Turbo-supercharging and aircraft performance

[joeblogs]

Hohun pointed out there was no way to map from just an engine model number to the actual performance of a particular vintage of the P47C or P47D, which had many variations within these designations.

White's new book resolves this. He has tables with every variation of the plane. He has about 25 separete versions of the P47D for example, complete with performance specs.

As Hohun guessed, improvements in the GE turbo were introduced along the way, even though the Pratt & Whitney engine designation did not change. The first significant improvement to the turbo, plus ADI, emerges on the P47D-5-RE (Evansville). The P47D-10-RE goes to the R2800-63, which was specifically designed to incorporate ADI and there is another new model of the turbo.

The first significant performance change occurs with the P47D-22-RE, I suspect because the propeller length was increased nearly a foot.  The P47D-25-RE gets a bigger fuel tank, more ADI capacity, a bubble canopy and I believe loses the turtleback. The P47D-28 gets P-38 type dive flaps plus new ailerons. This is continued in the D30 model, which was the most numerous of the D varients.

And so it goes... Curtiss built some P47Gs, and these were regarded as dogs. The P47N gets an improved turbo, a lighter airframe, and a lot more gas. Several more improvements to the turbos are introduced throughout the N series.

-blogs

[Captain Virgil Hilts]

The P-38 had only a single stage single speed crank
driven centrifugal supercharger, there were no multi
speed superchargers on Allison engines, hence the
lack of power at high altitudes for anything but the
P-38 turbocharged engines.

The GE turbochargers were:

B-2  120#of air per minute 21,300 RPM
B-13 130# of air per minute 21,300 RPM
B-33 165# of air per minute 24,000 RPM

These same turbochargers were used on several different  planes, including the P-38 and the P-47, and on the the B-17
and the B-24 as well.

Each model was a detail improvement on the previous model, with improvements in efficiency and maximum turbine RPM. They
were all very similar, and in fact, to my knowledge, were interchangeable as far as the physical installation goes.
However, due to power differences, they had to be installed in matched pairs on the plane.

Unlike modern installations, the GE turbochargers in use then
were not specifically designed for a particular engine with a particular power level. They were designed around a general power level alone. The GE turbocharger was at best about 65% efficient, as opposed to the current efficiency of correctly sized turbochargers that reach or at least aproach 90%.

The limiting factor with the Allison was octane. The turbocharger provided air at a rate regulated to equal sea level. Allison actually slowed down the mechanical supercharger because it could make enough boost with the turbocharger to exceed the limitations of the available fuel. When they slowed the mechanical supercharger down, the RPM required of the turbine in the turbocharger to maintain sea level pressure increased, and exceeded the limit. The wastegate could not dump enough boost from the turbocharger to keep the pressure down at low altitudes, so the mechanical supercharger had to be slowed down.

[joeblogs]

At least for some of the production models, the P47 was getting a "C" series turbo. Improvements were often manifested in the control mechanism rather than the turbo itself.

-blogs

Originally posted by Captain Virgil Hilts
The P-38 had only a single stage single speed crank
driven centrifugal supercharger, there were no multi
speed superchargers on Allison engines, hence the
lack of power at high altitudes for anything but the
P-38 turbocharged engines.

The GE turbochargers were:

B-2  120#of air per minute 21,300 RPM
B-13 130# of air per minute 21,300 RPM
B-33 165# of air per minute 24,000 RPM

These same turbochargers were used on several different  planes, including the P-38 and the P-47, and on the the B-17
and the B-24 as well.

Each model was a detail improvement on the previous model, with improvements in efficiency and maximum turbine RPM. They
were all very similar, and in fact, to my knowledge, were interchangeable as far as the physical installation goes.
However, due to power differences, they had to be installed in matched pairs on the plane.

Unlike modern installations, the GE turbochargers in use then
were not specifically designed for a particular engine with a particular power level. They were designed around a general power level alone. The GE turbocharger was at best about 65% efficient, as opposed to the current efficiency of correctly sized turbochargers that reach or at least aproach 90%.

The limiting factor with the Allison was octane. The turbocharger provided air at a rate regulated to equal sea level. Allison actually slowed down the mechanical supercharger because it could make enough boost with the turbocharger to exceed the limitations of the available fuel. When they slowed the mechanical supercharger down, the RPM required of the turbine in the turbocharger to maintain sea level pressure increased, and exceeded the limit. The wastegate could not dump enough boost from the turbocharger to keep the pressure down at low altitudes, so the mechanical supercharger had to be slowed down.

[pasoleati]

Originally posted by Captain Virgil Hilts
The P-38 had only a single stage single speed crank
driven centrifugal supercharger, there were no multi
speed superchargers on Allison engines, hence the
lack of power at high altitudes for anything but the
P-38 turbocharged engines.

The GE turbochargers were:

B-2  120#of air per minute 21,300 RPM
B-13 130# of air per minute 21,300 RPM
B-33 165# of air per minute 24,000 RPM

These same turbochargers were used on several different  planes, including the P-38 and the P-47, and on the the B-17
and the B-24 as well.

Each model was a detail improvement on the previous model, with improvements in efficiency and maximum turbine RPM. They
were all very similar, and in fact, to my knowledge, were interchangeable as far as the physical installation goes.
However, due to power differences, they had to be installed in matched pairs on the plane.

Unlike modern installations, the GE turbochargers in use then
were not specifically designed for a particular engine with a particular power level. They were designed around a general power level alone. The GE turbocharger was at best about 65% efficient, as opposed to the current efficiency of correctly sized turbochargers that reach or at least aproach 90%.

The limiting factor with the Allison was octane. The turbocharger provided air at a rate regulated to equal sea level. Allison actually slowed down the mechanical supercharger because it could make enough boost with the turbocharger to exceed the limitations of the available fuel. When they slowed the mechanical supercharger down, the RPM required of the turbine in the turbocharger to maintain sea level pressure increased, and exceeded the limit. The wastegate could not dump enough boost from the turbocharger to keep the pressure down at low altitudes, so the mechanical supercharger had to be slowed down.

It might be worth checking where was the wastegate as the wastegate controlled the amount of exhaust gas to turbine, not the amount of induction air to carburettor.

And exactly how did Allison slow down (=reduce the rpm of the impeller) the mech stage? After all, the mech stage has fixed gear ratio...

[Captain Virgil Hilts]

Originally posted by pasoleati
It might be worth checking where was the wastegate as the wastegate controlled the amount of exhaust gas to turbine, not the amount of induction air to carburettor.

And exactly how did Allison slow down (=reduce the rpm of the impeller) the mech stage? After all, the mech stage has fixed gear ratio...

Yes, the wastegate controls the amount of exhaust to the turbine. But that in turn controls the amount of boost to the intake. At least it does on my turbocharged Buick TType Regal, and every other turbocharged setup I've ever worked on (a lot). By the wategate "dumping boost" I mean the wastegate slowed the turbine down. "Dumping boost' is a common term among those of us who race turbo cars. If the wastegate is too small, it cannot allow enough exhaust to bypass the turbine, and therefore cannot "dump enough boost". It is a real problem on compound supercharged engines, as well as multistage turbocharged engines (pulling tractors and such).

Allison changed the fixed supercharger drive ratio. There were at least two or three ratios used in the 6:1-8:1 range. They used the 6:1 (approximate) to reduce boost at sea level because the turbocharger made too much boost when combined with the supercharger. Since the wastegate could not open enough to dump enough boost, they were forced to slow down the supercharger.

[pasoleati]

Do check the gear ratio of the V-1710-111/113. It is not 6:1, not even near.

And be specific what you write about. Lots of common misunderstandings in common aviation  literature would have been avoided if the author had written clearly and CORRECTLY.  

And in how many WW Two docs you have read about dumping the boost? Better keep the language proper and not degenerate it to automobile level.

[gwshaw]

Originally posted by pasoleati
Do check the gear ratio of the V-1710-111/113. It is not 6:1, not even near.

And be specific what you write about. Lots of common misunderstandings in common aviation  literature would have been avoided if the author had written clearly and CORRECTLY.  

And in how many WW Two docs you have read about dumping the boost? Better keep the language proper and not degenerate it to automobile level.

The V-1710-F5/10 in the P-38F/G used a gear ratio of 7.48:1 IIRC, good to increase map from about 30 in Hg @ the carb intake to approx 50 in Hg in the manifold. The F17/30 increased the ratio to 8.1:1, good for a little over 60 in Hg when fed 30 in from the turbo.

Non turbo-supercharged Allisons went from somewhere around 6.0:1 for the early airship engines IIRC, to 8.77:1 for the C-series, 8.8:1 for the F3/4 to 9.6:1 for the F20. The F21 in the A-36 used the same 7.48:1 as the P-38 engines.

Greg Shaw