PSRU design advantages

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

"Richard Lamb" wrote

So it *can* be done.
(The cannon firing thru the prop! )

I was curious to see if Bell had reduced the shaft RPM between the engine
and gearbox, but it looks like 1:1 there.

Yep. The normal speed reduction unit for the prop is taken off, and put at
the end of the shaft.

That might have been of interest to the OP, since his setup will likely
drive the shaft at prop rpm (after the psru).

Gonna take one tough (probably spelled h.e.a.v.y) shaft for that
service...


Are there any others?

There are numerous dual rotar sling wings that have an interconnected rotor
shaft, but they are usually turbine engines. One example is the Osprey.

The normal Allison AC engine also had an active fluid torsional resonance
reducer at the non driven end of the crankshaft on the engine, and a
torsional reducer coupling (rubber) on the drive end. I could not find
that info about the aircobra, but I'll bet they are there on that
application, also.
--
Jim in NC

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

Suppose the prop shaft is to be just long enough for the gear belt pulley
and the neccessary bearings - say 10 inches. But the engine flywheel
pulley is to be 4 - 6 feet below the prop shaft. The idea is to use a
very large multi-blade carbon fiber prop turning 800 - 1000 RPM driven by
a 4 cyl Soob turning at best power RPM. The idea is to get best thrust in
the 0 - 60 knot range. The airframe configuration is a prop over tail
boom pusher - an ultralight on steroids. (BTW, I'm not looking for a long
engine life under these conditions. I'll treat the Soob as a throwaway
power plant.)

Strange, but I am considering a system, very much like that. Flying boat,
with the engine in the fuselage, instead of on a pylon.

I'm thinking there isn't too much torsional vibration concern with very
short shafts, high reving engine and a stiff carbon fiber prop. The prop
will be seeing 6 - 7 power pulses per rev from the high reving Soob.

On that, I would agree. I believe you will have a new problem, though.

I believe that you will have a problem with the belt vibrating, like a
guitar string. At some speeds, it will get to the same resonance of the
belt, and slap like the devil. I have a 20" band saw that does this every
time, as it slows down, right before it stops.

One way to deal with this is to make the string (belt) shorter, with some
idler pullies. At least that is my plan. It would be a good idea to make
the distances between the plies a little different, or it will have a strong
tendency to vibrate as a complete string, at a higher fundamental (lowest)
frequency.

It is the same (in a way) as muffler design. The volume in the different
chambers is a little different, so when the frequency is resonating in one
chamber, it will not be resonating in the other chamber, thus more
frequencies are muffled.
--
Jim in NC

"Morgans" kirjoitti
...

"Richard Lamb" wrote

So it *can* be done.
(The cannon firing thru the prop! )

I was curious to see if Bell had reduced the shaft RPM between the engine
and gearbox, but it looks like 1:1 there.

Yep. The normal speed reduction unit for the prop is taken off, and put
at the end of the shaft.

That might have been of interest to the OP, since his setup will likely
drive the shaft at prop rpm (after the psru).

Gonna take one tough (probably spelled h.e.a.v.y) shaft for that
service...


Are there any others?

There are numerous dual rotar sling wings that have an interconnected
rotor shaft, but they are usually turbine engines. One example is the
Osprey.

The normal Allison AC engine also had an active fluid torsional resonance
reducer at the non driven end of the crankshaft on the engine, and a
torsional reducer coupling (rubber) on the drive end. I could not find
that info about the aircobra, but I'll bet they are there on that
application, also.
--
Jim in NC

The Bell P-39 Airacobra V-1710 engine does have a direct drive from the
crankshaft to the 8 foot extension-shaft. This coupling certainly may have
some kind of a damper unit? The rotation speed reduction is done in the nose
section PSRU unit. The extension-shaft does have a support bearing unit in
the middle of the shaft.

Here's some nice pictures about the powerplant/driveline combo
http://www.aviation-history.com/engines/allison.htm

"However, there were problems with the complex nose-mounted reduction gear,
which caused reliability problems and resulted in fairly low serviceability
rates as compared with other fighters"
(http://home.att.net/~jbaugher1/p39_1.html).

JP

("Richard Lamb" wrote)
[150+ lines of quoted text snipped]

My pardon, sir!


Speaking of reduction units ...! g


Montblack :-=)
Franz Liebkind: Der Führer does not say, "Achtung, baby."
The Producers (1968)

"Morgans" wrote in message
...

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

Suppose the prop shaft is to be just long enough for the gear belt pulley
and the neccessary bearings - say 10 inches. But the engine flywheel
pulley is to be 4 - 6 feet below the prop shaft. The idea is to use a
very large multi-blade carbon fiber prop turning 800 - 1000 RPM driven by
a 4 cyl Soob turning at best power RPM. The idea is to get best thrust
in the 0 - 60 knot range. The airframe configuration is a prop over
tail boom pusher - an ultralight on steroids. (BTW, I'm not looking for
a long engine life under these conditions. I'll treat the Soob as a
throwaway power plant.)

Strange, but I am considering a system, very much like that. Flying
boat, with the engine in the fuselage, instead of on a pylon.

I'm thinking there isn't too much torsional vibration concern with very
short shafts, high reving engine and a stiff carbon fiber prop. The prop
will be seeing 6 - 7 power pulses per rev from the high reving Soob.

On that, I would agree. I believe you will have a new problem, though.

I believe that you will have a problem with the belt vibrating, like a
guitar string. At some speeds, it will get to the same resonance of the
belt, and slap like the devil. I have a 20" band saw that does this every
time, as it slows down, right before it stops.

One way to deal with this is to make the string (belt) shorter, with some
idler pullies. At least that is my plan. It would be a good idea to
make the distances between the plies a little different, or it will have a
strong tendency to vibrate as a complete string, at a higher fundamental
(lowest) frequency.

It is the same (in a way) as muffler design. The volume in the different
chambers is a little different, so when the frequency is resonating in one
chamber, it will not be resonating in the other chamber, thus more
frequencies are muffled.
--
Jim in NC

A low RPM high thrust prop on a high thrust line would be ideal for a
seaplane.

Actually this is not too different than some motorgliders with the engine
buried in the fuselage and the prop on a retractable pylon. The tooth belts
have only one or two idler pulleys. I figgured on at least two idlers to
maintain belt tension and to damp belt vibrations.

I want the engine in an external conformal pod below the fuselage for
accessability and to locate it below the pilot for survivability reasons.
I'd want the whole engine/cooling system in this pod so it could be removed
as a unit. The prop and drive belt would remain with the airframe.

Bill D

Montblack wrote:
("Richard Lamb" wrote)
[150+ lines of quoted text snipped]

My pardon, sir!



Speaking of reduction units ...! g


Montblack :-=)
Franz Liebkind: Der Führer does not say, "Achtung, baby."
The Producers (1968)


Resonance, dude!

Morgans wrote:


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

Suppose the prop shaft is to be just long enough for the gear belt
pulley and the neccessary bearings - say 10 inches. But the engine
flywheel pulley is to be 4 - 6 feet below the prop shaft. The idea is
to use a very large multi-blade carbon fiber prop turning 800 - 1000
RPM driven by a 4 cyl Soob turning at best power RPM. The idea is to
get best thrust in the 0 - 60 knot range. The airframe configuration
is a prop over tail boom pusher - an ultralight on steroids. (BTW,
I'm not looking for a long engine life under these conditions. I'll
treat the Soob as a throwaway power plant.)


Strange, but I am considering a system, very much like that. Flying
boat, with the engine in the fuselage, instead of on a pylon.


Curious...
Tell me why?
What is the advantage?
(not argumentative, please - just curious)

If I absolutely *had* to do that, I'd think two stages.

HTD belt on the first stage from the engine, and either HTD, or Vopar type
chain for the final. Come to think of it, probably have to be chain for the
final drive due to the length of the thing...

But I'm back to the original question - why?

Wouldn't that put the engine where you would want to put people?

Richard

Bill Daniels wrote:

"Morgans" wrote in message
...

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


Suppose the prop shaft is to be just long enough for the gear belt pulley
and the neccessary bearings - say 10 inches. But the engine flywheel
pulley is to be 4 - 6 feet below the prop shaft. The idea is to use a
very large multi-blade carbon fiber prop turning 800 - 1000 RPM driven by
a 4 cyl Soob turning at best power RPM. The idea is to get best thrust
in the 0 - 60 knot range. The airframe configuration is a prop over
tail boom pusher - an ultralight on steroids. (BTW, I'm not looking for
a long engine life under these conditions. I'll treat the Soob as a
throwaway power plant.)

Strange, but I am considering a system, very much like that. Flying
boat, with the engine in the fuselage, instead of on a pylon.


I'm thinking there isn't too much torsional vibration concern with very
short shafts, high reving engine and a stiff carbon fiber prop. The prop
will be seeing 6 - 7 power pulses per rev from the high reving Soob.

On that, I would agree. I believe you will have a new problem, though.

I believe that you will have a problem with the belt vibrating, like a
guitar string. At some speeds, it will get to the same resonance of the
belt, and slap like the devil. I have a 20" band saw that does this every
time, as it slows down, right before it stops.

One way to deal with this is to make the string (belt) shorter, with some
idler pullies. At least that is my plan. It would be a good idea to
make the distances between the plies a little different, or it will have a
strong tendency to vibrate as a complete string, at a higher fundamental
(lowest) frequency.

It is the same (in a way) as muffler design. The volume in the different
chambers is a little different, so when the frequency is resonating in one
chamber, it will not be resonating in the other chamber, thus more
frequencies are muffled.
--
Jim in NC


A low RPM high thrust prop on a high thrust line would be ideal for a
seaplane.

Actually this is not too different than some motorgliders with the engine
buried in the fuselage and the prop on a retractable pylon. The tooth belts
have only one or two idler pulleys. I figgured on at least two idlers to
maintain belt tension and to damp belt vibrations.

I want the engine in an external conformal pod below the fuselage for
accessability and to locate it below the pilot for survivability reasons.
I'd want the whole engine/cooling system in this pod so it could be removed
as a unit. The prop and drive belt would remain with the airframe.

Bill D



Ok, that was the quick answer

Ricahrd

"Morgans" wrote in message
...

"Richard Lamb" wrote

So it *can* be done.
(The cannon firing thru the prop! )

I was curious to see if Bell had reduced the shaft RPM between the
engine
and gearbox, but it looks like 1:1 there.

Yep. The normal speed reduction unit for the prop is taken off, and put
at
the end of the shaft.

That might have been of interest to the OP, since his setup will likely
drive the shaft at prop rpm (after the psru).

Gonna take one tough (probably spelled h.e.a.v.y) shaft for that
service...


Are there any others?

There are numerous dual rotar sling wings that have an interconnected
rotor
shaft, but they are usually turbine engines. One example is the Osprey.

The normal Allison AC engine also had an active fluid torsional resonance
reducer at the non driven end of the crankshaft on the engine, and a
torsional reducer coupling (rubber) on the drive end. I could not find
that info about the aircobra, but I'll bet they are there on that
application, also.
--
Jim in NC

Sounds like a pretty sure bet to me. BTW, Thanks--I had wondered what
Allison did to eliminate resonance.

Peter