Improve That Design: How Aircraft Could Have Been Made Better

P-39 Expert · Aug 9, 2019

swampyankee said:
Because of its configuration, the P-39 had serious problems with growth, in general, and in re-engining. Since the size of a drive shaft is dictated by vibration properties and torque, and the path of the shaft has to be straight from the engine output to the propeller hub. This may require modifying the pilot so the shaft can pass through his chest or abdomen (not recommended) or modifying the Merlin for a remote gearbox (recommended). How difficult this would be, I have no knowledge. Obviously, a gearbox would also be needed. The Merlin and the Allison had different firing orders, which would have made differences in the forcing function for the shaft, which would require, at a minimum, analysis and testing. A Merlin P-39 would require, at a minimum, significant redesign of the fuselage center section and transmission from the engine to propeller; while this redesign could be done without slowing down production, introducing this into production would require new production tooling.

You still have an aircraft with severely constrained internal fuel capacity (the engine is right where you want to put fuel tanks) and a tendency to have its center of gravity move aft as combat proceeds, not infrequently resulting in flight characteristics that are somewhat unpleasant. While the P-39 was not as terrible as some have made out, it was at least a half-generation behind the P-51 (and F4U and P-47) and even the derivative design, the P-63, was no advance over the P-51 (and F4U and P-47).

I don't believe the P-39 needed the Merlin (especially with the mods necessary) as much as it needed a weight reduction in those early planes with the -35 engine with 8.8 gears. Those models had empty weights of 5409#-5462# and with a 1700# load would have weighed 7100# to 7162# and would have performed just fine for '41 and '42, see the P-39C at 7075# in wwiiaircraftperformance.org. The P-39M/N/Q with the later -85 engine with the 9.6 gears didn't really need a weight reduction but sure didn't need the drag of the gondola .50s on the Q. Growth of the P-39 could have consisted of the -93 engine with mechanical second stage supercharger (in production from April '43) and, yes, substitution of 50 gallons of fuel in place of wing guns. No major structural changes were necessary and the result would have been a two stage P-39(R?) with 170 gallons of internal fuel at a weight under 8000#. Quite a hot rod considering the P-63 weighed 8900# and the P-51B weighed 9600# and didn't reach combat until December '43 (call it '44).
The P-63 was another puzzle, why expand the wing size to 248sqft and only put 132 gallons if fuel in it? Remove the drag inducing gondola .50 caliber MGs and virtually the entire wing except landing gear would be available for fuel. And three drop tank locations. Without the wing guns the P-63 was competitive with the P-51B. I guess the Russians didn't need more fuel. Any of these P-39/63s would easily outclimb the F4U or P-47 of 1943 at all altitudes. The AAF just didn't seem to want the obvious improvements available to the P-39.

XBe02Drvr · Aug 9, 2019

P-39 Expert said:
The AAF just didn't seem to want the obvious improvements available to the P-39.

They just didn't believe there could be a silk purse hiding in that sow's ear. Why throw good money after bad?
Can you spell S K E P T I C A L?
Cheers,
Wes

P-39 Expert · Aug 9, 2019

Right, Bell only built 9500+ P-39s and another 3300 P-63s. That's a lot of bad money.

pinsog · Aug 9, 2019

Where was the turbocharger located on the XP-39?

Kevin J · Aug 9, 2019

P-39 Expert said:
Right, Bell only built 9500+ P-39s and another 3300 P-63s. That's a lot of bad money.

Bell had one very very satisfied customer in the USSR, who preferred their product over the Curtis's one. Just think, without the USSR, maybe no decent Bell helicopters in the Korean and Vietnamese wars.

wuzak · Aug 9, 2019

pinsog said:
Where was the turbocharger located on the XP-39?

Under the engine.

pinsog · Aug 9, 2019

wuzak said:
Under the engine.

Thank you.

I think adding a turbocharger to a P39 would be far easier than changing to a Merlin.

Put the turbocharger back under the engine behind the radiator. Delete wing intakes for radiator and replace them with another fuel tank in the wing roots. Belly scoop replaces wing root intakes. Intercooler on the sides behind engine with scoops like on P38. Probably be slower down low but should still be really fast up high. 2 50's and a 20mm. Wouldnt be the best against fighters due to engine location, but should be great against bombers with engine in the back.

Shortround6 · Aug 9, 2019

There were not a bunch of Merlins sitting around in crates waiting to be used. Allocations of engines were often made months if not a year prior to actual construction/installation in an aircraft. The US used up the vast majority of it's initial allotment of Merlin engines (3000 out of the first 9000 made) on the P-40F and L, there may have been a few hundred or so that the US let the British have, exact number of spares is not common knowledge. Neither is the numbers involved in the follow up contracts. In 1942 and early 1943 every Merlin powered P-39 is a Merlin powered P-40 NOT built. For the Merlin P-38 fans every early Merlin P-38 is TWO Merlin P-40s not built. That contract was fulfilled in the Spring of 1943 and Packard went on to build well over 40,000 more Merlins for both the British Commonwealth and the US.

There was a very real shortage of two stage Merlins in the late spring, early summer of 1943 with several hundred P-51 airframes waiting for engines. Packard was cranking out single stage engines at a rate over 35% higher than over the winter of 1942/43 but most of those were going to the British.

It is going to take a lot of rework to get a Merlin into a P-39, the motor mounts are the least of the problems. You need to change the supercharger cover which is the carb mount and inlet elbow so you can use a downdraft carb. otherwise you are mucking about with the radiator flap/door and a bottom inlet that is guaranteed to pick up every rock, pebble, piece dirt, stray nut or bolt on the whole airfield. You need to change the sizes of the radiator and oil coolers and that is for a single stage Merlin. The list goes on.

Shortround6 · Aug 9, 2019

pinsog said:
Thank you.

I think adding a turbocharger to a P39 would be far easier than changing to a Merlin.

Put the turbocharger back under the engine behind the radiator. Delete wing intakes for radiator and replace them with another fuel tank in the wing roots. Belly scoop replaces wing root intakes. Intercooler on the sides behind engine with scoops like on P38. Probably be slower down low but should still be really fast up high. 2 50's and a 20mm. Wouldnt be the best against fighters due to engine location, but should be great against bombers with engine in the back.

Bell made two mock ups of quicky installations of turbos on existing P-39s, There was NO Probably be slower down low. They were both 30-40mph slower under 15,000ft. Maybe a better more integrated job could have done better but getting high altitude performance at the cost of being out run by Zeros at low altitude doesn't really gain you much.

pinsog · Aug 9, 2019

Shortround6 said:
Bell made two mock ups of quicky installations of turbos on existing P-39s, There was NO Probably be slower down low. They were both 30-40mph slower under 15,000ft. Maybe a better more integrated job could have done better but getting high altitude performance at the cost of being out run by Zeros at low altitude doesn't really gain you much.

Agreed. Any pics of those? I assume you mean later model P39's like P39D or later?

Shortround6 · Aug 9, 2019

Somewhere in many threads on the P-39 there is at least one of them, One mock up put the turbo and intercooler in pod on the centerline replacing the drop tank or bomb.
It is a mock up so they never fitted the intended exhaust manifolds/pipes.

The second one was truly ugly with most of the hardware over the engine (blocking the rear view) with a large scoop on each side (Dumbo ears).

pinsog · Aug 9, 2019

Shortround6 said:
Somewhere in many threads on the P-39 there is at least one of them, One mock up put the turbo and intercooler in pod on the centerline replacing the drop tank or bomb.
It is a mock up so they never fitted the intended exhaust manifolds/pipes.

The second one was truly ugly with most of the hardware over the engine (blocking the rear view) with a large scoop on each side (Dumbo ears).

That defies explanation. Did Bell employ engineers? Did they fire them and replace them with children with crayons? That looks like the losing entry to 'draw a new Star Wars ship' in a kindergarten class.

swampyankee · Aug 9, 2019

pinsog said:
That defies explanation. Did Bell employ engineers? Did they fire them and replace them with children with crayons? That looks like the losing entry to 'draw a new Star Wars ship' in a kindergarten class.

The XP-39's turbo installation was, as I recall very bad, as in with such high drag that removing it improved performance and operational ceiling. Turbo installations of the time were quite bulky, as evidenced by the P-47 or, more closely applicably, the P-38. Maybe Bell did have trouble getting engineers. After all, there was a bit of demand for engineers at the time, what with the P-51, F4U, TBF, P-47, ....

Shortround6 · Aug 10, 2019

pinsog said:
That defies explanation. Did Bell employ engineers? Did they fire them and replace them with children with crayons? That looks like the losing entry to 'draw a new Star Wars ship' in a kindergarten class.

It was an attempt to fit a turbo with the least amount of change to the existing aircraft (disruption of production) but fitting turbos in small aircraft was not easy.

Zipper730 · Aug 10, 2019

Shortround6 said:
I am taking a wild guess but manifold pressure would be measured at the manifold (after the supercharger and/or intercooler on the two stage Merlins) and before the intake port on the cylinder head.

It instinctively sounds right, but things aren't always how you'd expect.

Also, this is something that I'm not sure I got a clear answer on: Was it known at the time (not now) that a higher-geared supercharger (i.e. one for higher altitude) would need an exponential amount of power (and structural strength) to provide the forces to drive it by engineers at the time?

Zipper730 · Aug 10, 2019

Now this is kind of just being silly, so it sounds completely retarded, I won't be all that surprised. I remember the sidewinder-supercharger arrangement seen on the F4U-5, and I'm wondering when the first time somebody thought of that idea was, and if anybody thought of a turbocharger that ran sideways? That or a twin-turbo?

XBe02Drvr · Aug 10, 2019

Zipper730 said:
Was it known at the time (not now) that a higher-geared supercharger (i.e. one for higher altitude) would need an exponential amount of power (and structural strength) to provide the forces to drive it by engineers at the time?

When you're dealing with a compressible fluid such as air, just about all parameters are going to be exponential in nature. That's never been a secret. Now a precise determination of the strength and likely breaking point of those gears may have required more calculating power than was available in those pre-computer days. Also, all humans, even mighty engineers, are capable of "brain farts".
Cheers,
Wes

pinsog · Aug 10, 2019

Shortround6 said:
It was an attempt to fit a turbo with the least amount of change to the existing aircraft (disruption of production) but fitting turbos in small aircraft was not easy.

My response was tongue in cheek, but I know you know that. The pic you showed, in my opinion, is such a poor design for adding a turbo that it really, to me, seems like a 'why even bother' attempt. Literally 'let's glue a turbocharger to the belly' sounds like a joke one of the engineers told at the bar after too many beers. But I do understand they were lacking space in an already small airframe and didn't want to slow production

wuzak · Aug 11, 2019

Zipper730 said:
Also, this is something that I'm not sure I got a clear answer on: Was it known at the time (not now) that a higher-geared supercharger (i.e. one for higher altitude) would need an exponential amount of power (and structural strength) to provide the forces to drive it by engineers at the time?

Yes.

wuzak · Aug 11, 2019

Zipper730 said:
Now this is kind of just being silly, so it sounds completely retarded, I won't be all that surprised. I remember the sidewinder-supercharger arrangement seen on the F4U-5, and I'm wondering when the first time somebody thought of that idea was, and if anybody thought of a turbocharger that ran sideways? That or a twin-turbo?

There is this drawing from Flight of a Merlin fitted with turbos.
Rolls-Royce Merlin XX Turbo-Supercharger Cutaway Drawing | #4956259 | Media Storehouse Print Store

Not sure if it was an actual Rolls-Royce proposal, or something Flight came up with on their own.

The B-29 had two turbochargers, one either side of the nacelle, behind the engine.

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