PSRU design advantages

"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

"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

"Bill Daniels" bildan@comcast-dot-net wrote

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.

One word. HEAVY ! ! !
--
Jim in NC

Peter Dohm wrote:
snip
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. :-(

I suspect that electronics help.
Instrumenting the shaft, to measure resonances in real time is no longer
prohibitively expensive.
I suspect a belt PSRU - if properly configured could act to decouple the
prop from the engine/shaft somewhat.
Add one or more rotational vibrational dampers - fill the shaft with
oil? And trim.

Ian Stirling wrote:

Peter Dohm wrote:
snip

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. :-(


I suspect that electronics help.
Instrumenting the shaft, to measure resonances in real time is no longer
prohibitively expensive.
I suspect a belt PSRU - if properly configured could act to decouple the
prop from the engine/shaft somewhat.
Add one or more rotational vibrational dampers - fill the shaft with
oil? And trim.



Best tool available to the amateur is a variable speed strobe - Party Light!

That way you can actually look and SEE what's happening.


Richard

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.


Continental GO-300 (Cessna 175).
Lycoming GO-435 (Navion).
--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.

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

"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

"Peter Dohm" wrote in message
...
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.


GO-300, GO-435, GO-480, etc. There are lots of geared flat aircraft
engines. Note that all of the ones I mentioned are sixes. Fours are
tougher and twins are about impossible. Gearbox design is pretty critical.
Also, all of these engines got a bad reputation from pilots who didn't know
how to fly them. You never want to unload the gears. Put them is an
unloaded situation and the gears will lash with each power pulse and quickly
eat the gearbox.

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )

ADK wrote:
IF you had to design a PSRU, to drive a pusher propellor via shaft, what
would your experience dictate? Thinking along the lines of a gearbelt, chain
or gear. Please, I would appreciate the collective experience available on
this group. I have decided on the aircraft, but want to make it the most
reliable and safest it can be.


For the sake of Peter, IT DOESN'T MATTER!!

For the energy to transfer to the prop, you have to attach the engine to
the prop. The engine doesn't produce smooth even power. It produces a
series of pulses. If the frequency of the pulses resonates with the
prop or shaft, it will store a little bit of each pulse as "spring
energy". This type of energy is stored by deflecting (ie, bending) the
prop or shaft. The prop stores it and then immediately tries to release
it by unbending. If the next engine pulse comes along at just the right
time, the new "spring energy" will be added to the previous "spring
energy" and the prop will bend a little more. This continues until the
prop or shaft has as much "spring energy" as it can phyiscally hold, and
then the element just vibrates. Eventually, the prop or shaft gets
tired of all the bending and unbending and just gives up (ie, breaks).

Making the pulses smaller doesn't help for the most part. All that does
is cut down on the amount of "spring energy" added with each pulse. A
smaller pulse will take 2000 pulses to fill the prop with "spring
energy" vs 1000 with a unmodified pulse. Whoop-te-do! What difference
will that make with the engine turning 2000RPM and four pulses per round.

Any one of the gearboxes you mentioned made to work safely, and each has
a set of advantages and disadvantages that are well known and easily
engineered around. The type of gearbox has nothing to do with torsional
resonance. Will not mitigate torsional resonance. Will not
cure/alleviate/lesson or bypass torsioanl resonance. Torsional
resonance is a totally different issue.

You didn't tie gearbox type and torsional resonance together directly,
but many people have in the past, and it's just self-deception. Any of
the gearboxes you mentioned can be as safe and dependable as any of the
others, if engineered properly.


--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."