Why does airspeed change when I adjust the prop?

On Tue, 16 Jan 2007 01:42:31 -0800, chris wrote
(in article om):


Mxsmanic wrote:
chris writes:

Are you perhaps pulling the RPM back too much and seeing a decrease in
airspeed?

The redline RPM is about 2500, and I pull it back to about 2000-2200.
My IAS then drops by 10 kts or so. The engine is less noisy. I don't
adjust the throttle (which in most cases is fully forward) or mixture
(adjusted for slightly ROP). The fuel rate drops a lot with the RPM,
but the manifold pressure doesn't seem to move much, or I haven't
looked at it closely enough.

Takeoff and landing of course are with the prop set to full fine

Yes, for take-off and landing I make sure the props and mixture are
set fully forward again.

Manifold pressure is set by the throttle.. And you need to pull the
throttle back before pulling the prop control back, and vice versa for
increasing RPM


Um, well not quite. People get confused about that when learning to use a
constant speed prop. You pull the manifold pressure back to an inch less than
where you finally want it to be simply because when you then reduce the rpm
the manifold pressure has a tendency to increase by that amount. You do the
opposite when increasing rpm. Increase the rpm first and then the manifold
pressure so that you don't have to go back and adjust the manifold pressure
again. Has little to do with being kind to the engine and everything to do
with minimizing your fiddling with the controls.