P-39 Turbocharged Prototype?

[Shortround6]

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.

 

[pbehn]

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.

 

[Zipper730]

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...

 

[wuzak]

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).

 

[Zipper730]

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...

 

[Shortround6]

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.

 

[MiTasol]

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

 

[Zipper730]

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".

 

[Zipper730]

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...

 

[Zipper730]

Message Deleted

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

 

[Zipper730]

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