Peter Dohm wrote:
"Richard Lamb" wrote in message
k.net...

Peter Dohm wrote:


"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




Rotax - the 912/914

Jabaru - (but the 6 cylinder will be a better seller - IMHO)

Believe it or not, a few VW's with belts.

And a couple of Subes with Rotax B boxes scabbed on.

The one that DIDN'T work was the Geo Metro 3-banger (broke the crank).

But that issue was already known - don't cut off any flywheel on 3 holers.
With the full flywheel, the 3 cylinder runs fine.


Richard


OK, you caught me fair and square on poor phrasing. I tend to think of
higher power applications, but you are right that some of the more
conservative and lower powered systems with flywheels still in place and a
little looser coupling seem to run quite reliably. I don't know how much
power is lost to friction, but some of the v-belt reduction drives even seem
to work quite reliably without any external crankshaft support!

Peter



You didn't follow the link that blueskies posted, didja Peter.

The BD-5 story - in all it's glory! And a few other odds and ends,
That was not a high powered setup, but kicked a bunch of engineers around.

http://www.prime-mover.org/Engines/T.../contact1.html


Richard