"Peter Dohm" wrote in message
...

"Bill Daniels" bildan@comcast-dot-net wrote in message
...
The basics:

Piston engines produce more power per pound if they rev higher. (HP = RPM
x
torque/5252)
Propellers are MUCH more efficient if they turn slow.
This begs for a PSRU.
BUT, a PSRU adds weight, cost and complexity.
Resonances, particularly torsional resonances are a real problem.
Lots of examples of PSRU's on 12, 14 and 18 cyinder engines
Few workable examples with fewer cylinders suggesting PSRU's don't like
power pulses.
If a shaft has a strong resonant fundamental, don't excite it or lower
the
fundamental below the input frequency.
Tuning a PSRU/shaft/propeller system is like tuning a piano - it's an art
not a science.

The 9 cylinder 1820 and 1840 CID radials used on B-17's were geared
approximately 16:9. However, your point is well taken, and I also am
unable
to name any 4 or 6 cylinder engines that have stood the test of time with
reduction drives.

I also believe that tuning any drive system, including a PSRU, is a
science--when fully understood. And therein lies the rub: There's plenty
left to learn--especially if it must also be light. So, in practice, you
are right--it is still an art. :-(

Peter

You're right. I forgot that there were some successful 9 cyl geared
engines. The radials used planetary gears in the nosecase. I like
planetaries since there's a lot of tooth engagement to carry the power yet
they tend to be compact and light.

I suppose...you could use a hydro drive. Turn a pump with the engine and
use a hydraulic motor to turn the prop. Some type of pressure regulator
could smooth the pressure to the prop motor. Might work for a really slow
turning prop.

Bill D