P-39 Turbocharged Prototype? (1 Viewer)

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Feb 7, 2013
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.
xp39-17.jpg
 
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!
 
What was the wing-area difference of the XP-39 and regular P-39 variants?
 
Whoops, my mistake.
The NACA doc shows same wing span (34 ft) for the XP-39 as it was for plain-vanilla P-39s.
 
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

unit-jpg.jpg

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

All that said, as it appears that
  1. Air is drawn in through a pair of ducts on the lower fuselage
  2. Airflow goes through the turbocharger and is boosted in air-pressure
  3. 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
  4. The airflow is mixed with fuel just before entering the engine supercharger, providing an additional boost to the airflow.
  5. Fuel air-mixture is then routed to the cylinders, where they are compressed and burned
  6. 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
  7. 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
  8. 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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  1. 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)
  2. Presumable air-fuel mixture flows through the turbocharger and is compressed
  3. 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.

allison_v1710.jpg

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.
 
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).
 
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.
 
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?

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?
 
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:

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.


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.
 
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.
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
 
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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.


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.
 
the XP-39 used 4 pipes.

For the exhaust? Weird...

It's not actually weird.

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

VELAM12PPExhaust-2.jpg


maxresdefault.jpg


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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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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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

f4uis-gif.gif


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

b17_169.jpg


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.
 
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.
1.jpg

2.jpg
 

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