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Matt Whiting · January 19th 06, 09:05 PM posted to rec.aviation.misc,rec.aviation.piloting

wrote:

Rolling off RPM may or may not roll you down the torque curve. If you
are running at an RPM above the torque peak, reducing RPM might actually
increase the torque available.

Reducing RPM on a fixed-pitch prop will reduce the torque. Reducing
RPM using the prop control on a constant-speed prop could increase
torque as the RPM drops, depending on the engine's torque curve.

Not in all conditions. In SS level flight, yes.

The torque curve is affected by volumetric efficiency, which is
a bigger factor in high-RPM engines, less so in slow-turning aircraft
engines. As RPM rises, the cylinder can't achieve anything near
atmospheric pressure at the bottom of the intake stroke so that the
amount of fuel/air mix is progressively reduced, and any horsepower
increase with rising RPM is due to RPM only. The turbo or supercharger
is the solution to the probem. In light airplanes, the turbo is more
normally employed to alleviate altitude losses.

It's been a while since I studied engines in any detail, but I believe
there is more to it than just VE. I don't believe that bearing friction
is linear with RPM for example. Also, speed of the flame front becomes
and issue at higher RPM. I believe the drop-off in torque with RPM is a
function of a number of factors. VE is dominant, but not the only one.
Even turbocharged engines have a torque peak with a drop-off at some
point.

Matt

Matt