High altitude & RPM

[abripl]

At higher altitudes (non-turbo piston engine) is it more efficient for
power/economy to use medium or max RPM? There is much less air there
and if the RPM is too high will it be starving more than at lower RPM?

[Bill Daniels]

"abripl" > wrote in message
oups.com...
> At higher altitudes (non-turbo piston engine) is it more efficient for
> power/economy to use medium or max RPM? There is much less air there
> and if the RPM is too high will it be starving more than at lower RPM?
>

I have flown light airplanes over the Rocky Mountains for a long time. I
always flew as high as possible at full throttle and as lean as possible.

In fixed-pitch airplanes. The cruise RPM stayed about the same at all
altitudes since the power absorbed by the prop declined with altitude at
about the same rate as engine power. Even with full throttle, the power was
usually something like 50 - 60% at altitudes up to FL 180.

With a CS prop, I ran full throttle and as low an RPM as the handbook
settings allowed. I used full throttle to reduce pumping losses and as lean
as the engine would take without undue vibration to conserve fuel. Low RPM
improved prop efficiency. My engines always lasted way beyond TBO.

Bill Daniels