PSRU design advantages

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

Peter Dohm wrote:

"Richard Lamb" wrote in message
link.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


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

"Morgans" wrote in message
...

"ADK" wrote in message
news:X6TXf.28774$%H.11944@clgrps13...
This is probably going to open old wounds. What I would like is
experienced input on the advantages, for economic, efficiency and
longevity etc. of different types of redrives.

I am leaning towards a cog-belt reducer in a 6 cylinder, liquid cooled,
configuration driving a long drive shaft to the prop.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Warning,Warning, Danger, Danger, Will Robinson!

Yep, that is a really big can of worms.

The redrive is not as big a problem as you think. You get into a mess
when you start talking about long drive shafts.

Torsional resonance has brought many of the great minds of the flying
industry to their knees. No joke. I don't have all of the links at hand,
but someone here does. Start by googling torsional resonance.

Then, be afraid. Be very afraid.
I you don't get afraid, keep looking, until you get afraid, because you
need to get afraid, or you don't understand the problem.
--
Jim in NC


The long driveshaft is a problem. Unlike a car, in an aircraft you put the
driveshaft in between the crankshaft and the flywheel. Not the place for a
driveshaft. My recommendation is to couple the driveshaft to the
engine/reduction unit with one of the shot filled fluid couplings. They
kill any feedback and pretty well damp the reinforcement that makes the
driveshaft go totally bonkers. Then use the largest diameter tubular
driveshaft you can fit into the space available. That will tend to put the
resonant frequencies into a range you won't pass through or run in normally.

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )

"Capt. Geoffrey Thorpe" The Sea Hawk at wow way d0t com wrote in message
...
snip

That was a problem on some of the early T-18's that were using cut down
propellers - resonance was fatiguing the propeller which would then shed a
blade, which would then encourage the engine to attempt to part company
with the airframe...


That used to happen so frequently with Formula air racers where they chopped
the props for the O-200's to get the RPM up to get additional HP out of that
little engine that they started using a safety cable around the engine.
That way when the prop shed a blade and the engine jumped out of the engine
mount the cable held it somewhere near where it was supposed to be so you
could get the airplane on the ground in a survivable fashion!

Not something you want to play with normally.

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )

"Richard Lamb" wrote in message
ink.net...
Big John wrote:

ADK

Look up Molt Taylor and his Aerocar system. He used a pusher
arrangement and seemed to have most of the problems solved with long
shaft back to prop.

His bird may still be flying some place to exhibitions?

Am sure someone will jump in here and give data on his bird and how he
coupled shaft to engine with a "power glide" clutch or some such. It
allowed a small amount of slippage at each power stroke to prevent the
pulse being transmitted to drive shaft and prop as I recall.

Best of luck with a difficult problem.

Big John

Ok,

I think it's also used on the Imp and Mini-Imp.


The "clutch" consists of two (wavy surfaced) plates with lead shot
loaded between them.

As the thing spun up, centrifugal force packed the shot solid, but
there was enough "give" with the shot to absorb the "jerk" (4th
derivative?).


Richard

no idea why that came out in past tense...


It is indeed used on the Imp and the MiniImp. Molt used it on most of his
designs and spent quite a few years getting the bugs worked out of it. It
is NOT "two wavy surfaced plates" but just a little different.

The driven part is a cylindrical case with a charge of shot in it. When the
case is driven the "fluid" shop is packed tightly against the outer diameter
of the spinning cylinder. The output shaft has a single "wavy" plate on the
end of it. This plate is enclosed in the cylinder with the shot. As the
shot gets packed into the rim of the case it grips the plate and transmits
the power to the output shaft. If you try to drive it backwards all the
plate does it turn in the loose shot and warm it up a bit. Like a "sprague
clutch" from a helicopter, it only transmits power in one direction. By
varying the load of shot in the cylinder you can vary the coupling
coefficient and "set" the breakaway torque for the unit. It cannot transmit
damaging torques from torsional vibration back through the coupling because
the output shaft "breaks away" before a crippling torque is reached. You
don't want it to break away at too low a torque either, or you will
basically have a "slipping" clutch in your drive train.

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )

"George" wrote in message
m...
Richard Lamb wrote:
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.

"ADK" wrote in message
news:X6TXf.28774$%H.11944@clgrps13...

This is probably going to open old wounds. What I would like is
experienced input on the advantages, for economic, efficiency and
longevity etc. of different types of redrives.

I am leaning towards a cog-belt reducer in a 6 cylinder, liquid cooled,
configuration driving a long drive shaft to the prop.




The collective experience is zilch = nada = squat = undefined.

THAT is what everybody had been trying to tell you.

Wait a second. Look around the airport.

How many shaft driven propellers do you see?

Have you ever seen?

If you are heart set on doing it, I sincerely wish you luck.

But I can't offer any further advice - 'cuz they ain't none...



Richard


Richard,

Didn't the military do this once?? Seems there was the P-39 Aircobra,
shaft driven from a rear mounted engine?? Are the gray cells working that
far back??

Not that it would be applicable to an experimental, but at least It was
once done?

George

Yep. And Molt Taylor did it on several different airplanes and with several
different engines. There have been several pushers with engines mounted
near the CG and the props back on the tail with long drive shafts. Several
of them worked very well. There is a gain in efficiency when you do that.
Unfortunately the increased weight of the drive train and the additional
cost and complexity of the requisite drive train generally overcame any
efficiency gain and none of them has ever gone into "production."

The P-39 was a special case. It had an aft mounted engine and a big
driveshaft that passed between the pilots legs! It scared a lot of pilots
thinking about what they would lose if the drive shaft pickled on them.
They also used the drive shaft for a gun barrel for a large bore cannon so
that it could fire straight ahead through the spinner and be easy for the
pilot to aim. Just point the airplane at your target and cut loose. The
additional weight of the complex drive train raised hob with the performance
and our pilots didn't like them at all. The Russian pilots loved them.
They could aim the whole airplane fairly well and when you ran out of cannon
shells they would just ram the enemy fighter. That brought down both of
them, but the russians didn't mind. They were fighting in their own
backyard and the German's were not.

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )

"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 )

"Richard Lamb" wrote in message
k.net...

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


I used to use a tool called a "Strobotach" which was a variable speed strobe
unit used for a non-contact tachometer.
Very useful for analyzeing periodic motions. :-)

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )

Ernest Christley wrote:
Ian Stirling wrote:

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

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


That'll spot ordinary vibrations.
Torsional ones are a little bit harder.

Especially if you want, as you probably should, a graph of maximum stress
anywhere in the shaft/PSRU/Prop system vs RPM.

A few fine white lines down the length of the shaft will clear up that
problem.

Will they?
It'll obviously show huge torsional movement, but many, especially
shorter shafts may fail before it becomes visible.

"Highflyer" wrote in message
...

"Richard Lamb" wrote in message
ink.net...
Big John wrote:

ADK

Look up Molt Taylor and his Aerocar system. He used a pusher
arrangement and seemed to have most of the problems solved with long
shaft back to prop.

His bird may still be flying some place to exhibitions?

Am sure someone will jump in here and give data on his bird and how he
coupled shaft to engine with a "power glide" clutch or some such. It
allowed a small amount of slippage at each power stroke to prevent the
pulse being transmitted to drive shaft and prop as I recall.

Best of luck with a difficult problem.

Big John

Ok,

I think it's also used on the Imp and Mini-Imp.


The "clutch" consists of two (wavy surfaced) plates with lead shot
loaded between them.

As the thing spun up, centrifugal force packed the shot solid, but
there was enough "give" with the shot to absorb the "jerk" (4th
derivative?).


Richard

no idea why that came out in past tense...


It is indeed used on the Imp and the MiniImp. Molt used it on most of his
designs and spent quite a few years getting the bugs worked out of it. It
is NOT "two wavy surfaced plates" but just a little different.

The driven part is a cylindrical case with a charge of shot in it. When
the
case is driven the "fluid" shop is packed tightly against the outer
diameter
of the spinning cylinder. The output shaft has a single "wavy" plate on
the
end of it. This plate is enclosed in the cylinder with the shot. As the
shot gets packed into the rim of the case it grips the plate and transmits
the power to the output shaft. If you try to drive it backwards all the
plate does it turn in the loose shot and warm it up a bit. Like a
"sprague
clutch" from a helicopter, it only transmits power in one direction. By
varying the load of shot in the cylinder you can vary the coupling
coefficient and "set" the breakaway torque for the unit. It cannot
transmit
damaging torques from torsional vibration back through the coupling
because
the output shaft "breaks away" before a crippling torque is reached. You
don't want it to break away at too low a torque either, or you will
basically have a "slipping" clutch in your drive train.

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )



I can really only agree with part of that. I did not reread the entire
brochure, but the clutch itself does not appear to be unidirectional, and it
is not intended to be a torsional dampener. However, the soft start
certainly would have eliminated resonance at srating motor speed, which
proved to be the most vexing problem in the Contact! article regarding the
BD-5. It could have made that other problems a lot easier to solve as well.

I have never personally seem any of Molt Taylor's aircraft, and I don't know
which specific parts he used, but a Dodge Flexidyne brochure is available
at: http://www.dodge-pt.com/pdf/brochure...s/dmr_1513.pdf

Peter

JP wrote:


The P-39/P-63 examples can't probably be compared directly with this matter
in question. These designs both have a large reduction gear casing in front
of the plane. The support structure for this PSRU looks very firm. Perhaps
the safety cage type center fuselage structure has something to do with the
lack of severe resonance problems? Any known resonance problems with these
aircrafts?

JP

I doubt any such information would be of value here since the those
PSRU has a gun barrel through it. Assuming any contact between the gun
and the PSRU any resonance solutions would be different.

Dan, U.S. Air Force, retired