# P-39 Turbocharged Prototype?



## MilitaryAttractions (Aug 14, 2016)

I'm interested in learning more about the XP-39 prototype that used a turbocharger. I'm told that the additional equipment (intercoolers and exhaust manifolds I guess) created too much drag. Does anyone have any more information on this? Any pictures and diagrams that show the system in detail?

about the only thing i've been able to find is a few pictures like this one that show the addition of the air ducts on the side of the fuselage.


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## GregP (Aug 15, 2016)

This was all fairly well-documented in here some years back. You can try searching for it. I can't exactly recall where, but it started out in the regular aviation forum , not the technical forum.

Good luck!


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## tomo pauk (Aug 15, 2016)

This is the latest thread:
https://ww2aircraft.net/forum/threads/xp-39-wind-tunnel-tuft-tests.44989/

Also discussed here:
XP-39: pros cons

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## Zipper730 (Dec 13, 2017)

What was the wing-area difference of the XP-39 and regular P-39 variants?


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## tomo pauk (Dec 14, 2017)

Wing area was 213 sq ft for the XP-39, 197.7 sq ft for the P-39s.


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## Shortround6 (Dec 15, 2017)

I think you are getting confused. AHT shows a "gross" wing area of 213.22 sq ft for the P-39 series and a "Net" wing area of 197.7 sq ft.
Subtract area of the fuselage from the gross area to get the Net area ( almost impossible from drawings)

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## tomo pauk (Dec 15, 2017)

Whoops, my mistake. 
The NACA doc shows same wing span (34 ft) for the XP-39 as it was for plain-vanilla P-39s.


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## Zipper730 (Dec 15, 2017)

MilitaryAttractions said:


> I'm interested in learning more about the XP-39 prototype that used a turbocharger. I'm told that the additional equipment (intercoolers and exhaust manifolds I guess) created too much drag. Does anyone have any more information on this? Any pictures and diagrams that show the system in detail?


Here's the intercooler arrangement





The engine would be right between the exhaust troughs and the manifolds, above the turbo.

All that said, as it appears that

Air is drawn in through a pair of ducts on the lower fuselage 

Airflow goes through the turbocharger and is boosted in air-pressure

Air then flows through the supercharger air-cooler, which is mounted inside a duct on the aircraft's right side; outside airflow moving through the duct carries away the heat from the airflow before it reaches the engine
The airflow is mixed with fuel just before entering the engine supercharger, providing an additional boost to the airflow.

Fuel air-mixture is then routed to the cylinders, where they are compressed and burned
Heat from the engine is carried away by a liquid coolant, which flows through a radiator duct on the other side, where airflow carries the heat away, at which point it is routed back through the engine to repeat the cycle

Exhaust flow travels out of each of 12-cylinders, and is routed through four manifolds, which are then either allowed to bypass the turbine at low altitudes, or is directed through the bucket-wheel (a turbine), driving it

Airflow escapes the aircraft through the bottom (I'm not sure whether it is through four exhaust pathways, one, or some number between).
On the bright side it doesn't seem to require air purely for inter-cooling (most aircraft with inter-coolers draw air from another source, route it through the intercooler, and then get rid of it), though from what I was told, you need around 3 times the amount of airflow for cooling than just the carburetor

On the downside, the exhaust flow path seems far from the ideal: For starters, most turbochargers are located behind the engine, not right below. Other problems with the radiator's and supercharge air cooler appear to be a lack of diverter for each, and cowl-flaps to vary the area (and vlocity)


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## Shortround6 (Dec 15, 2017)

Zipper730 said:


> Air is drawn in through a pair of ducts on the lower fuselage (at what point fuel is introduced does not appear to be made)
> 
> Presumable air-fuel mixture flows through the turbocharger and is compressed
> Presumable air-fuel mixture then flows through the supercharger air-cooler, which is mounted inside a duct on the aircraft's right side; airflow through the duct carries away the heat from the airflow before it is directed into the V-1710's single-stage supercharger (not shown, but present)




All in error except the last line. The carburetor was mounted on the engine driven supercharger.





Carb is the black device mounted on the supercharger. There seems to be a box blocking off the carb. 



> 7.Airflow escapes the aircraft through the bottom (I'm not sure whether it is through four exhaust pathways, one, or some number between).


the XP-39 used 4 pipes. 




> On the bright side it doesn't seem to require air purely for inter-cooling (most aircraft with inter-coolers draw air from another source, route it through the intercooler, and then get rid of it), though from what I was told, you need around 3 times the amount of airflow for cooling than just the carburetor



Not sure what you mean here, the XP-39 used an air to air intercooler. Unfortunately it was nowhere near big enough. Especially for climbing. Best climb speed for most fighters was under 1/2 of max speed and that means 1/2 the cooling air per minute going through the inter-cooler.


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## wuzak (Dec 16, 2017)

Zipper730 said:


> On the bright side it doesn't seem to require air purely for inter-cooling (most aircraft with inter-coolers draw air from another source, route it through the intercooler, and then get rid of it), though from what I was told, you need around 3 times the amount of airflow for cooling than just the carburetor



What are you talking about?

The intercooler duct feeds only the intercooler.

Coolant radiators are located elsewhere, as are the cold air intakes (shown on your diagram).


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## wuzak (Dec 16, 2017)

Shortround6 said:


> Not sure what you mean here, the XP-39 used an air to air intercooler. Unfortunately it was nowhere near big enough. Especially for climbing. Best climb speed for most fighters was under 1/2 of max speed and that means 1/2 the cooling air per minute going through the inter-cooler.



The intercooler undercooled at climb speeds but overcooled at high speeds.

They managed to make it not work over a wide range of conditions.


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## Zipper730 (Dec 16, 2017)

Shortround6 said:


> All in error except the last line. The carburetor was mounted on the engine driven supercharger.


So the fuel was mixed in at that point?


> the XP-39 used 4 pipes


For the exhaust? Weird...


> Not sure what you mean here, the XP-39 used an air to air intercooler. Unfortunately it was nowhere near big enough. Especially for climbing.


The airflow path way seemed to be from the underside to the turbo, through the cooler...


> Best climb speed for most fighters was under 1/2 of max speed and that means 1/2 the cooling air per minute going through the inter-cooler.


I thought the airflow increased to the square of velocity?



wuzak said:


> What are you talking about?
> 
> The intercooler duct feeds only the intercooler.


I figured the airflow to the engine was drawn in through the cold air ducts, then was compressed by the turbo, then went through the cooler, and to the engine: I was under the impression that most coolers that were air-to-air drew in air from the outside, ran it through the structures in the cooler (which would carry away the heat), so as airflow went across them, it would be cooled down...


> The intercooler undercooled at climb speeds but overcooled at high speeds.


You'd fix that with a flap right?


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## wuzak (Dec 16, 2017)

Zipper730 said:


> I figured the airflow to the engine was drawn in through the cold air ducts, then was compressed by the turbo, then went through the cooler, and to the engine: I was under the impression that most coolers that were air-to-air drew in air from the outside, ran it through the structures in the cooler (which would carry away the heat), so as airflow went across them, it would be cooled down...



Which is correct, but not what you said:



Zipper730 said:


> n the bright side it doesn't seem to *require air purely for inter-cooling (most aircraft with inter-coolers draw air from another source, route it through the intercooler, and then get rid of it)*, though from what I was told, you need around 3 times the amount of airflow for cooling than just the carburetor



The other source is the outside air, which is routed through the inetrcooler.




Zipper730 said:


> You'd fix that with a flap right?



It would need a variable outlet, a variable inlet, or both, to control the mass flow through the intercooler. The size of the intercooler is possibly too small, the control mechanism (most likely an outlet flap) would open up to allow cooling at low speed and high power, and then close down to make sure the airflow is the right amount and not too much.


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## Zipper730 (Dec 16, 2017)

wuzak said:


> Which is correct, but not what you said


I made a guess looking at the diagram (kind of followed the path of where the flow would go)


> The other source is the outside air, which is routed through the inetrcooler.
> 
> 
> > The size of the intercooler is possibly too small, the control mechanism (most likely an outlet flap) would open up to allow cooling at low speed and high power, and then close down to make sure the airflow is the right amount and not too much.
> ...


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## wuzak (Dec 16, 2017)

Zipper730 said:


> The intercooler air came from the cold-air intake? If so, I'm guessing 3/4 went to cooling and the remaining 1/4 to the engine?



The cold air intakes were shown in the diagram you presented facing forward and feeding the turbo.

The intercooler was mounted to the side at the rear of the engine.

The intercooler had its own duct. I don't believe that the intake air for the turbo came from that duct, and the diagram seems to confirm this.

So 100% of the air that went into the cold air ducts went into the engine. And 100% of the air entering the intercooling duct was feed through the intercooler.




Zipper730 said:


> So the solution would have been to put a diverter in place; then enlarge the cooler, and draw additional cooling air for this task; then add flap to control the airflow



Diverter?

A properly shaped duct would help - a divergent duct leading to the intercooler and a convergent duct after the intercooler with a adjustable door to control the mass flow.

Like the radiator ducts of the Mustang. Or even the Spitfire (which had a two position outlet).

The duct probably also required a boundary layer splitter or bleed.


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## wuzak (Dec 16, 2017)

Shortround6 said:


> the XP-39 used 4 pipes.





Zipper730 said:


> For the exhaust? Weird...



It's not actually weird.

For a V12 the exhausts are often grouped in sets of three.











This is because of the gas dynamics and the firing order.

The problems with the XP-39 design are that there are sharp corners in the ducting, which causes pressure losses in the system, which means the exhaust has less energy when it gets to the turbine wheel.

A similar issue is in the intercooler to engine loop.

There is a sharp corner with a change in cross-section feeding the intercooler and another at the outlet. There is a further sharp corner where the intercooler outlet feeds into the engine carburetor. All causing a pressure loss, as will the intercooler.

This will all cost performance.

It is also to be noted that the elbow that fed air from the carburetor into the V-1710's supercharger was also quite sharp and cause performance losses.

The early Merlin superchargers had a similar issue, but once the elbow was made less tight there was a few thousand feet gain in full throttle height.

Note that the XP-37 had a different set-up, although it and the XP-39 used the same turbo, at least originally. The turbo was mounted under the engine, as in the XP-39, but the exhaust was ducted forward to the front of the engine by a single header on each side, after which the right hand header crossed over the nose case (the long nose case) to meet the left hand header and form a single exhaust, which went down to the turbo.

Note also that the exhaust arrangement was left to the airframe manufacturers. Allison did not do it, instead supplying flange plates for the exhausts, to which the manufacturers would add their header/ejector exhausts.

And Allison did not run a turbo with a V-1710 on the test bench before the XP-37 flew, and maybe not before the XP-39 flew.

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## Zipper730 (Dec 16, 2017)

wuzak said:


> The cold air intakes were shown in the diagram you presented facing forward and feeding the turbo.


Yes


> The intercooler was mounted to the side at the rear of the engine.


Yes


> The intercooler had its own duct. I don't believe that the intake air for the turbo came from that duct, and the diagram seems to confirm this.


I see that airflow went through the intercooler duct from front to back, but I was under the impression that most intercoolers would tap air from another location (say the wing's leading edge), and route it through the air-passageways in the duct, so that when the airflow from the first stage of supercharging flows through the intercooler on it's way to the engine, it's cooled down before being compressed by a second stage...


> Diverter?


A separation between the airframe and duct to remove turbulent airflow.


> A properly shaped duct would help - a divergent duct leading to the intercooler and a convergent duct after the intercooler with a adjustable door to control the mass flow.


Yup


> Like the radiator ducts of the Mustang. Or even the Spitfire (which had a two position outlet).


Yes, though the P-38's scoops look like they'd be decent candidates too.


> Note that the XP-37 had a different set-up, although it and the XP-39 used the same turbo, at least originally. The turbo was mounted under the engine, as in the XP-39, but the exhaust was ducted forward to the front of the engine by a single header on each side, after which the right hand header crossed over the nose case (the long nose case) to meet the left hand header and form a single exhaust, which went down to the turbo.


The left and right side formed one duct which fed the turbo?


> Note also that the exhaust arrangement was left to the airframe manufacturers.


That's interesting!


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## wuzak (Dec 16, 2017)

Zipper730 said:


> I see that airflow went through the intercooler duct from front to back, but I was under the impression that most intercoolers would tap air from another location (say the wing's leading edge), and route it through the air-passageways in the duct, so that when the airflow from the first stage of supercharging flows through the intercooler on it's way to the engine, it's cooled down before being compressed by a second stage...



Some aircraft fed air from the leading edge to the air to air intercooler.

Such as the F4U






But the intercoolers were right there, near the wing root.

The B-17 did too, but the intercooler was in the nacelle, behind teh spar






A duct from the wing leading edges to the intercooler on the XP-39 would have been difficult considering the position of the intercooler and the things in the way, such as the engine.

The intercooler duct was one of the big drag producers on the XP-39, that's why it, and the turbo, had to go.


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## wuzak (Dec 16, 2017)

wuzak said:


> Diverter?





Zipper730 said:


> A separation between the airframe and duct to remove turbulent airflow.



One would hope that the duct was not placed in a location where it sees much turbulent airflow.


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## MiTasol (Dec 16, 2017)

Maybe diagram 1 below for the standard production P-39, when used with the excellent diagram of Zippers in post 8, will help.
The green shows the airflow to the engine through the carburettor, and to the oil coolers. The turbocharger intakes and ducting would have been similar to the oil cooler intakes and ducting shown. Carburettors were always mounted on the engine in turbo installations. 
Diagram 2 below is the early P-38 installation.

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## Shortround6 (Dec 17, 2017)

Zipper730 said:


> So the fuel was mixed in at that point?


That is what carburetors do. 
They meter (add the appropriate amount) and mix the fuel and air. 



> I thought the airflow increased to the square of velocity?



Drag goes up with the square of the velocity, airflow goes up in proportion to the velocity. 

The XP-39 had an intercooler that was too small. In part because they were trying to keep the weight down (XP-39 was at least 10% over weight) . However this causes a large amount of pressure drop across the intercooler. Read drag for pressure drop. Any radiator, oil cooler or intercooler (or air cooled engine baffles) is going to have air moving slower/at lower pressure on the outward side than on the intake wide. However a low pressure drop means there is only a small reduction in the velocity of the air moving through the cooling device. The larger the pressure drop the larger the change in the speed of the airflow and the more drag. However this is only one measurement. Is a 2in pressure drop over 4 sq ft radiator better or worse than 4in pressure drop over 2 sq ft radiator? wer get into the squares of velocity and the fact that the edge areas of the cooling device don't work as well as the core (inner areas) so it is not that simple.

On the XP-39 they were hoping that the intercooler would remove 50% of the heat added by the turbo. What they got was a 25% reduction in heat added in level flight and a 12% reduction in climb. 

As an example lets say that the turbo added 100 degrees F to the intake air at 12,000ft. they were hoping to get that down to a 50 degree rise. What they got was a 75 degree rise in level flight and an 88 degree rise in climb. Since the hotter air meant they were running closer to detonation they had to limit the boost/power of the engine which hurt the performance. 
at around 23,000ft you might have had a 200 degree F temperature rise. which is only partially offset by the 39-40 degree drop in air temperature. 

Please remember that TWO stage supercharged Turbo aircraft and intercoolers were in their infancy at this time. Still in the cradle. While the US had built dozens of turbo charged planes at this point the vast majority had been single stage engines. Turbos added to an engine with no engine driven supercharger so the temperature rise wasn't as big a problem. There wasn't going to be a second supercharger adding another several hundred degrees to the intake charge.


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## pbehn (Dec 17, 2017)

I believe there is another aspect to the radiators performance. The air may be much colder at high altitude but there is much less of it, taken to the extreme the dark side of the moon is circa -173C but a radiator doesn't work at all.

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## Zipper730 (Dec 17, 2017)

Shortround6 said:


> Drag goes up with the square of the velocity, airflow goes up in proportion to the velocity.


I didn't know that...


> The XP-39 had an intercooler that was too small. In part because they were trying to keep the weight down (XP-39 was at least 10% over weight).


I did some checking and it would appear they expected a weight of 5550 pounds.


> On the XP-39 they were hoping that the intercooler would remove 50% of the heat added by the turbo. What they got was a 25% reduction in heat added in level flight and a 12% reduction in climb.


What was the typical efficiency figures seen at the time?


> As an example lets say that the turbo added 100 degrees F to the intake air at 12,000ft. they were hoping to get that down to a 50 degree rise. What they got was a 75 degree rise in level flight and an 88 degree rise in climb. Since the hotter air meant they were running closer to detonation they had to limit the boost/power of the engine which hurt the performance.


That I get just fine


> at around 23,000ft you might have had a 200 degree F temperature rise. which is only partially offset by the 39-40 degree drop in air temperature


So you see around 160-161 degrees...


> Please remember that TWO stage supercharged Turbo aircraft and intercoolers were in their infancy at this time. Still in the cradle. While the US had built dozens of turbo charged planes at this point the vast majority had been single stage engines.


So it was a naturally aspirated engine with ducting to drive the turbo, and feed the air through the turbo, then the engine?


> Turbos added to an engine with no engine driven supercharger so the temperature rise wasn't as big a problem. There wasn't going to be a second supercharger adding another several hundred degrees to the intake charge.


And intercooling...


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## wuzak (Dec 18, 2017)

Shortround6 said:


> However a low pressure drop means there is only a small reduction in the velocity of the air moving through the cooling device. The larger the pressure drop the larger the change in the speed of the airflow and the more drag.



A large pressure drop across the cooler probably means the speed through the cooler is larger. 

The aim, generally, is to slow the air down as much as possible in the duct (by using a divergent duct), and then speed it up again after (with a convergent duct).

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## Zipper730 (Dec 18, 2017)

wuzak said:


> A large pressure drop across the cooler probably means the speed through the cooler is larger.
> 
> The aim, generally, is to slow the air down as much as possible in the duct (by using a divergent duct), and then speed it up again after (with a convergent duct).


That makes sense...


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## Shortround6 (Dec 18, 2017)

The idea is that the speed of the air through the radiator matrix causes drag, and goes back to the drag increasing with the square of the speed. 
Yes, if you can slow the speed of the air going through the matrix you will get less drag, a lot less, You also need enough air (mass not volume) to do the cooling required flowing through the matrix and doing it at a speed that allows for the transfer of heat. 
the pressure drop is sort of a measure of the drag.


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## MiTasol (Dec 22, 2017)

For the benefit of those who do not know the basics of supercharging and turbocharging, that mechanical superchargers are an integral part of the engine *(except for Aux Stage Blowers)*, carburettor placement, etc, and the effect of engine and supercharging variables on airframe design I have uploaded a 1943 Allison publication with links to two other editions that covers all these subjects in a single 86 page Readers Digest size booklet.

Engine design as related to airplane power : with particular reference to performance at varying alt

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## Zipper730 (Jan 14, 2018)

tomo pauk said:


> Whoops, my mistake.


Okay, so the operational P-39's had a wing area of 197.7 square feet excluding the fuselage, 213.22 with the fuselage, and a wing-span of 34'0" feet?

What's the figures for the XP-39? It's area was larger and the wing was 35'10".


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## Zipper730 (Jan 14, 2018)

wuzak said:


> What are you talking about?


Basically, the amount of air drawn through the cold air passage fed the engine, and that air flowed through the intercooler duct. I was under the impression that inter-coolers needed about 2-3 times the airflow that would be required for the carburetor itself.

Except in this case, the air flowing through the intercooler duct probably was at least 2-3 times the amount of air that the cold air intake provided. I basically misunderstood what was being discussed.


> The intercooler undercooled at climb speeds but overcooled at high speeds.


So, if you were to venture a guess

Would you say an outlet flap would have been essential to have improved the cooling across the speed range?
If you can make an estimate on the depth of the intercooler depicted early in the thread, and from that how much additional area would be required?



> The intercooler was mounted to the side at the rear of the engine.


The right side, the left side on the XP-39 was the engine radiator


> A properly shaped duct would help - a divergent duct leading to the intercooler and a convergent duct after the intercooler with a adjustable door to control the mass flow.


That seems straight-forward.


> The problems with the XP-39 design are that there are sharp corners in the ducting, which causes pressure losses in the system, which means the exhaust has less energy when it gets to the turbine wheel.


Was there enough room in the aircraft for more curved ducts?


> A similar issue is in the intercooler to engine loop.


Was there enough room to admit more curvature?


> And Allison did not run a turbo with a V-1710 on the test bench before the XP-37 flew, and maybe not before the XP-39 flew.


The P-38 still managed to make it all work...


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## Zipper730 (Mar 24, 2018)

Message Deleted



FLYBOYJ said:


> .


I'm not sure how to tag you, but could you erase this post?


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## Zipper730 (Mar 24, 2018)

The Allison V-1710 was designed so that either an extra supercharger stage or a turbosupercharger could be bolted on right?


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