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)

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Put a wheel in the tail and use the front wheel to adjust CoG in flight, yet another win-win.
 
By the way, the P-39 is not all that much smaller in the engine bay area than the P-63. But it IS a slight bit smaller and the Aux-stage Allison does not fit. The Aux supercharger would be up against a lateral brace and bulkhead (sort of a bulkhead, anyway). The fuselage is simply not big enough for the Aux S/C to fit inside, but it COULD be knocked out for a fit and the bulkhead COULD be moved. That would mess up the airflow around the area and the CG would be aft of the gear, but it COULD be done.

To fix these somewhat minor inconveniences, they made the P-63. If has very little in common with the P-39 except the general layout.

P-39:
http://www.aviastar.org/pictures/usa/bell_p-39.gif


P-63:
http://www.aviastar.org/pictures/usa/bell_p-63.gif

If you go look carefully at a 3-view of the P-39 and P-63, notice the placement of the wing.

In a P-39, the tip airfoil maximum thickness runs through the bulkhead line that is just behind the rear of the door. In the P-63, the tip airfoil maximum thickness runs through the middle of the exhaust manifolds. From the top view, the P-39 looks like the wing is almost exactly halfway along the fuselage, but it actually is located forward of halfway. From the top view of a P-63, the wing is definitely placed more rearward than on a P-39.

What happened is what Drgondog said, the CG moved aft with an aux-stage Allison installed, so the landing gear had to be moved aft. That's very hard to do without moving the wing, too. While it was somewhat POSSIBLE to move a few things around inside a P-39 and shoehorn in an aux-stage Allison, it was not really possible to move the wing attach point without major reconstruction. Hence, the P-63 design to accomplish all the changes required.

It may not seem obvious, but the Aux-stage supercharger, with shaft, comes in at between 220 and 250 pounds. Let's call it 200 pounds, just for the sake of discussion.

In the P-39, there is no structure where the Aux-stage would go if it could be fitted inside. The airplane was designed for 8 gs and an overload of 12 gs at some design weight. The Aux-stage sits behind the V-1710 power section a distance of some 2 feet or so. At 12 gs, we have about 2,400 pounds (200 x 12) cantlevered aft of the power section. To support that, you need a structure that also can withstand the stress of landing many times over with no maintenance (how many engine mount systems require regular maintenance?). A load-bearing structure doesn't come without some weight penalty. The middle structure is one of the primary differences between the P-39 and P-63 structure.

Why am I wasting my time doing this?
 
I'd like to suggest the mother of all threads which will easily carry us into the next century:

"Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained - Color pics For a Model I'm Building"


[h3][/h3]
 

The one thread to rule them all? Smeagol wants the Precious!!!!

 
Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained)
GregP said:
I can give you P-39 and P-63 pics, but I just don't have time today. Will try my best before the weekend.
Click to expand...
Greg - you did an excellent job of distilling a complex discussion to one that 'shall not be spoken or written of" MAY actually comprehend.

That said, the fallback after your gentle conversation will be to remind us that warm up, taxi, take off. form up and climb is not explicitly stated in the Op Manual - and 'let the combat radius discussion begin anew'...
 
Greg - great information and despite what our narcissistic friend wants to believe or says I appreciate your inputs and efforts!
 
On a serious note I found this on Joe Bauger's site Wartime Service of P-39 with USAAF
The 31st Fighter Group was provided with Airacobras in Southern England in August of 1942. Between August and October of 1942, the Group participated in missions against enemy targets in France. The Group suffered heavy losses in air-to-air combat against the Luftwaffe, and the 31st FG re-equipped with Spitfire Mk Vs.

I cant find any reference to the 31st fighter group using anything other than Spitfires in that period.
 
I forgot to add some information about the "shoehorn an Aux-stage Allison into a P-39 post. The P-39Q-1 has an empty weight of 6,400 lbs and holds 87 gallons of internal fuel. Gross weight was 8,350 lbs. Per the manual. At 15,500 feet, at military power, it burns 138 US gallons of fuel per hour. The -35 engine was rated at 1,000 hp at 10,800 feet and 2,600 rpm.

A P-63A-1 has a gross weight of 8,410 lbs, holds 136 US gallons of internal fuel, and can carry a 64-gallons belly tank and/or a 75-galloins wing tank. They apparently needed more fuel than a P-39 did. The -93 engine was rated at 1,000 hp at 20,000 feet and 2,600 rpm.

The density of air at 10,800 feet is .001713 slugs / ft3​. The density of air at 20,000 feet is .001268 slugs / ft3​. To get the same air at 20,000 feet that you had at 10,800 feet (to make that 1,000 hp), you need an airflow of about 35% more at 20,000 feet than at 10,800 feet. So, you are looking at something like 35% more fuel flow except for the fact that the internal horsepower needed to drive 2 stages is way more than required to drive one stage. Let's be conservative and say you need 75% more internal horsepower to drive 2 stages than to drive one stage.

If you go look at an engine book, you will find out that the fuel flow will materially increase. Yet another reason you could not modify a P-39 with a 2-stage engine ... there is not enough fuel to do anything except fight very close to the home airfield if you increase the fuel consumption by a substantial amount. The P-39 was already woefully short on range. Add in even a 35% higher fuel flow at 20,000 feet, never mind the extra due to internal power required to drive the aux stage, and you will not have enough range to be useful, even in a tactical situation, in a P-39 airframe, assuming you could move the wings aft as required.

We have yet another reason they designed the P-63! Added fuel, decent CG position, and the ability to be a fighter at 20,000 feet ... not just to fly there.
 
Last edited:
Two VCs 8-19-42 in P-39s, 307FS. Last before November 1942 in Spits/MTO. They participated in CAS during Dieppe raid.
Notable: first Mustang victory credit in ETO, last P-39 victory credit in ETO.
 
IFR means, "I Follow Roads."
IMC means, "I Might Climb."
VNE means, "Very Nearly Extinct."
ETC means, "Extreme Technical Crap."

The best acronym I ever saw was FASOTRAGRULANT.

I saw it on a sign at Patuxent River Naval Air Station. Even though it sounds like something you might eat in a Yugoslavian restaurant, it turned out to mean, "Fleet Avionics Squadron Operational Training Group Atlantic." Still, it sounds like a curse word from some long gone demented vernacular tongue ...

One of the better signs is coming into Reno Stead airport, "Fosdick Fullfillment." Not too sure what Fos if filling, but it made me laugh all the way to the gate on the way to the races.
 
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