F4U-5 Supercharger Question

[BuzzLightyear]

The -5 Corsair was equipped with the excellent P&W R-2800-32W radial with the "Sidewider" auxillery supercharger.

Graham White's R-2800: Pratt and Whitney's Dependable Masterpiece (page 201) indicates that the the -32W used two stages of supercharging: an auxillery stage and main stage. The main stage supercharger always rotated at a fixed ratio. And as with other P&W two stage engines, the auxillery stage could be slected in neutral, low blower and high blower.

Hmmm, that suggests the auxillery stage was a two-speed unit like as other Corsair engines had.

However, in the same book (page 267) the blower ratio is shown to be Main: 6.7:1 min, Main: 9.6:1 max.. This seems misleading. First of all, the main stage rotated at a fixed ratio. So, do these numbers really represent the range of speeds of the auxillery blower? I think they do. And if they do, it suggests the auxillery stage was a variable speed unit, not a two speed unit.


Now, the F4U-5 Pilot's Handbook (section 1-2) says the airplane is powered by...with an automatically controlled two-stage, variable speed supercharger.

So, am i correct to assume that White is in error about the nature of the -32W's auxillery stage supercharger, and that it really is a variable speed unit with a minimum ratio of 6.7:1 and a maximum ratio of 9.6:1?

[GregP]

Hi Buzz,

Why don't you discuss blowers a bit with us.

A supercharger does the same thing a turbocharger does: it compresses air for use by the intake system. The supercharger is driven by the crank via gears or a belt, and a turbocharger has a thing called a waste gate that really is a vavle in the exhaust that routes a certain amount of the exhaust (or perhaps all of it) through a turbine on a common shaft with a second turbine that is used to compress the intake charge, usually beforer it is mixed with fuel.

There are 3 types of superchargers: screw type, roots, and centrifugal. Roots uses two interlocking rotating bars to compress air and is most often found today on American NHRA Top Fuel Dragsters. Screw type superchargers use a big screw inside a tight-fitting housing that turns toward the intake to move air, and centrufugal superchargers are basically a turbocharger without the waste gate. The crank drives the turbine to compress the intake charge.

An engine is an air pump, pure and simple, nothing more. If you pump more air through it, you get more power. Air and fuel need to be mixed in a certain ratio. Usually between 10:1 and 14.7:1 is about right, depending on whether you want to run rich or lean. 10:1 is rich; that is, 10 parts air to one part fuel. 14.7 to 1 is lean; same gas but more air.

My chevrolet Camaro SS runs about 13.5:1 or so and runs very well, thank you. The mixture stays pretty constant from idle to over 6000 rpm.

My question comes in here soon.

First, I have flown in a WWII T-6 Texan trainer. It has a 600HP radial on it, but it is not supercharged or turbocharged. I have ridden in a P-51, but was not briefed on Merlin operation, so I don't know if it was supercharged or not. We never got above 10,000 feet anyway.

The F6F Hellcat and the F4U COrsair used the same powerplant and propeller for a great deal of their service lives, the P&W R-2800. I understand they both had three conditions in which they could be run: Main stage, low blower, and high blower.

My own understanding is as follows:

Main stage is unsupercharged, that is the blower was running, but was routed away from the intake. In the Main stage, I understand the Corsair used ram air to get more power, but suffered carburetor icing more often as a result. The Hellcat used either an updraft or a downdraft carb and did NOT use ram air in the Main stage because Grumman was concerned about carb ice.

Low blower is a supercharged stage. So is high blower. It was my understanding that both of these employed the same impeller, but used different gears to drive it ... more slowly for low blower and faster for high blower. Makes sense to me.

Question: Exactly what IS a 2-stage supercharger anyway?

A 2-Speed supercharger seems simple ... change gears for the compressor turbine. But 2-stage?

Could you give a SIMPLE explanation that denottes the difference between 2-stage and 2-speed?

Thanks!

[BuzzLightyear]

In simplest terms, two stage supercharging means the engine has the benefit of two blowers/compressors - in some arrangement or another. It really just boils down to that. I'll write in terms of US WWII aero engines only.



Single stage supercharger:

This type of supercharger is behind the engine and was geared to it. It runs whenever the engine runs - sometimes at a constant ratio (as in a single stage/single speed(ratio) setup) or at 2 ratios (as in a single stage/two speed(ratio) setup). The Allison V-1710 in the P-39, P-40, P-51/P-51A used this type of supercharger - the Allison V-1710 was of the single stage/single speed variety.

The F4F Wildcat's R-1820 also used this type of supercharger, although it was of the single stage/two speed variety. Additionally, the P-40F with the Merlin V-1650 engine used a single stage/two speed supercharger.






Two stage supercharger:

This type of supercharging consists of an engine stage supercharger as described above, but before the air reaches it, it passes through a 1st stage supercharger - or auxillery stage supercharger.

There were 3 different types of auxillery supercharging on US fighter:

The first was the setup used on the Merlin P-51. Its supercharger used two blower discs, one behind the other, mounted on a common shaft. As a result, they both turned at the same speed.

The second was the variety used in the F6F and F4U. These planes had an engine stage supercharger that always ran at a fixed ratio, but there was a second blower as well that which either didn't run, or did run at 2 different ratios depending on altitude. Air was fed thru this supercharger before it reached the engine stage supercharger.

And last, but not least, was the turbosupercharger. Plane that used these - P-47, P-38 - both had the engine stage supercharger, but before air reached it, it was fed fed through and coimpressed by the turbosupercharger.

That's about it. As I'm sure you know, the planes that used 2 stage supercharging also used various types of intercoolers or aftercoolers, and fuel was mixed in to the air at different points.

Do I pass?

[GregP]

Yup.

And thanks, Buzz.

I was curious since a supercharger is so simple (as is a turbocharger), I wondered why we would complicate it with multiple stages. The answer is more power.

If I am not mistaken, the practical limit for piston engines made for aircraft use was about 6500HP, after that level, you were using the extra power to haul the weight of the engine around.

The practical limit for NON-aircraft use, however, is enormous. Most supertanker oil tankers use deisel engines of 6 cylinders, but the bore and stroke are 6 feet by 7 feet, so the displacement is huge. These "big" engines also get turbocharged, and the blower for them is VERY interesting.

[Wuzak]

It was my understanding that the two stages of the Merlin superchargers (Merlin 61 on) had different gearing. They then had a high and a low speed operation.

Also that the turbocharged Allisons (and R2800s I guess) replaced the single stage supercharger with a single stage turbosupercharger.

Can anybody confirm or deny?

[Mark J]

As far as I know, the Merlin 60 series had a two stage supercharger comprising two compressor vanes in series on the same shaft. (They may have been of different size in a conical shaped housing which could explain the stage gearing idea but I don't think so). The supercharger was driven by a two speed gearbox.
Not sure about the Allisons.

cheers

[DoBravery]

Greg,
If cars were planes, we'd be in the same squadron.

1994 Camaro Z-28
:)