XP-39 II - The Groundhog Day Thread

[Peter Gunn]

Follow up to my previous post:

XP-39 II

View attachment 598971

Basic weight and balance - weight x arm = moment

Moments on chart;

Wheels up - 1016062

Wheels down - 1011053

My math;

7570 x 134.22 = 1016045.4 wheels up

7570 x 133.56 = 1011049.2 wheels down

Looks like you could save 61.5lbs by leaving the engine and gear box oil out, I mean who needs that right?

 

[XBe02Drvr]

Being a Southern Gentleman, I choose pistols suh...

Yours:
View attachment 598955


Mine:
View attachment 598956

You chose the weapons, I choose the attire. Agreed?
Yours:

Mine:

DEAL?

 

[P-39 Expert]

Hey Stig1207!

Overclaiming? Surely you jest.

Never happened, especially on the Russian Front where Stalin never executed anyone for lack of performance!

"OK, you two guys get one rifle and 5 rounds. When the first man is killed, the second man takes his rifle and continues fighting! In the Soviet Army, it's hard not to be a hero!"

Supposedly Stalin also said "It takes a very brave man to be a coward in the Soviet army." Referencing the "Political" officers in the rear who shot anyone retreating.

 

[P-39 Expert]

It is interesting to compare models' weights as tested. Most test reports tell you that they have ammo on board, etc. On some reports you need to click on the "See entire report HERE" to get the weights.
The 'as tested' weights grew from 6592 in the YP-39 to 7871 in the P-39Q. That's a significant change at just under 20%, while bhp went up from 1090 to 1420, ~30%.


Model...........weight as tested in pounds.........bhp................max speed.................time to 15,000'

YP-39..............................6592.......................1090..................368 mph....................4.6 min

P-39C.............................6689........................1150..................379 mph...................~4.3 min

P-39D.............................7525........................1150.................Not given....................Not given

P-39D.............................~7800......................1170.................368 mph....................5.7 min

P-39D.............................7450........................1172.................370 mph....................Not given

P-39M.............................7430........................1420.................385.5 mph.................4.38 min

P-39N.............................7274........................1420.................398.5 mph.................4.15 min

P-39N.............................7301........................1390................. Not given..................3.82 min

P-39Q.............................7871........................1405.................385 mph....................4.5 min

P-400 British tests..........7845........................Not given...........355 mph...................~5.3 min


Source: P-39 Performance Tests

Agree. The YP had a derated engine because the -35 had not passed the 150 hour test and didn't have self sealing tanks and armor plate/glass.
The C model had the standard -35 engine but did not have self sealing tanks and armor plate/glass. Some sources say it had the rear armor glass behind the pilot's head.
The D model had the -35 engine, self sealing tanks and 240lbs of armor plate/glass and was the first model to be considered combat ready.
My climb comparison was between the C and D since they had the same engine, propeller, horsepower etc, only difference was weight and internal equipment.

 

[Ivan1GFP]

Agree. The YP had a derated engine because the -35 had not passed the 150 hour test and didn't have self sealing tanks and armor plate/glass.
The C model had the standard -35 engine but did not have self sealing tanks and armor plate/glass. Some sources say it had the rear armor glass behind the pilot's head.
The D model had the -35 engine, self sealing tanks and 240lbs of armor plate/glass and was the first model to be considered combat ready.
My climb comparison was between the C and D since they had the same engine, propeller, horsepower etc, only difference was weight and internal equipment.

Hello P-39 Expert,

At the risk of resuming the last world war, have you considered the possibility that the P-39C was much better balanced and may have flown "better" than the P-39D?

You may not like where this might lead but here goes anyway:
From the discussions thus far, the general consensus and analysis is that even with late model P-39N and P-39Q in normal loaded condition, the CoG was fairly close to its aft limits.
My own belief is that earlier models were substantially worse in their balance because of different locations of equipment and the CoG at basic weight was 1-3 inches FURTHER AFT than the late models.

Note that in the typical testing that was conducted, the CoG was typically much further forward (24-25% MAC) than it the weight and balance charts would indicate for normal loaded weight. I am not sure how this was done, but it MIGHT have improved performance.

I had also wondered why the designers at Bell had been so stupid as put together the Airacobra in such a way that its CoG was always at or near its aft CoG limit.
Thinking about it a bit more, I don't believe they actually screwed up in that aspect.
Look over the armament of the P-39C. Note that although it carried no armour and probably not the Gearbox Armour that you dislike so much that we all believe was necessary to maintain proper balance, it had an extra pair of .30 cal MG in the nose along with 300 rounds of ammunition per gun. It carried about 30 pounds less ammunition for the 37 mm but the net effect is a lot more weight up front where it was needed for balance.
This might have substantially improved its flying qualities as compared to the later P-39D.

- Ivan.

 

[GregP]

Source are detailed-but-unsubstantiated claims over the internet about Soviet aerial victories.

All the "lists" I have seen of Soviet victories have been rather detailed, but low on primary sources. Yet ALL show the P-39 as being used by very many pilots with more than 30 victories in P-39s. If you add them all up, you get more victories than the P-51 has in air-to-air combat ... at the cost of very vague sources for the data. Almost all of the lists I got back in the day were internet addresses ending in " .RU".

There is absolutely nothing wrong with Russian data, and they would have been worth saving if the sources were known and actually available to the public. Telling me the source is in a semi-publically-available archive in Moscow at some address doesn't help me get to the source, but that's what I got back in the 1980s / 1990s.

I have several books on Soviet WWII aircraft, all of which are currently in storage. Again, the sources are there, but unavailable to westerners, AFAIK. Even though I was interested, I wasn't about to buy a ticket to Moscow and show up at an archive on the vague chance I'd be admitted.

 

[fubar57]

Have you seen this site Greg? Soviet top Aces of WWII rating

 

[P-39 Expert]

Hello P-39 Expert,

At the risk of resuming the last world war, have you considered the possibility that the P-39C was much better balanced and may have flown "better" than the P-39D?

Is that what made it climb 1000fpm better than the D? I think it was because it was 800+lbs lighter.

You may not like where this might lead but here goes anyway:
From the discussions thus far, the general consensus and analysis is that even with late model P-39N and P-39Q in normal loaded condition, the CoG was fairly close to its aft limits.
My own belief is that earlier models were substantially worse in their balance because of different locations of equipment and the CoG at basic weight was 1-3 inches FURTHER AFT than the late models.

Need to prove that.

Note that in the typical testing that was conducted, the CoG was typically much further forward (24-25% MAC) than it the weight and balance charts would indicate for normal loaded weight. I am not sure how this was done, but it MIGHT have improved performance.

Need to prove that too.

I had also wondered why the designers at Bell had been so stupid as put together the Airacobra in such a way that its CoG was always at or near its aft CoG limit. Bell was a real moron. Designed a single engine fighter around a 37mm cannon and a tricycle landing gear.
Thinking about it a bit more, I don't believe they actually screwed up in that aspect.
Look over the armament of the P-39C. Note that although it carried no armour and probably not the Gearbox Armour that you dislike so much that we all believe was necessary to maintain proper balance, it had an extra pair of .30 cal MG in the nose along with 300 rounds of ammunition per gun. It carried about 30 pounds less ammunition for the 37 mm but the net effect is a lot more weight up front where it was needed for balance.
This might have substantially improved its flying qualities as compared to the later P-39D.

The .30s in the nose weighed about 100lbs with ammunition. Less the additional 30lbs for the additional 15 37mm rounds and the net decrease is about 70lbs. The nose armor plate was between 70lbs and 100lbs depending on the model. About the same weight.

- Ivan.

Please expand above.

 

[Peter Gunn]

You choose the weapons, I choose the attire. Agreed?
Yours:
View attachment 599041

Mine:
View attachment 599042

DEAL?

Agreed:

Yours:

Mine:

Rats... are they talking about the P-39 again?

 

[Stig1207]

Source are detailed-but-unsubstantiated claims over the internet about Soviet aerial victories.

All the "lists" I have seen of Soviet victories have been rather detailed, but low on primary sources. Yet ALL show the P-39 as being used by very many pilots with more than 30 victories in P-39s. If you add them all up, you get more victories than the P-51 has in air-to-air combat ... at the cost of very vague sources for the data. Almost all of the lists I got back in the day were internet addresses ending in " .RU".

There is absolutely nothing wrong with Russian data, and they would have been worth saving if the sources were known and actually available to the public. Telling me the source is in a semi-publically-available archive in Moscow at some address doesn't help me get to the source, but that's what I got back in the 1980s / 1990s.

I have several books on Soviet WWII aircraft, all of which are currently in storage. Again, the sources are there, but unavailable to westerners, AFAIK. Even though I was interested, I wasn't about to buy a ticket to Moscow and show up at an archive on the vague chance I'd be admitted.

So the Soviets got such an impreesive performance by removing some excess weight from the P-39? More victories than the P-51!
I would then rather suggest a more plausible reason: the Soviet pilots were simply superior to the American pilots. Perhaps we should add to the good old 'Overpaid, oversexed, overthere'.... overrated?

 

[Ivan1GFP]

Hello P-39 Expert

Is that what made it climb 1000fpm better than the D? I think it was because it was 800+lbs lighter.

There were obviously some other differences besides the difference in weight. Unless of course you are not following the explanations that have already been presented. What do YOU think the differences were?

Need to prove that.
.....
Need to prove that too.

I don't have to prove a thing if you actually read the reports you are selectively quoting from.

Bell was a real moron. Designed a single engine fighter around a 37mm cannon and a tricycle landing gear.

Bell actually didn't do that. Bell designed a single engine INTERCEPTOR around a 37 mm cannon. When that failed, he attempted to make it fill the role of a fighter but it didn't have the stretch for the extra equipment and armour.

The .30s in the nose weighed about 100lbs with ammunition. Less the additional 30lbs for the additional 15 37mm rounds and the net decrease is about 70lbs. The nose armor plate was between 70lbs and 100lbs depending on the model. About the same weight.

I don't think you understand. This isn't the P-39D.
Is the nose armour plate or any other armour plate installed in the P-39C?

- Ivan.

 

[P-39 Expert]

Hello P-39 Expert



There were obviously some other differences besides the difference in weight. Unless of course you are not following the explanations that have already been presented. What do YOU think the differences were?

That was my point in comparing the C with the D. There were no differences except internal. Same outside, engine, propeller, horsepower. Only difference was weight.

I don't have to prove a thing if you actually read the reports you are selectively quoting from.

Please stop with the handling issues. You keep trying to make this plane unflyable when it was obviously easy and pleasant to fly.

Bell actually didn't do that. Bell designed a single engine INTERCEPTOR around a 37 mm cannon. When that failed, he attempted to make it fill the role of a fighter but it didn't have the stretch for the extra equipment and armour.

You're splitting hairs. Interceptor vs fighter.

I don't think you understand. This isn't the P-39D.
Is the nose armour plate or any other armour plate installed in the P-39C?

No nose armor in the C. You were talking about the difference in weight in the nose, I was explaining that it was about the same after deducting the .30s and adding the nose armor.
Some reports say that the C did have the armor glass behind the pilot, some don't.

- Ivan.

Please expand above.

 

[GregP]

Thanks for that Fubar 57. Yes, I have seen it. Again, no mention of sources except thanks to Alexey Koliush, Alexander Abramov, Igor Utkin, Andrey Mikhailov, Alexander Melnikov, Allan Magnus, Hakkan Gustavson at the bottom of the data.

Not sure if the Soviet victories match with the Magnus family website, which I have liked for years, as I have not compared them. But that would at least make me feel better about the data. Again, no way to break out the P-39 victories unless the P-39 was the only aircraft they flew. So, it's interesting, but hardly authoritative.

You can say the same about a lot of data on Soviet activities. Perhaps getting identified as a data source in the former Soviet Union might have been fatal? I can't say and would speculate that the data are difficult to find since so little data with primary sources has been forthcoming from the former Soviet Union after it's collapse. I doubt anyone in Russia is all that interested in releasing the data to the western public.

 

[P-39 Expert]

I tossed my numbers when this discussion seemed over.

Look at Greg's calculator, he's got it down pretty good and you're able to play with some of things we discussed.

The error on the page I mentioned, I'll post later

I tried to calculate how far aft the CG moved if the nose armor (71lbs) was removed. I came up with:

Weight 7570lbs CG Arm 134.22 Moment 1016045 From the P-39Q weight chart
Less 71lb Arm 21 Moment 1485
New Wt. 7499lbs New CGArm 135.29 New Moment 1014560

New CG arm calculated by dividing new moment 1014560 by new weight 7499lbs.

New CG is 1.07 inches aft of old CG (134.22-135.29=1.07).

MAC is 80.64", CG limits 23% to 31% of MAC, or 5.6". 1.07" should be within the CG limits. Sorry I can't make the columns line up.

 

[GregP]

The main difference between the P-39C and P-39D was the addition of self-sealing fuel tanks to the P-39D, but you cannot take data from one airplane and transfer it to another one in another test. You've already been told that. If you want to see the difference, you have to add weight to the same airframe and attempt to test at the same or very nearly the same atmospheric conditions, before too long a time has gone by and things have started wearing out ... like a propeller that is operated on rough strips will lose some thrust over time with use and, if the dust is coral, the engines will wear out 10 times faster or more due to the dust alone.

We already told you most of this, P-39 Expert, and we already told you they didn't typically vary weight in WWII tests because they were interested in the performance of production aircraft with typical war loads, not confirmation of aerodynamic formulas that they KNEW were correct. You either have to go out and buy a P-39 and do the tests yourself or you have to be satisfied with the real data available or in aerodynamic formulas that everyone knows are correct. Making up your own comparisons with data unrelated to real aerodynamic performance won't make them correct.

Maybe I'll stop responding to trolling. I can't believe it's gone 43 pages! ... but it has. Some good stuff in here, but enough would seem to be enough.

I apologize for putting it that way since I am not a moderator in here ... just my impatience showing through. So, cheers and good luck to you, P-39 Expert.

Whatever you do, don't so this to you P-39 / P-63.

It killed Mike Carroll when he did it! He struck the stabilizer when he bailed out and never opened his chute.

 

[XBe02Drvr]

Agreed:

Yours:

View attachment 599062

Mine:

View attachment 599063



Rats... are they talking about the P-39 again?

You defile the honor of the southron gentleman you claim to be! You made your weapons choice in post 837, and are honor bound to stick with it, else I have no obligation to defend my honor against a disgraced, dis-honored item of southron trash! This disreputable affair is dismissed! My Airacobra squadron is on it's way to strafe your hillbilly shack.

 

[P-39 Expert]

The main difference between the P-39C and P-39D was the addition of self-sealing fuel tanks to the P-39D, but you cannot take data from one airplane and transfer it to another one in another test. You've already been told that. If you want to see the difference, you have to add weight to the same airframe and attempt to test at the same or very nearly the same atmospheric conditions, before too long a time has gone by and things have started wearing out ... like a propeller that is operated on rough strips will lose some thrust over time with use and, if the durst is coral, the engines will wear out 10 times faster or more due to the dust alone.

I'm simply determining how weight affects climb on one airplane. Same engine, propeller and horsepower at the same altitude. In the same airframe. Both planes were virtually brand new right off the production line. Only difference was internal equipment resulted in different weights.

We already told you most of this, P-39 Expert, and we already told you they didn't typically vary weight in WWII tests because they were interested in the performance of production aircraft with typical war loads, not confirmation of aerodynamic formulas that they KNEW were correct. You either have to go out and buy a P-39 and do the tests yourself or you have to be satisfied with the real data available or in aerodynamic formulas that everyone knows are correct. Making up your own comparisons with data unrelated to real aerodynamic performance won't make them correct.

This is just a comparison of one plane with two different internal loads. Very simple. The lighter one outclimbed the heavier one by 1000fpm. No atmospheric conditions will make up for 1000fpm of climb. Divide the 1000fpm by the weight difference 836lbs and each pound of weight cuts climb by 1.2fpm. It can't be any simpler than that. Now we can determine how much climb will be improved by removing certain redundant or unneeded items.

Maybe I'll stop responding to trolling. I can't believe it's gone 43 pages! ... but it has. Some good stuff in here, but enough would seem to be enough.

No disrespect meant, but I am not trolling. I'm presenting facts as determined by official military tests of one airplane.

I apologize for putting it that way since I am not a moderator in here ... just my impatience showing through. So, cheers and good luck to you, P-39 Expert.

And the very best of luck to you GregP. Thanks for your help and information.

Please expand above.

 

[fubar57]

You defile the honor of the southron gentleman you claim to be! You made your weapons choice in post 837, and are honor bound to stick with it, else I have no obligation to defend my honor against a disgraced, dis-honored item of southron trash! This disreputable affair is dismissed! My Airacobra squadron is on it's way to strafe your hillbilly shack.

Gentlemen....gentlemen. Lets's make this fair.....kinda. Choose one but be warned. One has fired off all it's nose ammo and is now dangerously unstable

​

In hindsight I should have made one Russian

 

[GregP]

P-39 Expert, I can only answer with a suitable story:

Wants pawn term, dare worsted ladle gull hoe lift wetter murder inner ladle cordage, honor itch offer lodge dock florist. Disk ladle gull orphan worry ladle cluck wetter putty ladle rat hut, an fur disk raisin pimple colder Ladle Rat Rotten Hut.

Wan moaning, Rat Rotten Hut's murder colder inset, "Ladle Rat Rotten Hut, heresy ladle basking winsome burden barter an shirker cockles. Tick disk ladle basking tutor cordage offer groin-murder hoe lifts honor udder site offer florist. Shaker lake! Dun stopper laundry wrote! An yonder nor sorghum-stenches, dun stopper torque wet strainers!"

"Hoe-cake, murder," resplendent Ladle Rat Rotten Hut, an tickle ladle basking an stuttered oft. Honor wrote tutor cordage offer groin-murder, Ladle Rat Rotten Hut mitten anomalous woof. "Wail, wail, wail!" set disk wicket woof, "Evanescent Ladle Rat Rotten Hut! Wares are putty ladle gull goring wizard ladle basking?"

"Armor goring tumor groin-murder's," reprisal ladle gull. "Grammar's seeking bet. Armor ticking arson burden barter an shirker cockles."

"O hoe! Heifer blessing woke," setter wicket woof, butter taught tomb shelf, "Oil tickle shirt court tutor cordage offer groin-murder. Oil ketchup wetter letter, an den - O bore!"

Soda wicket woof tucker shirt court, an whinney retched a cordage offer groin-murder, picked inner widow, an sore debtor pore oil worming worse lion inner bet. Inner flesh, disk abdominal woof lipped honor bet an at a rope. Den knee poled honor groin-murder's nut cup an gnat-gun, any curdled dope inner bet.

Inner ladle wile, Ladle Rat Rotten Hut a raft attar cordage, an ranker dough belle. "Comb ink, sweat hard," setter wicket woof, disgracing is verse. Ladle Rat Rotten Hut entity bet rum an stud buyer groin-murder's bet.

"O Grammar!" crater ladle gull, "Wood bag icer gut! A nervous sausage bag ice!"

"Battered lucky chew whiff, doling," whiskered disk ratchet woof, wetter wicket small.

"O Grammar, water bag noise! A nervous sore suture anomolous prognosis!"

"Battered small your whiff," insert a woof, ants mouse worse waddling.

"O Grammar, water bag mousy gut! A nervous sore suture bag mouse!"

Daze worry on-forger-nut gulls lest warts. Oil offer sodden, thoroughing offer carvers an sprinkling otter bet, disk curl and bloat-thursday woof ceased pore Ladle Rat Rotten Hut an garbled erupt.

Mural: Yonder nor sorghum stenches shut ladle gulls stopper torque wet strainers.


The truth is somewhere in Little Red Riding Hood, spoken with an accent.

 

[FLYBOYJ]

I tried to calculate how far aft the CG moved if the nose armor (71lbs) was removed. I came up with:

Weight 7570lbs CG Arm 134.22 Moment 1016045 From the P-39Q weight chart
Less 71lb Arm 21 Moment 1485
New Wt. 7499lbs New CGArm 135.29 New Moment 1014560

New CG arm calculated by dividing new moment 1014560 by new weight 7499lbs.

New CG is 1.07 inches aft of old CG (134.22-135.29=1.07).

MAC is 80.64", CG limits 23% to 31% of MAC, or 5.6". 1.07" should be within the CG limits. Sorry I can't make the columns line up.

You'll be within limits but you better not fire off any ammo or consume much fuel. Use the excel spread sheet Greg put together, he nailed it from what I can see.

Armor removed

No Nose Ammo, Full tanks

No nose ammo, half tanks