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

SaparotRob · May 19, 2021

Okay everybody, let's all kick in a couple hundred grand US to the Planes Of Fame Museum in Chino, California. GregP (not Greg) and his guys build and rebuild these two airplanes to the mods suggested here and see if we can do what Bell, the USAAF, the RAF, the RCAF, the RAAF couldn't. I'm sure we can get one of our Forum members with a pilot's license drunk enough to take up the modified 'Cobras and see if it would work. We'll settle it once and for all. Then we can fix the Botha.

GregP · May 19, 2021

Actually, the P-63 is at Yanks Air Museum, located on the same airport. I am a member of both museums. Planes of Fame has a P-39 and Yanks has a P-39 and a P-63. They are just about to have first-run on the P-63 engine any day now. Actually, it HAS run, but not in the airplane connected to the propeller. It was run in on a test stand by Joe Yancey.

In point of fact, most bulkheads are hollow. That is, they aren't solid across the entire fuselage. So, it is possible and even likely the engine mounts on the two airplanes are the same since the engine mounts plates are integral to the engine block. It is highly unlikely that the aux stage is left hanging out in space all by itself, supported only by a driveshaft housing. See below.

So, while the power section engine case block, nose box and integral supercharger are all supported by the engine mounts, it is also likely the case that the auxiliary supercharger stage is supported by a mount that is simply not present on the P-39. I'll check on this next Tuesday and report back.

By the time an aux-stage Allison was available, nobody in the USAAF wanted the P-39. So, it would never have had the aux-stage Allison fitted in the first place. The P-63 was an update of the P-39, longer and bigger, and had very little in common with the P-39 save some few components that could be shared. None of the airframe was interchangeable. But, you all know this and an argument saying the Allison 2-stage could be mounted in the P-39 ignores reality completely. It was a non-starter.

glennasher · May 19, 2021

I feel sorry for the womenfolk who turned in their aluminum pots and pans for recycling into P-39s and P-63s, it must have been pretty upsetting to waste all those good cooking utensils for that dog of an airplane.

Ovod · May 19, 2021

glennasher said:
I feel sorry for the womenfolk who turned in their aluminum pots and pans for recycling into P-39s and P-63s, it must have been pretty upsetting to waste all those good cooking utensils for that dog of an airplane.

Did they use recycled scrap aluminium to build aircraft in the war? They don't use scrap metal to build aircraft today, it' got to be "virgin" aluminium, straight from the ground.

You're probably being a a bit negative about Bell aircraft from the war - it served its role quite well during the war, especially on the Eastern front.

pbehn · May 19, 2021

Ovod said:
Did they use recycled scrap aluminium to build aircraft in the war? They don't use scrap metal to build aircraft today, it' got to be "virgin" aluminium, straight from the ground.

You're probably being a a bit negative about Bell aircraft from the war - it served its role quite well during the war, especially on the Estern front.

There was a famous propaganda campaign "throw in all your pots and pans, we'll make them into Spitfires" by Beaverbrook in UK. It was great for getting everyone feeling involved, it resulted in mountains of pots pans and lots of other stuff that wasnt made into Spitfires. Pans into Planes – thewartimekitchen.com

P-39 Expert · May 19, 2021

FLYBOYJ said:
22Gallons of fuel (1/4) tank puts it out of C/G

Now something to consider - This data was taken from the P-39Q flight manual. The manual did not show what radio (or IFF) was installed for weight and balance purposes, that would be specific to the aircraft and found in the maintenance records that follow the aircraft.
View attachment 623943

Like I said, without fuel, nose armor and nose ammunition the CG is .5416" aft of the aft limit. A hair over a half inch. Like I also said, if the 71lb nose armor is removed an adjustment would be necessary like moving the radio in the aft fuselage. I always said that a CG adjustment would be necessary and suggested the radio could be moved to offset the deletion of the nose armor.

P-39 Expert · May 19, 2021

GregP said:
Here's an accurate scale representation of the P-39Q and P-63A.

View attachment 623941
The P-39Q is 30" 2" long and the P-63A is 32" 8" long. In the above pic, I lined up the front of the exhaust manifolds since they are bolted to the engine. I went to the aft part of the air intake. Note the length added to the P-63 is aft of the windscreen.

As it happens, we have a P-63 in the final stages of work just before first engine run. We also have a P-39, complete with external engine and nose case. I volunteer every week on Tuesday, so I was there yesterday, but I was working on parts for another aircraft. Next week, I'll get measurements of the same areas shown above. Now, our P-63 is not using a 2-stage V-1710, but the engine compartment is stock. The engine we are using is a 100-series and makes about 1,600 hp at full MAP. Of course, it will very rarely see 1,600 hp and will likely be operate at the 1,200 - 1,400 hp max limit when it flies, and only at an airshow at that. Most of the time, it should see that limit only on takeoff and initial climbout.

Easy since we have the subjects right there. Talk with you then.

Cheers.

Your photo shows the P-39 engine compartment to actually be a few inches longer than the P-63, probably the result of slightly different scale. Both are exactly the same length.

Marcel · May 19, 2021

pbehn · May 19, 2021

Marcel said:
View attachment 623958

Looks like he needs some weight in his tail.

Marcel · May 19, 2021

pbehn said:
Looks like he needs some weight in his tail.

Did you see that canon in the nose? That's at least 2 inches....

P-39 Expert · May 19, 2021

Shortround6 said:
Another yes, no and maybe.

The coolant may well have weighed 149lbs but it was not all (or even most of it) in the "tank".
The tank is an overflow tank to hold the expanded fluid as the engine heats up.
Most of the coolant is in the engine block and the radiator with a bit in the lines connecting them.

Much like many cars have a small plastic tank near the radiator with a hose connecting the tank and the radiator at the pressure cap. As the coolant gets hot and expand the over flow goes through the tube and into the "expansion tank" and once the coolant cools it is syphoned back into the cooling system.

At any rate, there is no 149 lbs to move, Modern Prestone coolant weighs about 10lbs per gallon. I think that Allison may have been running a 70-30 mix?

Tank is only going to hold a few gallons even when full.

Silly me, assuming that the coolant was in the coolant tank.

From Design Analysis of the P-39 Airacobra "The Prestone expansion tank is so arranged that when filled to the level of the filler neck, when the airplane is in its normal rest position it will contain it's proper amount of coolant and the proper amount of expansion space."

FLYBOYJ · May 19, 2021

P-39 Expert said:
Like I said, without fuel, nose armor and nose ammunition the CG is .5416" aft of the aft limit. A hair over a half inch. Like I also said, if the 71lb nose armor is removed an adjustment would be necessary like moving the radio in the aft fuselage. I always said that a CG adjustment would be necessary and suggested the radio could be moved to offset the deletion of the nose armor.

Not that fast...

We still don't know how much the IFF removed would do for the C/G as we don't know the arm and weight - additionally we also don't know what part of the radio configuration will negate ALL equipment from the radio bay in the empennage, so this is still an enigma. At this point it's about the operator's configuration of the aircraft. Did the Russians put any equipment in the empennage radio bay? My feeling is if you have anything in that bay you're going to have C/G issues or come close to the most aft C/G in the conditions stated.

A hair over a half inch will not cause the aircraft to fall out of the sky (if you fly straight and level and do NOT do any form of aerobatics) but it will be more than likely against normal operating regs of the day to fly it that way as the aircraft is already tail heavy in it's normal configuration.

P-39 Expert · May 19, 2021

AHT says the IFF radio weighs 110-130lbs and it is farther away from the CG than the nose armor plate so it should move the CG forward within range.

GregP · May 19, 2021

Reference post #839.

No, it doesn't.

pbehn · May 19, 2021

FLYBOYJ said:
You are to a point - to make a difference you have to take weight out of the tail

Funnily enough the Spitfire had the CoG issues when they inserted bigger engines, they just put circa 90lbs (5x 17.5) in the tail of a MkIX and around 150lbs ( if I remember correctly) in the MkXIV. No fuss at all, carry on chaps.

GrauGeist · May 19, 2021

P-39 Expert said:
Silly me, assuming that the coolant was in the coolant tank.

From Design Analysis of the P-39 Airacobra "The Prestone expansion tank is so arranged that when filled to the level of the filler neck, when the airplane is in its normal rest position it will contain it's proper amount of coolant and the proper amount of expansion space."

Notice the key word "expansion tank"?
That's not a reservoir or storage unit, it's a tank that allows for the coolant someplace to go as the operating temps increase.
It is essentially the same as the plastic bottle that's attached to you car's radiator - which is usually 1/3 to 1/2 full.

FLYBOYJ · May 19, 2021

P-39 Expert said:
AHT says the IFF radio weighs 110-130lbs and it is farther away from the CG than the nose armor plate so it should move the CG forward within range.

It should if it was installed to begin with. If the W&B configuration shown in the POH was meant not to include the IFF, we're back to square one.

Now with all this said, what we're showing "could have" been easily figured out by the Soviets. Why did they leave the armor plate in the nose while removing the IFF? My feeling is to move the C/G forward making the aircraft handle better...

And again - did they put anything in the radio bay?

Shortround6 · May 19, 2021

Later planes may have changed a bit but expansion tank capacity 3 gallons. About 30lbs at most. plus the weight of the tank.

nuuumannn · May 19, 2021

Right then! Glad that's sorted (again... eyeroll). So, how 'bout that Fw 187, eh guys...

Greg Boeser · May 19, 2021

Why are we arguing about putting in a different engine in the P-39 when Bell actually designed a successor aircraft to the P-39 to hold that very engine?
It's like saying the B-17 would have been a super bomber if Boeing had only put R-3350s in it.

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

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