XP-39 Airplane in wind tunnel

[Elmas]

omissis
What changes could have been made? I had actually thought of creating a thread about theoretical ways WWII airplanes could have been made better with the knowledge of the time.

No changes, unfortunately. No matter how pretty P-39 could have been, the mass distribution was flawed, and there were no modifications that could have improved the design.
USAAF was more than happy to get rid of it at the first useful opportunity, giving them as a present to a low-ranking Allies...

 

[pbehn]

No changes, unfortunately. No matter how pretty P-39 could have been, the mass distribution was flawed, and there were no modifications that could have improved the design.
USAAF was more than happy to get rid of it at the first useful opportunity, giving them as a present to a low-ranking Allies...

Even if the mass distribution was OK, if you change the engine substantially it isn't and you need a new airframe.

 

[Elmas]

Even without the turbocharger P-39 had a C.G dangerously aft, so I can imagine how it could be with turbocharger in the rear of the fuselage.
I'm inclined to think that stripping P-39 of the turbo was a technique to get an aeroplane "more or less" flyable at the expense of high flying performance.
But it is not only , as I stated before, just a matter of "static balance" but also of "dynamic balance".

 

[pbehn]

Even without the turbocharger P-39 had a C.G dangerously aft, so I can imagine how it could be with turbocharger in the rear of the fuselage.
I'm inclined to think that stripping P-39 of the turbo was a technique to get an aeroplane "more or less" flyable at the expense of high flying performance.
But it is not only , as I stated before, just a matter of "static balance" but also of "dynamic balance".

I was thinking more along the lines of fitting the Merlin, most allied aircraft had some engine change or other, with the P-39 it required a complete re design to do it.

 

[Shortround6]

I don't know why people want to reinvent the wheel.

The P-63 was a redesigned P-39. It solved the handling problems.

it also used no parts in common with the P-39 except screws and rivets.

 

[pbehn]

I don't know why people want to reinvent the wheel.

The P-63 was a redesigned P-39. It solved the handling problems.

it also used no parts in common with the P-39 except screws and rivets.

Could you just slot a Griffon or Sabre in there Mr Bell.

 

[wuzak]

Even without the turbocharger P-39 had a C.G dangerously aft, so I can imagine how it could be with turbocharger in the rear of the fuselage.
I'm inclined to think that stripping P-39 of the turbo was a technique to get an aeroplane "more or less" flyable at the expense of high flying performance.
But it is not only , as I stated before, just a matter of "static balance" but also of "dynamic balance".

The turbo was below the engine, not in the aft fuselage.

 

[Elmas]

The turbo was below the engine, not in the aft fuselage.

Of course. But it was always behind the C.G.

 

[Zipper730]

The P-63 was a redesigned P-39. It solved the handling problems.

Why didn't the USAAF buy the P-63?

 

[Zipper730]

Pratt & Whitney 2 stage engines used a hydraulic clutch to change supercharger drive gears.

So that was about friction reduction?

The torque converter/fluid coupling drove the supercharger directly.

Okay, I got you

 

[wuzak]

So that was about friction reduction?

No, it operated like a friction clutch.

 

[Zipper730]

No, it operated like a friction clutch.

I'm confused...

 

[pbehn]

Why didn't the USAAF buy the P-63?

To replace what? Months before the start of the jet age?

 

[Shortround6]

Why didn't the USAAF buy the P-63?

Who do you think paid for over 3300 of them?

Design work started in early 1941, The US army ordered two Prototypes in June of 1941.

Out of 3300 the Russians got just over 2400.

and NO, not all of the rest were the famous pinball target aircraft.

 

[wuzak]

I'm confused...

Rolls-Royce used friction clutches (like what is used in a manual/stick shift car) to change supercharger gears. One for each gear. To change the gear the clutch in the current gear is released, while the one for the other gear is engaged, slipping as the supercharger is brought to the speed of the new gear.

Pratt & Whitney used fluid couplings to do the same thing. Once the supercharger is up to speed the fluid coupling does not vary its ratio.

Rolls-Royce engineered a small amount of slippage in the supercharger drive clutches to reduce the effects of torsional vibration from the crankshaft affecting the supercharger gears (that is, reduced shock loadings). The fluid coupling in Pratt & Whitney's superchargers would naturally have some slippage.

A variable speed fluid coupling has a variable speed ratio. This is done by controlling the amount of fluid on the casing. This is different to a torque converter, which varies its speed ratio depending on load.

 

[chris mcmillin]

What I always thought was the turbo in a ventral scoop with intakes for the inter/after coolers and an oil to coolant heat exchanger for the oil cooling and the deeper profile allowing a better velocity drop for the radiators and a controlled exit like the NAA design.
This way the original fuse shape would be unchanged but the power would be maintained and cooling drag optimized if not reduced.

Fuel capacity and wing armament would still be a problem. With the wing as thin as it is, the armament could be removed and fuel capacity increased by another cell in the ammo and breech areas. Two .50's and a 37 mm seemed to be capable enough for those high scoring Russian guys! (I'm pretty sure the Airacobra is the highest scoring allied fighter, at 50 kills.)

Oh, maybe wing pylons for drops too. In the high 20's fuel burn is really low, like 45 gph at 300 miles true. Two 50 gallon drops would be more than 2 hours endurance increase. An escort fighter before the Mustang!

Chris...

 

[Shortround6]

Might have been faster at high altitude, was 30mph slower at low altitudes. This was a mock up.
P-39s were running on the margin for cooling both the glycol mixture and and the oil as it was. Trying to cool an 1150-1425hp engine at 25,000ft needs bigger radiators or a lot of attention to the radiators and ducts.

The space in the outer wings doesn't work well for fuel if you are using self sealing tanks. the tanks wind up rather thin and spread out for the amount of fuel they hold meaning even the empty tanks are rather heavy.

You are also forced into using under wing tanks/bombs, perhaps not a bad thing but that much more stuff to sort out.

The P-39 was already a pint and a half (750ml) in a pint (500ml) bottle, trying to stuff the turbo in means you are going for the full quart (1000ml) in the pint (500ml) bottle.

 

[chris mcmillin]

By my personl point of view, P-39 was a mess not for aerodynamics, but for the distribution of the masses, expecially consumables.
That affected quite a lot Static balance (Moment of first order, expecially after ammunition expended) but even more the Moment of second order, namely the "Moment of inertia" of the airplane, making handling tricky, to say the least.

Moment of inertia - Wikipedia

I dont see any real evidence for balance problems, more like hearsay and myth written by uninformed writers.

The cannon brass is retained after expending the lead, the cannon and guns are always retained, the balance is figured for full and empty on all consumables, ammo, fuel, drops, gear up and down. Just to add, this is a front line fighter that near 10,000 were built. All of these fantasy aft CG problems were nothing but, real engineers built this fighter, real test pilots did the engineering flight test, it wasnt pulled out of someone's rear end... like much that is written about them certainly. The idea it was as you describe is ridiculous.

When I read these handling reports from keyboard jocks online, I always wonder how much time they have in the Airacobra. You? How much time in any airplane?
If not just your opinion, how about a paper written by a combat pilot or commander? Like Kit Carson did, better yet the paper by Buzz Wagner. Real pilots, in era, flying combat airplanes...

The problems with the Airacobra were spares, gasoline quality and lack of support in the PNG. Any problems it had were doubled by these things. By comparison, the P-51 had many problems that had engineering resources thrown onto it plenty fast because of it's time period and political position because of high ranking AAF people pushing it. Shedding tails, shedding wings, and actual aft CG instability were addressed quick and plenty. Not so with the Bell contingent in 1942.

Chris...

 

[pbehn]

Fubar, put your overalls on, we are swapping tanks and guns again.

 

[chris mcmillin]

Might have been faster at high altitude, was 30mph slower at low altitudes. This was a mock up.
P-39s were running on the margin for cooling both the glycol mixture and and the oil as it was. Trying to cool an 1150-1425hp engine at 25,000ft needs bigger radiators or a lot of attention to the radiators and ducts.

The space in the outer wings doesn't work well for fuel if you are using self sealing tanks. the tanks wind up rather thin and spread out for the amount of fuel they hold meaning even the empty tanks are rather heavy.

You are also forced into using under wing tanks/bombs, perhaps not a bad thing but that much more stuff to sort out.

The P-39 was already a pint and a half (750ml) in a pint (500ml) bottle, trying to stuff the turbo in means you are going for the full quart (1000ml) in the pint (500ml) bottle.

No, no, I meant a real ventral scoop designed with the engineering elegance of the NAA west coast guys. You obviously missed my point. One, okay... P-51D shaped, ventral scoop architecture in which turbo, inter and after coolers and glycol radiator was internally ducted, with proper, low drag intake and adjustable outlet. Oh, and sorry, the 2,200 hp P-51H's -9 had the oil cooled by the coolant/oil heat exchanger. Eliminated weight and drag and area of another radiator.

See, I've actually installed thin bags. These fuel tanks are actually real. It could've been done, that is why I wrote what I wrote. What, belts and rails aren't heavy, along with four rifle caliber useless guns, give me the gas.

Under wing stores are not something odd.

Yes, the airplane was little and that is why it turned out the way it did. I'm just giving my 2 cents, and they are shiny.

Chris...