XP-39 Wind Tunnel Tuft Tests

[Zipper730]

Yes, some versions of the V-1710 two stage engine did.

What aircraft used these?

But definitely not for the air cooled engines.

That makes sense

 

[wuzak]

XP-51J?
F-82 maybe.

 

[Shortround6]

I would note that trying to use the cooling system of the engine is pretty much useless to the intercooler. The US was trying to reduce the temperature of the air entering the Carburetor on turbo-ed engines to a max of 100 degrees F. They didn't always meet this goal but trying to use engine coolant wasn't going to work as engine coolant was usually near boiling even when cruising. A liquid intercooler needs it's own pump, radiator and lines.
On the P-51 normal carb inlet temperature was 15C to 40C normal with 50C max. Coolant temp was 60 C minimum for take-off, 100-110C normal and 121 C max or 125C max for -9 engines.

 

[Zipper730]

Surface cooling only worked for racing planes.

It was sort of a joke, though (from a purely intellectual standpoint) I was curious how much drag it would reduce.

Poor damage resistance and high maintenance would definitely qualify: Still, from an intellectual standpoint, how much drag does it reduce?

the two mock ups Bell tried with add on turbos were 30-40mph slower than standard P-39s at low altitude.

The P-39 could do how much at sea-level?

Even cutting that to 20mph with a lot of attention to detail leaves the P-39 with little speed advantage at low altitude over the Zero.

Yeah, it's almost the same

Liquid cooling helps with packaging, it is a lot easier to run a few fluid filled lines than large air ducts.

The idea was based on smaller size: A pound and a half of bricks versus a pound of cotton.

Yes the intecooler may be able to be made smaller but now you need the intercooler radiator and you need XXX number of pounds per minute of cooling air to cool the inter-cooler fluid.

Worked decently on the P-51 and Spitfire…

I would note that trying to use the cooling system of the engine is pretty much useless to the intercooler. The US was trying to reduce the temperature of the air entering the Carburetor on turbo-ed engines to a max of 100 degrees F. They didn't always meet this goal but trying to use engine coolant wasn't going to work as engine coolant was usually near boiling even when cruising. A liquid intercooler needs it's own pump, radiator and lines.

The Merlin 60's did this right?

On the P-51 normal carb inlet temperature was 15C to 40C normal with 50C max. Coolant temp was 60 C minimum for take-off, 100-110C normal and 121 C max or 125C max for -9 engines.

Quite a difference in temperature...

 

[wuzak]

I assume this principle is based on reducing pressure and increasing speed correct?

The idea was to reduce pressure drop and this was achieved by using a divergent/convergent duct.

The duct leading to the radiator would be divergent, that is its cross sectional area would increase. Thus the air flow velocity would be reduced and the pressure drop across the cooler reduced. Then the outlet duct is convergent, that is the area reduces. That way the heated air from the radiator is accelerated and, hopefully, will provide some thrust.

This also has the benefit of keeping the air in contact with the radiator for longer, thus increasing heat transfer.

The radiator was a compromise between heat transfer and minimal pressure drop. You could reduce the latter by having a thinner radiator with wider spaced cooling passages/fins, but then you don't get the required heat transfer.

Duct design was important. if the divergence is too sever you will get flow separation and turbulence, reducing the effectiveness of the installation.

The RAE tested the Spiteful radiator and found issues with the boundary layer (the installation had no boundary layer bleed off) and separation, and that the top 20% of the radiator was not cooling at all.

They tried several different methods to improve the system. One was to lengthen the intake section, which had one of the best reductions in internal drag - but that was completely offset by an external drag increase.

Some systems were tried with boundary layer bleeds. One went across the top of the duct from front to back. Another released the boundary layer air through teh upper surface of the wing - drag was reduced, but so was the lift of the wing.

The best solution they found was guide vanes before and after the radiator, at about 20-25% of radiator height (from the top).

The Merlin 60's did this right?

No. The 2 stage Merlins and Griffons used a water/glycol mix for the intercooler fluid, but they did not use the engine coolant. The engine cooling system was separate from the intercooler system.

If the intercooler system was hit the engine could still continue, but boost had to be lowered.

 

[Zipper730]

The idea was to reduce pressure drop and this was achieved by using a divergent/convergent duct.

The duct leading to the radiator would be divergent, that is its cross sectional area would increase. Thus the air flow velocity would be reduced and the pressure drop across the cooler reduced.

Wait, I thought a divergent shape would cause a gain in pressure similar to a bell-mouth, not reducing a drop...

Then the outlet duct is convergent, that is the area reduces. That way the heated air from the radiator is accelerated and, hopefully, will provide some thrust.

Like a ramjet

This also has the benefit of keeping the air in contact with the radiator for longer, thus increasing heat transfer.

So slowing it allows more interaction with the radiator and that produces better transfer of heat, because the slower airflow also has higher pressure it's compressed more so when it's heated, it expands out with more force. If it's slowed too much, it won't be able to provide sufficient coolant (air) through the duct rapidly enough to carry away the heat, and if the compression makes excessive heat, it won't be cool enough to absorb the radiator heat?

The radiator was a compromise between heat transfer and minimal pressure drop. You could reduce the latter by having a thinner radiator with wider spaced cooling passages/fins, but then you don't get the required heat transfer.

I figure you'd want nice thin sheets spaced closely together for maximum heat-transferring surface area...

Duct design was important. if the divergence is too sever you will get flow separation and turbulence, reducing the effectiveness of the installation.

Okay

The RAE tested the Spiteful radiator and found issues with the boundary layer (the installation had no boundary layer bleed off) and separation, and that the top 20% of the radiator was not cooling at all.

When you say no boundary layer bleed off, do you mean like a splitter or diverter?

They tried several different methods to improve the system. One was to lengthen the intake section, which had one of the best reductions in internal drag - but that was completely offset by an external drag increase.

Some systems were tried with boundary layer bleeds. One went across the top of the duct from front to back. Another released the boundary layer air through teh upper surface of the wing - drag was reduced, but so was the lift of the wing.

The best solution they found was guide vanes before and after the radiator, at about 20-25% of radiator height (from the top).

Was this known to be an effective solution before the 1944?

No. The 2 stage Merlins and Griffons used a water/glycol mix for the intercooler fluid, but they did not use the engine coolant. The engine cooling system was separate from the intercooler system.

If the intercooler system was hit the engine could still continue, but boost had to be lowered.

Redundancy?

 

[tomo pauk]

XP-51J?
F-82 maybe.

V-1710 as installed in the P-82s were not intercooled, that was one of reasons they were so unreliable when trying to make promissed 2200 HP. Also a major reason for not being capable to beat Merlin 61 from 1942 with those V-1710s even in 1945 in altitude power.
Only the XP-51J was to feature intercooled 2-stage V-1710s.

 

[P-39 Expert]

I am always amazed at the idea that the P-39 could fit a two stage supercharger when the Bell engineers lengthened the fuselage by about two feet to make the XP-39E and the P-63. The XP-39E was actually ordered as the P-76 to the tune of 400 aircraft but contract was later cancelled or transfered to the P-63.
If the Bell engineers figured they needed bigger fuselage ( and relocated wing) to make it work I don't know why people think it would have been so easy to stuff the needed components in the existing P-39.

Not trying to argue, but the space for the engine/second stage mechanical supercharger in the P-63 is exactly the same size as the P-39. The P-63 moved the coolant tank from behind the engine to up behind the pilot's seat above the engine and put the second stage blower where the coolant tank had been. Same thing for the XP-39E. The fuselage was lengthened but it was the tail cone aft of the coolant tank, not the engine compartment.

 

[Shortround6]

P-39 fuselage drawing.

P-39 engine

P-63 engine. Please note the collection of machinery spaced off the rear of the engine.
Also please note the relocated position of the cockpit and engine of the P-63 compared to the P-39.

Bell had two chances at redoing the P-39, the P-39E (also with a longer fuselage and many other changes) and the P-63 (different wing too). and yet we modern day engineers are so much smarter that we can fit stuff in where large numbers of full time engineers could not??

 

[Shortround6]

P-39, note where the upward angle starts at the rear of the engine.

P-63

looks like the coolant tank was in front of the engine and not on top of it.

 

[Zipper730]

Not trying to argue, but the space for the engine/second stage mechanical supercharger in the P-63 is exactly the same size as the P-39. The P-63 moved the coolant tank from behind the engine to up behind the pilot's seat above the engine and put the second stage blower where the coolant tank had been.

Just out of curiosity, why this change in position?

 

[P-39 Expert]

View attachment 489169
P-39 fuselage drawing.
View attachment 489170
P-39 engine
View attachment 489171
P-63 engine. Please note the collection of machinery spaced off the rear of the engine.
Also please note the relocated position of the cockpit and engine of the P-63 compared to the P-39.

Bell had two chances at redoing the P-39, the P-39E (also with a longer fuselage and many other changes) and the P-63 (different wing too). and yet we modern day engineers are so much smarter that we can fit stuff in where large numbers of full time engineers could not??

Here are two drawings, one of the P-39 and one of the P-63. The distance from the front of the engine compartment to the back are both exactly 90.25 inches. They did lengthen the rear fuselage but it was aft of the engine compartment, not within the engine compartment.

They also did move the wing back a bit (or the fuselage up) on the P-39E and P-63 necessitating the lengthened tail cone for balance and stability.

 

[P-39 Expert]

Just out of curiosity, why this change in position?

They had to move the coolant tank in order to put the second stage supercharger there.

 

[Zipper730]

They had to move the coolant tank in order to put the second stage supercharger there.

No, I get that, what I'm wondering is if there was physical room off the bat to position the coolant tank forward, and arrange a turbocharger aft of the engine: Why did they place the turbo under the engine from the get-go?

 

[Shortround6]

There were more than just space/volume considerations.
weight/center of gravity comes into it. As does ducting, both exhaust and inlet plus inter-cooler

 

[P-39 Expert]

No, I get that, what I'm wondering is if there was physical room off the bat to position the coolant tank forward, and arrange a turbocharger aft of the engine: Why did they place the turbo under the engine from the get-go?

I believe so since the size of the engine compartment in both the P-39 and P-63 were almost exactly the same, and the P-63 moved the coolant tank up just ahead of the engine while the P-39 still had it behind the engine.

 

[Zipper730]

There were more than just space/volume considerations.
weight/center of gravity comes into it.

Do you have any idea how much the turbo would weigh?

As does ducting, both exhaust and inlet plus inter-cooler

I'm curious if there was enough space to put the turbocharger behind the engine and still have enough room for ducting and intercooler spacing?

 

[Zipper730]

I should have found this already: Where was the engine radiator on the XP-39

 

[pbehn]

When discussing the Meredith effect, as with everything there is no free lunch, on full power the water pumps on a Merlin consumed 40BHP.

 

[P-39 Expert]

I should have found this already: Where was the engine radiator on the XP-39

Coolant radiator was in left (facing forward) wing leading edge. Did not have an adjustable outlet.