Throttle sttings inches vs %

[Niah]

I was browsing through advice regarding p-40 engine management and there is one thing i am wondering about. How throttle setting in inches translates to throttle percentage? Does it directly translate between inches and % at all?

[BMA_FlyingShark]

I think it depends on what plane you're in.

For instance, 70% throttle may result in 50 inches in a P51 while the same throttle setting may result in 40 inches in the P40 (fictive numbers).

I Presume you mean inches of Manifold Pressure off course.

 

Have a nice day.

 

Edited March 15, 2021 by FlyingShark

[Charon]

4 hours ago, Niah said:

I was browsing through advice regarding p-40 engine management and there is one thing i am wondering about. How throttle setting in inches translates to throttle percentage? Does it directly translate between inches and % at all?

 

The P-40 (unlike other planes in the sim) lacks a manifold pressure regulator, and thus manifold pressure depends on throttle position, RPM, and altitude. In the real plane the RPM dependency actually reverses at about 12,000ft: below that altitude increased RPM results in decreased manifold pressure (the engine generates more suction in the intake manifold), while above that altitude, increasing RPM results in increased manifold pressure (because of increased supercharger effectiveness), although I don't believe this is modeled in the sim.

 

Since there's no direct translation between throttle position and inches of mercury, you should:

• When increasing power, first adjust RPM to the desired setting, then adjust the throttle to obtain the desired manifold pressure
• When decreasing power, first adjust throttle to approximately the desired manifold pressure, then decrease RPM to the desired setting, then fine-tune the throttle to obtain the desired manifold pressure.

Edited March 14, 2021 by Charon
Clarity

[CSW_Tommy544]

18 hours ago, Charon said:

 

The P-40 (unlike other planes in the sim) lacks a manifold pressure regulator, and thus manifold pressure depends on throttle position, RPM, and altitude. In the real plane the RPM dependency actually reverses at about 12,000ft: below that altitude increased RPM results in decreased manifold pressure (the engine generates more suction in the intake manifold), while above that altitude, increasing RPM results in increased manifold pressure (because of increased supercharger effectiveness), although I don't believe this is modeled in the sim.

 

Hmm, slightly off-topic here, but do you have any more information/material on why there is this inversion happening (in this case around 12,000 ft)? I imagine that altitude will be the critical altitude for that specific engine and its supercharger. I can understand that above the critical altitude, where the throttle is fully open, and by increasing the engine RPM, you also increase the supercharger RPM and thus creating higher pressurized air in the intake and so the manifold pressure will increase. The only reason that comes to my mind as to why this would be reversed below the critical altitude is that the throttle body is not fully open and constrains the air flow. Increasing the engine RPM in this case would increase the volume of air that can go through the engine, but by not opening the throttle more, you start to starve the engine of air a little. Increasing the engine RPM will still increase the supercharger RPM and so the air pressure at the throttle body will be higher so if my theory is correct, this increase of pressure at the throttle body, which is at some partially closed position must have a smaller effect than the pressure difference behind the throttle body caused by the engine wanting more air.

 

Feel free to correct me if my assumptions are wrong!

[Charon]

From page 10 of this PDF here: https://ww2aircraft.net/forum/threads/p-40-flight-manual.7478/page-2#post-390159

 

This is intended for later models, but engine in the P-40E we have is fundamentally the same. Unfortunately, it doesn't explain why this behavior occurs.