at this time or is this another conclusion you you have come to with no evidence to back it up?

 

[Shortround6]

And how are you going to compensate for the resulting rearward CG shift in a plane that's already borderline tail heavy? Move the battery and the radio boxes forward? Where you going to find room for them to gain back the moment arm you lost by deleting the armor? The farther the battery gets from the starter and generator, the fatter (and heavier) your cables are going to get and the more starting problems you're going to have.......... There's more to it than just yanking the armor.

The battery was already in the nose.

 

[Shortround6]

No idea, but I do know that they would need to be removed if fuel tanks were replacing them.

So another WAGS.

So, at one pound per gallon the new tanks would weigh 25 pounds each (for two)? The normal tanks in the P-39 weighed about 1pound per gallon. The weight would have increased by 300# of fuel and 50# of new tanks for a total of a 350# increase. Weight would have been reduced by the .30 cal guns 99#, ammo 77#, the unneeded nose and selected other armor 140#, and the gun mounts, chargers, heaters and ammo boxes. If these last items weighed 34# (8.4# per gun) then the weight penalty for the additional fuel and tanks would have been exactly zero.

How about actually looking up a few things before posting? the fuel system in a P-400 went about 267lbs, the one in a P-39D-2 went 291lbs so the the weight of the tanks was well over 2lbs per gallon even allowing for some heavy fuel lines and valves.

Somewhere I thought I saw that cutting the fuel tanks (3 cells taken out of each wing) cut the empty weight by about 100lbs, I could be wrong on that but your weight estimates for the tanks seem rather low. At any rate the P-40s saved 100lbs by taking out a single 37 gallon tank which was short and fat as wing tanks go.
You quickly jump to the low end of the range for a poorly shaped tank.

A little though exercise, you can make a "tank" that holds 8 cubic feet of volume by making a box 2 feet on each side. 4 square feet per side, six sides is 24 sq ft.
You can also make a "tank" that holds 8 cubic ft by making a box 8 ft long, 2 feet wide and 0.5 ft deep. you now have 42 sq ft of tank wall.
This is just to illustrate the principle. Sticking fuel tanks way out in the wings is a heavy way to store fuel.

I have never been in favor of less fuel for the P-39. With more than the standard 120gal the P-39's effectiveness would have been increased. The standard 120 gallons carried by the P-39 was more than was carried internally on Spitfires, 109s, (and the 190 if it's much larger engine is considered) and most Russian fighters.

Nobody is really in favor of less fuel, that "option" was usually forced by the need to cut weight to boost performance.

Yes the P-39 carried more fuel, 120 gallons vs 103 for Spitfire and 106.7 for the 109. I am not going to get all excited over the P-39 because it carried 16% more fuel than standard Spitfire.

 

[XBe02Drvr]

It would have never occurred to me when redesigning a WW2 fighter that they wanted the radio close to the center of the plane due to g forces.

Not to mention the multiplied dynamic lateral and vertical loads from abrupt maneuvering the farther these delicate electronics are from the CG.

 

[XBe02Drvr]

The battery was already in the nose

Thank you, Sr6, I stand corrected. Now Cobra 'spurt, where you going to find the extra moment to make up what you lost by stripping the nose armor? Huh?
Cheers,
Wes

 

[wuzak]

Yes the P-39 carried more fuel, 120 gallons vs 103 for Spitfire and 106.7 for the 109. I am not going to get all excited over the P-39 because it carried 16% more fuel than standard Spitfire.

Didn't the standard capacity become 87USG for later models?

 

[pbehn]

Meanwhile with the dear old Mustang/P-51 if you wanted 4 cannon you got 4 cannon, you want six 0.5 MGs you got them. More fuel? in the back or under the wings where would sir prefer? You may need some more oil Sir, we can sort that out for you too. Removing fuel, armour guns and other military equipment to increase performance is just trending away from being a military machine and towards a Reno racer.

 

[Jugman]

Didn't the standard capacity become 87USG for later models?

The P-39N through P-39Q-1 had a max internal capacity of 87USG. There were kits made to bring them back up to normal capacity.