PSRU design advantages

"Richard Lamb" wrote in message
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Peter Dohm wrote:
"Richard Lamb" wrote in message
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Peter Dohm wrote:

"Richard Lamb" wrote in message
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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


Actually I did, some months ago following an earlier post, and
subsequently
also learned that the Contact! article is quite famous. One of the more
interesting points was that trying to make the shaft and/or transfer
drive
more rigid was not helpful on the BD-5. Softening the system eventually
did
resolve the breakage problem within the drive train; but IIRC the drive
system to airframe resonance (evidenced initially by loosened rivets)
was
not fully resolved during the author's tenure. That was the article
that
really convinced me that I didn't necessarily know enough to design a
clutchless system with a high degree of confidence--even by leaving the
flywheel in place.

However, the set of books mentioned elsewhere in this thread, by Mr.
Horton,
could prove to contain the necessary formulas and explanations to reduce
this problem to a cookbook science. A quick web search confirmed his
belief
that one of the books may now be virtually unobtainable.

I am willing to entertain his book suggestion because, in my earlier
career
as an electronic technician, a technical tome entitled "Shielding and
Grounding Techniques in Instrumentation" made previously insurmountable
grounding problems easy to solve. It is probable that work on
mechanical
resonance, done for World War II, may have been covered in books
published
during the succeeding quarter century.

Peter



My pardon, sir!

Richard

No appology needed - I manage to miss plenty!

Peter