Torsional Vibration and PSRU Design

Dan Horton wrote:
Hello Charles,
According to Ker Wilson, prop flutter has no real impact on
torsional vibration. He could be wrong, but he devoted more than a
half century to the subject. Blade passing frequency, however,
apparently does come into play in some systems. So does whirl, but
that isn't the internet topic of the year.

Ahh, thank you, appreciate the confirmation.

Lucky dog, wish I had my own copy. I have to beg my local
librarian to get it from the UA library.

Dan


A quote:

"In most practical cases coupled axial/flexural modes occur
independently of coupled torsional-flexural modes since there is usually
no appreciable coupling whereby component harmonics of the shaft torque
are able to excite symmetrical blade vibration."

And to your earlier point:

"In aero-engine/airscrew systems there are, in general, two series of
excitations. The airscrew is one source, of aerodynamic origin, arising
from the passage of the blades through a non-uniform airstream, or due
to the airstream entering the airscrew disc obliquely when the aircraft
is executing certain manouevres ..... The other series originates from
the non uniform character of the engine torque."

Hence the blade passing frequency. Still the flexural properties of the
propellor are key in determining how the system will respond to the
excitations since the prop will resonate.

As far as modeling the propeller and determining its natural frequencies
(it has multiple as well) it appears to be a right bear. The shape is
complex and there are multiple modes of vibration and all of them have
to be adjusted for RPM because the stiffness varies with the centrifugal
force (the real kind). For an adjustable prop, the stiffness in the
plane of rotation changes with angle.

Charles

No joy there, the charts I have show the EA81 at over 200 ft-lbs at 3000
rpm, climbing from there and not dropping below 200 through 6000rpm.

Charles

Dan Horton wrote:
Gordon says:
It works because the springs have a preload of a certain force and
will compress only when torsional oscillations reach a certain
amplitude.

Gordon, this is YOUR lucky day! You've bumped into the only guy on
the net who has actually measured the spring rate of clutch disks.
Gosh, Subaru, Chevy truck, Ford truck, Suzuki, a few others too! Let's
look at the spring data for an EA81 2WD clutch disk. No trouble, got
it right here on my hard drive.

Everybody draw a plot, torque up the left side, degrees rotation
across the bottom. Ready? Draw a straight line from 0-0 to 40 ft-lbs
at 3.5 degrees, and from there, proceed straight to 162 ft-lbs at 6
degrees. At a tad past 6 degrees, the springs bottom and the spring
rate becomes near infinite.

Gordon, you got that? Please show us the "preload" that "will
compress only when torsional oscillations reach a certain amplitude".

Dan

It is descending into ridiculous semantics. Semantics, popular usage
notwithstanding, concerns itself with the notion that words have
specific meanings. So for example, when educated professors go to the
trouble of burdening something with the moniker of "fictitious
centrifugal force" they do so in the hopes that people will not in fact
mistake it for a real force. Reactive and fictitious centrifugal forces
are just a convenience for doing the math, and fictitious or not, the
phenomena is a result of inertia, not the cause of inertia.

Charles

Gordon Arnaut wrote:
Charles,

Actually there is more than one reality when it comes to centrifugal force,
namely the reactive centrifugal force and the fictitious centrifugal
force -- depending on what you want to use as your reference frame.

But this is quickly descending into ridiculous semantics. My original point
was that if you have a flywheel with enough inertia, it will be an effective
restraining force to act against excitations that would otherwise produce
vibration. Naturally, higher moment of inertia in a rotating object must
necessitate a higher centrifugal force. Saying that one causes the other is
quite meaningless, in a chicken and egg kind of way.

Regards,

Gordon.

"Dan Horton" wrote

Gordon, this is YOUR lucky day! You've bumped into the only guy on
the net who has actually measured the spring rate of clutch disks.
Gosh, Subaru, Chevy truck, Ford truck, Suzuki, a few others too! Let's
look at the spring data for an EA81 2WD clutch disk. No trouble, got
it right here on my hard drive.

Le'me see if I can predict what Gordon's reply will say. g

He will say something along the lines that "spring rate" is really not what
he was saying, and that you are needlessly being too picky with the
definition, or wording of your reply. He was, after all, just trying to
keep things from getting too technical.

Stay tuned!
--
Jim in NC

No joy there, the charts I have show the EA81 at over 200 ft-lbs at
3000
rpm, climbing from there and not dropping below 200 through 6000rpm.

Charles, better take a look at your units. The above would be
114hp at 3000 and 228hp at 6000.....a rather unusual EA81.

Dan

"Dan Horton" wrote:

Gordon, you got that? Please show us the "preload" that "will
compress only when torsional oscillations reach a certain amplitude".

I wonder if this might add something to the conversation:

http://www.international-auto.com/in...id=2600&cid=41

I recently bought an Alfa Romeo, and was intrigued to find one of
these in front of the first drive shaft (can't figure out why they
NEED two driveshafts on a short sports car, but that's a different
thread).

The transmission end hooks up with three bolts, the driveshaft with
the other three. Oh, and the metal band isn't there once it's
installed.

It seems to me that a device like this would probably give the effect
Gordon's looking for (since there's no "bottoming" of the spring, and
it's clear that the thing is designed to work in the power range we're
discussing (the Alfa Spider has around 120hp).

I'm guessing that this was added to the Alfa drivetrain to cure some
sort of resonance.

Mark "Mr. Flexible" Hickey

Dan,

How many springs are in the clutch disk? More than one right? I believe six
usually.

So multiply the spring rate that you measured by the number of springs. Also
you have taken a clutch disk from a fairly small 4-cylinder engine as an
example. I think the Ross gearbox uses a considerably beefier clutch.

In any case, what's your point? Are you saying that the system will not
change its critical frequency when the springs compress?

Also, just to clear things up about the prop and excitations, I see that you
didn't take issue what what I said, so I will assume that has been cleared
up to everyone's satisfaction.

Regards,

Gordon.







"Dan Horton" wrote in message
oups.com...
Gordon says:
It works because the springs have a preload of a certain force and
will compress only when torsional oscillations reach a certain
amplitude.

Gordon, this is YOUR lucky day! You've bumped into the only guy on
the net who has actually measured the spring rate of clutch disks.
Gosh, Subaru, Chevy truck, Ford truck, Suzuki, a few others too! Let's
look at the spring data for an EA81 2WD clutch disk. No trouble, got
it right here on my hard drive.

Everybody draw a plot, torque up the left side, degrees rotation
across the bottom. Ready? Draw a straight line from 0-0 to 40 ft-lbs
at 3.5 degrees, and from there, proceed straight to 162 ft-lbs at 6
degrees. At a tad past 6 degrees, the springs bottom and the spring
rate becomes near infinite.

Gordon, you got that? Please show us the "preload" that "will
compress only when torsional oscillations reach a certain amplitude".

Dan

Charles,

Thanks for that snippet from the Wilson book.

Please note the part where he says how the "flexural properties of the
propellor are key in determining how the SYSTEM (my emphasis) will respond
to the excitations since the prop will resonate."

Is this not exactly what I said about the prop beginning to resonate and
then cuasing something else in the system to break?

I have said all of this in my posts, with the exception of the part about
prop excitations arising from aircraft manouevers, which is really part of
the point about distubed flow.

Thank you for confirming the correctness of my position. For the record now,
I don't think there can be any serious question that the prop does not
contribute a very real component to the excitations side of the equation.

Regards,

Gordon.




"Charles Vincent" wrote in message
et...
Dan Horton wrote:
Hello Charles,
According to Ker Wilson, prop flutter has no real impact on
torsional vibration. He could be wrong, but he devoted more than a
half century to the subject. Blade passing frequency, however,
apparently does come into play in some systems. So does whirl, but
that isn't the internet topic of the year.

Ahh, thank you, appreciate the confirmation.

Lucky dog, wish I had my own copy. I have to beg my local
librarian to get it from the UA library.

Dan


A quote:

"In most practical cases coupled axial/flexural modes occur independently
of coupled torsional-flexural modes since there is usually no appreciable
coupling whereby component harmonics of the shaft torque are able to
excite symmetrical blade vibration."

And to your earlier point:

"In aero-engine/airscrew systems there are, in general, two series of
excitations. The airscrew is one source, of aerodynamic origin, arising
from the passage of the blades through a non-uniform airstream, or due to
the airstream entering the airscrew disc obliquely when the aircraft is
executing certain manouevres ..... The other series originates from the
non uniform character of the engine torque."

Hence the blade passing frequency. Still the flexural properties of the
propellor are key in determining how the system will respond to the
excitations since the prop will resonate.

As far as modeling the propeller and determining its natural frequencies
(it has multiple as well) it appears to be a right bear. The shape is
complex and there are multiple modes of vibration and all of them have to
be adjusted for RPM because the stiffness varies with the centrifugal
force (the real kind). For an adjustable prop, the stiffness in the
plane of rotation changes with angle.

Charles

Dan Horton wrote:
No joy there, the charts I have show the EA81 at over 200 ft-lbs at
3000
rpm, climbing from there and not dropping below 200 through 6000rpm.

Charles, better take a look at your units. The above would be
114hp at 3000 and 228hp at 6000.....a rather unusual EA81.

Dan

Your right, it was late. I was reading the chart I had wrong. It showed
torgue at the prop flsnge ( after reduction) plotted agsainst crankshaft
RPM. I don't have a chart for the naked engine.

Charles

Actually Gordon, the words you quoted were my words not Wilson's. You
will notice there are no quotaion marks around them in my original post.
The text with the quotation marks is from Wilson. The excerpt was
actually confirming Dan's contention that the excitation source was
disturbed airflow, that it does not originate in the prop. Take the
disturbed airflow away and the natural hysterisis of the prop and rest
of the system will cause it to return to normal. So while excitations
can enter through the prop or they can enter through the crank, these
two components don't create the excitation, they react to them. There
are components in a redrive system that can originate excitations
though, which is why if you just want to fly it is easiest to pick a
direct drive wooden prop snd go fly. Not guaranteed, but much simpler.

Charles



Gordon Arnaut wrote:
Charles,

Thanks for that snippet from the Wilson book.

Please note the part where he says how the "flexural properties of the
propellor are key in determining how the SYSTEM (my emphasis) will respond
to the excitations since the prop will resonate."

Is this not exactly what I said about the prop beginning to resonate and
then cuasing something else in the system to break?

I have said all of this in my posts, with the exception of the part about
prop excitations arising from aircraft manouevers, which is really part of
the point about distubed flow.

Thank you for confirming the correctness of my position. For the record now,
I don't think there can be any serious question that the prop does not
contribute a very real component to the excitations side of the equation.

Regards,

Gordon.




"Charles Vincent" wrote in message
et...

Dan Horton wrote:

Hello Charles,
According to Ker Wilson, prop flutter has no real impact on
torsional vibration. He could be wrong, but he devoted more than a
half century to the subject. Blade passing frequency, however,
apparently does come into play in some systems. So does whirl, but
that isn't the internet topic of the year.

Ahh, thank you, appreciate the confirmation.

Lucky dog, wish I had my own copy. I have to beg my local
librarian to get it from the UA library.

Dan


A quote:

"In most practical cases coupled axial/flexural modes occur independently
of coupled torsional-flexural modes since there is usually no appreciable
coupling whereby component harmonics of the shaft torque are able to
excite symmetrical blade vibration."

And to your earlier point:

"In aero-engine/airscrew systems there are, in general, two series of
excitations. The airscrew is one source, of aerodynamic origin, arising
from the passage of the blades through a non-uniform airstream, or due to
the airstream entering the airscrew disc obliquely when the aircraft is
executing certain manouevres ..... The other series originates from the
non uniform character of the engine torque."

Hence the blade passing frequency. Still the flexural properties of the
propellor are key in determining how the system will respond to the
excitations since the prop will resonate.

As far as modeling the propeller and determining its natural frequencies
(it has multiple as well) it appears to be a right bear. The shape is
complex and there are multiple modes of vibration and all of them have to
be adjusted for RPM because the stiffness varies with the centrifugal
force (the real kind). For an adjustable prop, the stiffness in the
plane of rotation changes with angle.

Charles