PSRU design advantages

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.

"ADK" wrote in message
news:3pGYf.26105$Ph4.10950@edtnps90...
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.

"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

There is nothing that eliminates a long shaft from the design of a
PSRU. Nonbelievers might be advised to consider ship propulsion; long
shafts, low cylinder counts, propellers operating in uneven flow, often
via a gearbox. Sound familiar?

The important issue is torsional stiffness of the shaft, not
length. A long shaft can be torsionally stiff or soft, depending on
diameter and material. The engineering process will tailor torsional
stiffness of the shaft (along with a number of other factors) to adjust
natural frequency.

The information you need is found in engineering texts, not on RAH.
The subject can be complicated, but there are no unknowns. You will
find most of the torsional vibration classics listed in the
bibliography of Taylor's "Internal Combustion....". Some texts, like
Wilson's "Practical Solution.." (the ultimate reference) will be
difficult to locate. Try a large university library. The best readily
available text (sort of the ultimate primer on all matters vibrational)
is JP DenHartog's "Mechanical Vibrations". You can buy it for less
than $15 at Amazon. Here is a short list:

CF Taylor, "The Internal-Combustion Engine in Theory and Practice",
1966 (vol. 1), 1968 (vol. 2), MIT Press

W Ker Wilson, "Practical Solution of Torsional Vibration Problems", 3rd
Ed, 5 Vols., 1956, 0412091100, Chapman & Hall

JP Den Hartog, "Mechanical Vibrations", 1956, 070163898, McGraw-Hill

My compliments to Mr. Christley, whose comment (re frequency) was a
sole beacon of accuracy.


Dan Horton

"Dan Horton" wrote in message
ups.com...
There is nothing that eliminates a long shaft from the design of a
PSRU. Nonbelievers might be advised to consider ship propulsion; long
shafts, low cylinder counts, propellers operating in uneven flow, often
via a gearbox. Sound familiar?

The important issue is torsional stiffness of the shaft, not
length. A long shaft can be torsionally stiff or soft, depending on
diameter and material. The engineering process will tailor torsional
stiffness of the shaft (along with a number of other factors) to adjust
natural frequency.

The information you need is found in engineering texts, not on RAH.
The subject can be complicated, but there are no unknowns. You will
find most of the torsional vibration classics listed in the
bibliography of Taylor's "Internal Combustion....". Some texts, like
Wilson's "Practical Solution.." (the ultimate reference) will be
difficult to locate. Try a large university library. The best readily
available text (sort of the ultimate primer on all matters vibrational)
is JP DenHartog's "Mechanical Vibrations". You can buy it for less
than $15 at Amazon. Here is a short list:

CF Taylor, "The Internal-Combustion Engine in Theory and Practice",
1966 (vol. 1), 1968 (vol. 2), MIT Press

W Ker Wilson, "Practical Solution of Torsional Vibration Problems", 3rd
Ed, 5 Vols., 1956, 0412091100, Chapman & Hall

JP Den Hartog, "Mechanical Vibrations", 1956, 070163898, McGraw-Hill

My compliments to Mr. Christley, whose comment (re frequency) was a
sole beacon of accuracy.


Dan Horton

You are very probably right--and it won't be the first time that I believed
that something was still a "black art" until I found out otherwise. For
years after I first became an electronic technician, I believed that about
grounding problems--and then I read a book titled "Sheilding and Grounding
Techniques in Instrumentation." Even 20 years ago, that book was long out
of print; but could still be obtained by special order from University
Microfilm. Almost miraculously, the problems went away!

After reading your post, I decided to look for the books you mentioned and
found that you were correct about the difficulty of locating W Ker Wilson's
book. That could indicate that it is the true source, as the dates
mentioned for earlier editions suggest, and therefore a custom reprint could
be worth every penny and more if a source is known.

The other two books seem to still be available, although I have no idea when
I might find time to read them...

Peter

"Dan Horton" wrote

There is nothing that eliminates a long shaft from the design of a
PSRU. Nonbelievers might be advised to consider ship propulsion; long
shafts, low cylinder counts, propellers operating in uneven flow, often
via a gearbox. Sound familiar?

I think you will find that they do it on ships, with pure weight. A big,
heavy, solid steel shaft. Very heavy! That is how they get the stiffness.

Also, the shaft turns very slowly, so there are many pulses per revolution;
more than you will get with a 4 or 6 cylinder, 4 cycle airplane engine, in
most cases.

I agree with the rest of your post; dig into the engineering text books.
--
Jim in NC

Jim in NC writes:
I think you will find that they do it on ships, with pure weight. A
big,
heavy, solid steel shaft. Very heavy! That is how they get the
stiffness.

Sheesh. Shaft weight is not a factor. And more stiffness may or
may not be desired.

Shaft stiffness is one of the parameters that may be adjusted (up OR
down) so that the system has a natural frequency not matched by a
significant exciting frequency. If the system is driven by internal
combustion, identifying the most significant exciting frequency is dead
simple. It is (RPM x #cyls)/120 = hertz for a 4-stroke and (RPM x
#cyls)/60 = hertz for a two stroke. Designing a system with a natural
frequency that does not match the exciting frequency identified by this
equation is easy IF the engine runs at one RPM only. It gets a lot
harder if you expect to use a wide RPM range.

Also, the shaft turns very slowly, so there are many pulses per
revolution;
more than you will get with a 4 or 6 cylinder, 4 cycle airplane engine,
in
most cases.

Sheesh again. Shaft rotational speed alone is not a factor.

Shaft speed AND number of propeller blades may be of interest if
disturbed flow is the source of an exciting frequency. "Pulses per
revolution" sort of defines the term ''order" as it is used in
rotordynamics (the number of times anything happens per revolution). A
handy term, nothing more. For example, a ship's shaft at 90 RPM turns
1.5 times per second. If it has a four-blade prop and a single source
of disturbed flow (perhaps a strut supporting the shaft), then the
disturbance is a 4th order event. Order times rotational speed per
second (4 x 1.5) means an exciting frequency of 6 hertz. In this case
let us hope the engineer designed a system with no natural frequencies
between 4 and 8 hertz.

Wnat another example of speed x order? Consider the cardan joint.

I agree with the rest of your post; dig into the engineering text
books.

I wish you well with them.

Dan

Dan Horton wrote:
There is nothing that eliminates a long shaft from the design of a
PSRU. Nonbelievers might be advised to consider ship propulsion; long
shafts, low cylinder counts, propellers operating in uneven flow, often
via a gearbox. Sound familiar?

Yes, but they use mass for damping like solid shafts. Weight is
nowhere near the issue it is for aircraft.

Dan, U.S. Air Force, retired

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.