"Dave Butler" wrote in message
news:1113855718.630798@sj-nntpcache-3...
Turbo Arrow wrote:

just a note, on decents, instead of reducign power to come down, you can
just
pull the prop back, it will cause more drag so you can decend but keep
engine
from shock cooling (BTW this is out of the manual)

"pulling the prop back" increases drag? Do you mean pulling on the
propeller control knob? How does that increase drag? Maybe you meant that
it reduces power?

Dave

No, it doesn't necessarily reduce power. Typically it will increase the
manifold pressure. If there is not a concomitant reduction in RPM the power
will increase. However, you will likely move the prop away from the optimum
pitch for the airspeed/RPM combination. This will reduce the effectiveness
of the horsepower to thrust conversion that you use the propellor for. That
means that more power will be required for the same thrust. This allows you
to maintain the thrust selected for the desired descent while producing
sufficient power to keep the cylinders warm and working.

However, I would not recommend that technique for all engines, even if it
would probably work alright for a geared engine. "pulling the prop back"
implies to me that you are increasing the pitch of the prop. With the
throttle setting unchanged this will result in a decrease in RPM and an
increase in Manifold Pressure. This moves you closer to the detonation
margin and could result in engine damage.

The normal procedure for increasing the pitch of a controllable or constant
speed prop is to reduce the throttle before increasing the pitch. With a
controllable prop you control the pitch directly. With a constant speed
prop you increase the pitch by reducing the RPM with the prop governor
control. High manifold pressure and low RPM is comparable to starting off
with your car in high gear. It lugs the engine.

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )