All moderate to high power geared piston engines and all turboprops have
gear reduction and CS props. It is not just about engine efficiency, it is
about prop efficiency over a range of airspeeds and altitudes. As airspeed
increases, AOA of the prop decreases so you can not have a prop which works
efficiently for takeoff and cruise unless the airplane is very slow. My
airplane advances 19 feet for each revolution of the prop at cruise vs. 0
feet at the beginning of the takeoff roll. Similiarly, the prop needs more
pitch when the air is less dense to be efficient..

Mike
MU-2

"Jay" wrote in message
om...
Thanks for your insight, which raise a few questions:

Apart from the geared Cessna (which isn't all that hi performance)
which aircraft have a PSRU AND a CS prop?

Which experimental aircraft has 2 speed gear boxes? I heard of a guy
flying a WW1 replical biplane on a Honda motocycle engine. He just
kept the original gear box and said he actually shifts gears depending
on if he's climbing or cruising.

My point about using an engine that can operate efficiently over a
large range of RPMs (like a modern automobile engine) is that the CS
prop is NOT as necessary although it certainly does help, no doubt
about it. Certainly you will get you peak horsepower at high revs,
but the moderm engine has a fatter torque curve due to being able to
change valve AND ignition timing in a manner optimum for the
particular revs it is at. The Lyco/Conti design takes a double hit
for operating at low revs, its off the peak HP point, and its timing
was peaked for a specific RPM.


"Bill Daniels" wrote in message
...
"Jay" wrote in message
om...
Seems to me that some of the benefits of the constant speed prop were
based on the limitiations of timing (ignition and valve) of the
Lyco/Conti engines. If your engine was designed to have a large
dynamic range of efficient operation, you won't need the articulated
prop as much.

Prop blades are just rotating wings. The goal is to run the blades at
their
most efficient angle of attack for the RPM and aircraft airspeed. The
performance of the prop is best at low RPM but the piston engine driving
it
is likely to be most efficient at a higher RPM. That is the reason that
high performance piston aircraft have both PRSU's and constant speed
props.

Some experimental powerplant/prop systems included a two speed gearbox
in
addition to the CS prop to run the engine at high RPM at takeoff and low
RPM
for cruise. These experimental engines also shifted the cam and
ignition
timing for the two PRSU ratios. This helped the prop blades stay at the
best AOA to maximize thrust and optimized the engine at two set points,
high
RPM for takeoff and low RPM for long range cruise. This was at the very
end
of large piston engine development and an attempt to wring the last bit
of
performance out of these monsters.

Having an engine with a wide "dynamic range" is nice for a car but less
useful for an airplane where it is best to optimize the engine for one
RPM
and let the CS prop and PRSU operate the prop in the most efficient way.

Bill Daniels