McDonnell XP-67

Zipper730 · Sep 19, 2016

I remember something to the effect that the McDonnell XP-67 Bat had a creative turbocharger arrangement in which the turbine was pointed rearwards with the exhaust from the engines blowing out the back to maximize thrust.

I'm curious if anybody else had thought of this up to this point?

T Bolt · Sep 19, 2016

I beleave the P-47's turbo exhaust at the underside of the rear fuselage added some thrust

wuzak · Oct 5, 2016

The earlier versions of the P-47 seem to have had the exhaust pointed straight down, the flight hood then turning that flow rearwards. I shouldn't think it the best solution for exhaust thrust.

The XP-47J had the turbo inclined so that the exhaust pointed down and to the rear. This should have improved the exhaust thrust. I am not sure if other late model P-47s had a similar arrangement.

The XP-67 differed in that the turbo was mounted so that its exhaust was parallel, or nearly so, with the direction of flight. This was better for exhaust thrust.

The XP-67 also had its wastegate exhaust pointing rearwards.

You can clearly see in this photo two exhausts in each nacelle.
http://www.warbirdsresourcegroup.org/URG/images/xp67-7.jpg

Below critical altitude some proportion of the exhaust must bypass the turbine to prevent it from overboosting/overspeeding. This provides additional thrust. At critical altitude the exhaust may match the turbine's requirements, but I am not sure if exhaust has to go through the wastegate to prevent overspeeding of the turbo at altitudes above critical. The higher the altitude the bigger the effect of exhaust thrust.

Zipper730 · Oct 11, 2016

wuzak said:
The XP-67 differed in that the turbo was mounted so that its exhaust was parallel, or nearly so, with the direction of flight. This was better for exhaust thrust.

The XP-67 also had its wastegate exhaust pointing rearwards.

Yeah, that's what I was getting at: The exhaust to the turbo is pointing rearwards and the wastegate is also pointing rearwards. I'm surprised nobody saw that potential earlier.

I've been told by the late 1930's the Navy was trying to determine thrust produced by exhaust stacks either to pump the cowl-duct and/or to push the plane (the F4U, if I recall used it's exhaust to pump the duct and re-accelerate the airflow through the rear part of the cowl after flowing through the cylinder heads).

Below critical altitude some proportion of the exhaust must bypass the turbine to prevent it from overboosting/overspeeding.

I never knew that, though I knew the airflow was somehow varied through the turbine. I thought the exhaust area was simply altered to vary the speed of the flow.

wuzak · Oct 11, 2016

Zipper730 said:
I never knew that, though I knew the airflow was somehow varied through the turbine. I thought the exhaust area was simply altered to vary the speed of the flow.

Pratt & Whitney tried that with the R-2800 and R-4360 VDT (Variable Discharge Turbines). DIdn't work terribly well, and probably needed FADEC to work properly. One R-4360 VDT was flown in a B-50 - the flight engineer had to constantly monitor the engine and adjust the nozzle.

The wastegate was the method used to control exhaust flow through the turbine, excess exhaust bypassing the turbine. And it is still done to this day - almost every turbo engine on the road has a wastegate.

The turbo engines in F1 have a different solution - they have a turbine that is big enough to cope with the entire exhaust and use a motor/generator unit to control the speed of the turbo. It is a turbo-compound. They still have a wastegate to dump the exhaust for short periods of extra power (the motor/generator is used to drive teh turbo in these situations).

In the Porsche 919 Le Mans car the turbo's wastegate feeds into a power recovery turbine, which is connected to a generator.

In WW2 era engines the Wright R-3350 had a turbo-compound version post war. It had three turbines, but these were not connected to compressors, but were mechanically linked to the crankshaft (with fluid couplings).

Allison also developed the V-1710-127 (-E27) which used the turbine from the C-series turbo (as used in P-47s) to feed power back to the crankshaft.

In these two cases it was the mechanical link which prevented the turbine overspeeding, so a wastegate was not required.

Shortround6 · Oct 11, 2016

Zipper730 said:
I've been told by the late 1930's the Navy was trying to determine thrust produced by exhaust stacks either to pump the cowl-duct and/or to push the plane (the F4U, if I recall used it's exhaust to pump the duct and re-accelerate the airflow through the rear part of the cowl after flowing through the cylinder heads).

It changed.