Torsional Vibration and PSRU Design

[Gordon Arnaut]

I hope ADK, who posted a question a few days ago on an interesting design
problem involving reduction gearing and a driveshaft got some insight from
the various postings.

The subject of torsional vibration quickly cam up and there was a lot of
stuff mentioned, but not really a simple and understandable explanation of
the forces at work.

Torsional vibration is not that hard to understand for the non-engineer, but
one must think of it in terms of excitation forces on one side of the
equation, and restraining forces on the other.

In a drivetrain system excitation forces come largely, but not solely, from
combustion firing events. Another contributor of excitation is imbalance in
a rotating or reciprocating mass. (Incidentally, it is impossible to
perfectly balance a reciprocating engine.)

Yet another source of excitation in airplanes is the spring effect of the
prop, where the blade tips whipsaw back and forth as they undergo
acceleration and deceleration due to the torque spikes of cylinder firing.

These various excitation forces work to induce vibration in the drivetrain
system and its various pieces.

Restraining forces counteract the tendency toward vibration. They consist of
three factors: stiffness, mass and dampening.

So what happens when torsional vibration starts to take place? First of all,
torsional vibration is a twisting vibration where the amplitude is measured
in degrees of rotation. It should not be confused with linear vibration,
which is the up and down and side to side shaking of an engine.

Torsional vibration in engines happens on a very small scale in terms of
overall movement. If a crankshaft were subjected to a twisting movement of
even a degree or two, it would not last more than a few minutes.

A large-diameter springy mass such a propeller will obviously be able to
exhibit considerably more twist at its tip, but there is still a limit to
how much it can twist before it will break.

Torsional vibration is simply the result of the various excitation forces
causing the drivetrain, or its constituent pieces to start twisting back and
forth.

The issue of frequency was brought up in the previous dicsussion, but there
were some misconceptions expressed.

As mentioned, every object in the engine -- or every other object for that
matter -- has its own natural "resonant" frequency. That is the frequency at
which a particular object will vibrate if it is acted upon by a single
excitation of sufficient force to displace it from its resting state.

Think of a diving board. If you jump on the diving board with enough force
to set it in motion, it will continue to vibrate at its natural resonant
frequency -- even if you depart the board after a single jump.

Just like the diving board, a crankshaft has its own natural resonant
frequency and will vibrate if it is disturbed with enough force. Where the
problem gets tricky is if that disturbing force is timed to exactly coincide
with the vibrational frequency. With each additional "push" coinciding
exactly with the oscillations, the amplitude starts to increase.

What that means in a twisting vibration is that the shaft will twist more
and more until it breaks.

So does this mean that frequency is somehow supremely important? That you
must make sure that your excitation frequency and your resonant frequency
must never coincide?

Quite simply, no. It is quite possible to have an engine with a resonant
frequency at a certain rpm and it will run all day at that rpm with no
problem.

How is that possible? This is where we go back to the basic equation of
excitation versus restraining forces.

If your excitation force is not strong enough to actually displace the
object, it cannot possibly set it vibrating. Think of a massive diving board
made for giants and a tiny person trying to jump on it. That litte
leprechaun is not going to set the board vibrating no matter how hard he
jumps on it. There are no worries about resonance or vibrations of any kind.

This is due to the restraining force of mass, which we mentioned at the
beginning. This shows that mass is important, unlike some erroneous comments
that mass doesn't matter. It should be intuitive that the mass of an object
and the force of the excitation must be proportional if there is going to be
any vibration created.

Stiffness is another restraining force. It acts similarly to mass in that it
makes it hard to disturb it from its natural state and therefore requires
that any excitation force will have to be much more powerful to produce any
kind of vibration. Think of a very stiff and rigid diving board. A small
child may not be able to deflect it at all.

And what happens if the excitation force is strong enough to overcome the
restrining forces of mass and stiffness? That's where damping comes in. If
the excitation force is strong enough to actually produce vibrations, you
can stop those vibrations if you apply enough dampening force.

Think again of the diving board. If you jump up and down on it to set it
vibrating, what happens if you then change your mind about jumping off and
use your knees as shock abosrbers to abort the jump? The board stops
vibrating.

It's the same with an engine. A flywheel is a simple example of a damping
device. It uses centrifugal force to counteract and overcome the twisting
acceleration imparted by the cylinder firing. The twisting force is not
strong enough to overcome the centrifugal force of the spinning flywheel so
vibrations can't get started.

It doesn't matter a whit that the frequency of of the engine at a particular
rpm may coincide exaclty with its resonant frequency. If the damping is
sufficient, frequency is meaningless.

Likewise for mass and stiffness. It doesn't matter if the frequency of the
excitations exactly coincides with the natural resonant frequency of the
engine if the engine is stiff and massive enough not to be disturbed.

Almost all four-cylinder engines operate perfectly well without any kind of
torsional damping device because of the simple fact that their short
crankshafts are quite stiff.

This despite the fact that they only have four cylinders so the torque
spikes are quite severe.

A six cylinder engine is actually more problematic because, while it has
smoother torque pulses, also has a longer and less stiff crankshaft.

A V-8 is even more of the same, but here the crankshaft is flexible enough
that it's natural frequency is now quite low and possibly even below idle
rpm.

So for the sake of ADK's question, he seems to have chosen the most
problematic configuration.

Also worth noting here is that when we talk about torsional vibration, we
are must examine the entire drivetrain system as a whole -- not just its
bits and pieces. As soon as you bolt anything to the engine, you have now
changed the vibrational characteristics of that engine, along with its
natural resonant frequencies. (Yes there can be more than one).

In the simplest aircraft propulsion system you have just the engine directly
driving a prop. That drivetrain will now have different vibration and
resonance issues than jsut the engine alone. And here is where it gets hard,
because it is almost impossible to predict what the vibrational
characteristics will be. The only way is to instrument and measure.

When you add a reduction drive, whether belt or gear, you have added an
additional component and have again changed the vibrational characteristics.
And then to add a driveshaft onto all that, well, you can certainly see that
this is adding layer upon layer of vibrational complexity.

So the simple conclusion is that this is a very problematic configuration.
That's not to say that it will be impossible to make work reliably, but it
is a considerable engineering challenge.

Some have made this kind of arrangement work. Taylor and the Mini-imp have
been mentioned and that's a good example. I don't know if this link was
already mentioned, but here it is:
http://www.mini-imp.com/index.htm

Dornier also created a very interesting axial thrust twin near the end of
WWII, the 335, which used a drivehsaft to turn the rear prop at the end of
the empennage. A marvelous design that performed exceptionally.
Incidentally, when Cessna came out with their axial-thrust Skymasters, they
gave it the 336 designation, followed by the 337, in an obvious nod to the
Dornier ancestry.

So these examples show that it can be done, but you will have to put your
thinking cap on and study up on those engineering texts.

Regards,

Gordon Arnaut.

[Ian Stirling]

Gordon Arnaut > wrote:
<snip>
> These various excitation forces work to induce vibration in the drivetrain
> system and its various pieces.
>
> Restraining forces counteract the tendency toward vibration. They consist of
> three factors: stiffness, mass and dampening.
<snip>
> As mentioned, every object in the engine -- or every other object for that
> matter -- has its own natural "resonant" frequency. That is the frequency at
> which a particular object will vibrate if it is acted upon by a single
> excitation of sufficient force to displace it from its resting state.

I'm going to have to disagree.

Stiffness, and mass are almost irrelevant to torsional vibration (and
any other sort).
Stiffness and mass change the resonant frequency.

I'd approach this from the 'Q' perspective. Q is 'quality factor', which
basically means how good something is at resonating.
A Q of 100 means that it will oscillate 100 times at its resonant
frequency before the oscilation decays to 27% of its original level (not
completely sure of the 27%).

Consider a mass on the end of a stiff wire hanging down from a fixed
point.
Twist and hold it.
There is now energy stored in the wire, in the form of deformation. If
you try to twist it too far, well, any structural element can only take so
much energy stored in it, and if you exceed that, it bends or breaks.

Let the weight go, and it'll spin clockwise, come to rest, then spin
back anticlockwise.
This is energy being traded from potential (stored in the wire) to
kinetic (movement) and back.
Let's call this amount of energy stored 'E'.

If the wire was a springy steel one, then the Q is probably quite high,
say 20.

In order to keep the mass spinning clockwise and anticlockwise, to the
same amount, you need to supply some energy.
Now, the higher the Q is, the less energy. E/Q, or E/20.
If you supply (at just the right times) 1 unit of torsional energy, the
system will build up so that it's storing 20 units in the vibration.

It's just like a swing - if it's got a heavy weight on it, but you push
it a little every time, it'll build up to a decent swing.

The other side to this is that as it's only storing 20 units of energy,
after a time, somewhere else that 1 unit is coming out, probably as heat,
maybe in another part of the system as vibration.

There are several ways to address this.

You can make sure that the resonant frequencies are never hit - which is
horribly hard in many cases, or make sure that the Q is very low, which
means low stored energy.

Adding mass, or stiffness does not alone change Q.

Taking the earlier posted example of the overheating clutch.

Let's say a 50Kw engine was pumping in 10KW of vibrational power, to a long
springy shaft, connected to a clutch.
Say the Q is 10, at the frequency that the engine is running at.

After a few moments, the vibrational power in the shaft is not 10Kw, but
100Kw, the vibrational torque stored in the shaft is 10 times higher than the
input torque, and the shaft is twisting back and forth much more than
expected.
But, the clutch is only rated for 70Kw maximum of torque, and starts to
slip back and forwards.
This damps the vibration in the shaft somewhat, but in the end, it ends
up with most of the 10Kw vibration output heating the clutch.

(I'm mixing power and energy, but I hope it's clear)

Now, you can fix this several ways.

Double the clutch size.
This will fix the immediate problem, but as the energy in the shaft now
is not being damped in the clutch, the vibration there will increase.
This may cause other things to break in the system, maybe the prop,
maybe the PSRU, maybe the pilot :)

Add a big sticker to the panel saying "Do not run for more than 10
seconds at between 2300 and 2400 RPM"
Maybe OK in some applications.

Make the shaft more or less stiff, or more or less massive.
This will change the resonant frequency, and may avoid the problem at
2340 RPM, but may add a new one at 3200.

Or, you add some sort of damping to the system, reducing the Q.

I need to go to bed, will add more tomorrow.

[Montblack]

("Ian Stirling" wrote)
[snip]
> Or, you add some sort of damping to the system, reducing the Q.
>
> I need to go to bed, will add more tomorrow.


....and we'll read it.

Thanks for posting. And thanks to Gordon, too.


Montblack

[cavelamb]

Gordon Arnaut wrote:
> I hope ADK, who posted a question a few days ago on an interesting design
> problem involving reduction gearing and a driveshaft got some insight from
> the various postings.
>
> The subject of torsional vibration quickly cam up and there was a lot of
> stuff mentioned, but not really a simple and understandable explanation of
> the forces at work.
>
snipped out the juicy part
>
> So these examples show that it can be done, but you will have to put your
> thinking cap on and study up on those engineering texts.
>
> Regards,
>
> Gordon Arnaut.
>
>
>

Fabulous piece of work, Gordon.

Very well written, understandable, and agrees with what I think I understand of
the subject.

There is a difference between being able to understand the subject matter and
being able to explain it well.

I think you've done both here.

Don't know what more to add to that.


Richard Lamb

[Morgans]

"cavelamb" > wrote

> Very well written, understandable, and agrees with what I think I
> understand of
> the subject.


There are parts, here and there that miss the mark, but mostly correct, and
well written.

Hopefully it will keep an unknowing experimenter from jumping in way too
deep, then finding out, far too late. It is a very tough subject, once you
actually start on the nuts and bolts part...
--
Jim in NC

[Morgans]

"Gordon Arnaut" > wrote

> Cog belts are noisy, tend to fail more quickly and are expensive -- and
> they are only needed in applications where synchronous operation is
> required, such as in driving camshafts.
>
> Today's multi-v belts -- the kind used to drive engine accessories on
> newer cars -- are highly efficient and can handle huge amounts of power,
> up to 1000 hp.

That is the first bad advice you have given.

It is interesting to note that the cog belt is the drive of choice, for
manufacturers of V-6 and V-8 redrives manufactured today. Perhaps they know
something that you don't?

Belts are lasting 200 hours with no sign of wear, BTW.
--
Jim in NC

[stol]

>Chain drives have the same effect.


>Belt and chain drives do impose side loads on the front crankshaft journal,
>however, so that is a negative point.


>Another issue is packaging

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

Well written explanation but this is incorrect. The sideload is imposed
on the rearmain journal of the crank, not the front one. Most redrives
that use the cog belt also incorporate an idler bearing outboard of the
lower sprocket, that prevents the sideload forces on the crank. Also I
think a multivee belt will not transfer 1000 hp, I don't think it would
evem work on my 330 hp+ auto engine powered plane. I will stick with my
cog belt, thank you.

Ben
www.haaspowerair.com

[stol]

>Chain drives have the same effect.


>Belt and chain drives do impose side loads on the front crankshaft journal,
>however, so that is a negative point.


>Another issue is packaging

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

Well written explanation but this is incorrect. The sideload is imposed
on the rearmain journal of the crank, not the front one. Most redrives
that use the cog belt also incorporate an idler bearing outboard of the
lower sprocket, that prevents the sideload forces on the crank. Also I
think a multivee belt will not transfer 1000 hp, I don't think it would
evem work on my 330 hp+ auto engine powered plane. I will stick with my
cog belt, thank you.

Ben
www.haaspowerair.com

[Dan Horton]

Gordon Arnaut wrote:
<<Another contributor of excitation is imbalance in a rotating or
reciprocating mass.>>

True, but it has limited power compared to gas pressure
oscillation. I was trying to keep things simple.

<<Yet another source of excitation in airplanes is the spring effect of
the prop>>

Naaaa. Propellers in disturbed flow can excite the system, but
usually the concern is the opposite. Might be a few rare causes. For
example, I've seen (with telemetry) a variation in torsional vibration
due to a propeller-powerplant whirl mode.

<<a crankshaft has its own natural resonant frequency and will vibrate
if it is disturbed with enough force.>>

Actually a crank has N-1 natural frequencies, where N equals # of
inertias. That would be four frequencies for a typical 4 cyl with 90
degree throws (4 crankthrows plus flywheel, 5 minus 1). However, to be
fair, when designing a PSRU you can model the crank and flywheel
assembly as a single mass moment of inertia, IF the crank is short and
stiff. The inaccuracy in F1 prediction will be small, like 200-300
RPM.

<<A flywheel is a simple example of a damping device. It uses
centrifugal force to counteract and overcome the twisting.....>>

A flywheel is an inertia. A damper is a device that removes energy
from the system, usually as heat. Think slipping clutch, slipping
v-belt, or viscous ring damper. Designed a viscous disk damper and ran
it parallel with a soft element in a drive a few projects back. It
shed a lot of heat, and telemetry said it damped resonant amplitudes
very well. The successful Raven drive for the 3 and 4 cyl Suzukis uses
a dry frictional damper.

Ahhhh, I'll let you correct the part about centrifugal force <g>

<<This shows that mass is important, unlike some erroneous comments
that mass doesn't matter.>>

Who said anything about mass? For the record, please note that the
previous comment was "Shaft weight is not a factor", the context being
ship propulsion. Shoot, I'm all for careful use of terms. In the
context of torsional vibration, what IS important is "mass moment of
inertia". And that ain't the same as mass or weight.

Ok, you argue that torsional problems can be eliminated through the
use of flywheel mass and stiff shafting. I argue that your approach
has severe drawbacks when applied to the subject at hand, a long shaft
aircraft system.

I agree that a large-inertia flywheel (which is not necessarily a
large-mass flywheel) always reduces vibratory amplitude. It may not
be reasonable to incorporate a huge flywheel inertia in an airplane
because of effect on (1) handling (remember the Sopwith Camel), as well
as (2) aircraft empty weight. You must use a moderate flywheel, a
compromise, not the infinite inertia you describe.

As for stiffness in the shafting that connects the inertias, what
magic did you have in mind? All practical shaft materials exhibit a
stress-strain relationship. I know of only one practical PSRU concept
that meets your goal of near infinite stiffness; it has no shafts at
all other than the crankshaft. Hardly the long shaft system under
consideration. With a shaft several feet long, some degree of twist is
physical reality.

Given that infinite stiffness is impossible in the long shaft
system, I'll tell you what you'll really get. A stiff shaft will raise
the system F1 so that it intersects the gas pressure oscillation order
somewhere up in the operating range close to peak torque. The system
will resonate into junk. The classic solution then tried by the
uninformed is to make it "stronger" (the result being stiffer), which
makes the problem worse. Near idle or below idle is where you want the
intersection of F1 and gas pressure frequency, because gas pressure
oscillation isn't very powerful at idle. You do that with a soft shaft
or rubber element, and note that it doesn't take a huge inertia to
smooth a small near-idle-speed oscillation. By tailoring frequencies,
we can get a practical, lightweight system.

Nobody can teach this subject in RAH posts. Hell, "Practical
Solution" is several volumes. What we can do is (1) direct folks to
quality reference material, and (2) quit telling them it is impossible.
I think I'll puke if I see one more guy reference the Hessenaur
article and declare "torsional vibration even beat Rutan". If somebody
had handed Burt the right books or introduced him to J.P. Den Hartog,
you can bet you wouldn't be reading that crap.

Dan

[Fly]

>
> Naaaa. Propellers in disturbed flow can excite the system, but
> usually the concern is the opposite. Might be a few rare causes. For
> example, I've seen (with telemetry) a variation in torsional vibration
> due to a propeller-powerplant whirl mode.
>

Hi Dan,
Can you say a little more about the example of the whirl mode you saw?

Kent Felkins
Tulsa


*** Posted via a free Usenet account from http://www.teranews.com ***

[Dan Horton]

Hi Kent,
Long time no talk. You still doing balance work?

Seemed to be a whirl mode much like that described for a radial with
a too-loose front propshaft bearing. This was with the 3 cyl Suzuki,
an engine with a natural wobble, again much like the radial. The PSRU
was a cantilever upper axle type, so just a little excess freeplay in
the bearing setup was enough to set it whirling at high power. The
torsional amplitude pulsed at about 2 hertz on the o-scope display.
Took awhile to realize what we were had. You could hear it in the prop
noise and see the whirl at night if you lit the prop disk with a flood.
Didn't explore it much as we had other stuff on the front burner.
Just got rid of the freeplay.

Dan

[Gordon Arnaut]

Ben,

I wasn't wrong about the front journal, it's just a matter of semantics.

When I said the front journal, I meant the one from which the drive pulley
is attached, obviously. Even if this is the rear journal on the engine as
mounted in the car, it is obviously the frontmost journal when a redrive and
prop is bolted to it.

And you are quite wrong about poly-v belts. They can indeed take upwards of
1000hp. I'm looking right now at the Hutchinson design manual and there are
belts available that will take 800 kilowatt, which is 1072 horsepower.

Also significant is the design rpm and duty cycle, both of which are very
high.

Cog belts are inherently inferior because the ribs acting against the pulley
sprockets produce more heat. They should only be used where you need a
synchronicity between the drive and the driven device. This is not the case
with a prop -- it does not need to be synchronized.

(Incidentally, "cog" is not even the correct terminology for these belts,
they are called synchronous belts. Cog belts are actually V-belts, with ribs
on their inside surface for heat dissipation, like fins on cylinder heads.)

Someone mentioned 200 hours for synchronous belts, which is nowhere near
what a properly desgned poly-v belt will provide.

Why are the "cog" belts so popular then? Who knows, but being popular does
not make it sound engneering.

Personally I would not use any commercially available redrive, gearbox or
belt, except for the ones offered by Rotax and Powersport, because they are
the only vendors who have actually done scientific torsional vibration
testing and measurement and have given prop moment of inertia ranges that
are known to be safe.

Regards,

Gordon.



"stol" > wrote in message
ups.com...
> >Chain drives have the same effect.
>
>
>>Belt and chain drives do impose side loads on the front crankshaft
>>journal,
>>however, so that is a negative point.
>
>
>>Another issue is packaging
>
> /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
>
> Well written explanation but this is incorrect. The sideload is imposed
> on the rearmain journal of the crank, not the front one. Most redrives
> that use the cog belt also incorporate an idler bearing outboard of the
> lower sprocket, that prevents the sideload forces on the crank. Also I
> think a multivee belt will not transfer 1000 hp, I don't think it would
> evem work on my 330 hp+ auto engine powered plane. I will stick with my
> cog belt, thank you.
>
> Ben
> www.haaspowerair.com
>

[Morgans]

"Gordon Arnaut" > wrote
>
> Personally I would not use any commercially available redrive, gearbox or
> belt, except for the ones offered by Rotax and Powersport, because they
> are the only vendors who have actually done scientific torsional vibration
> testing and measurement and have given prop moment of inertia ranges that
> are known to be safe.

I remember you, now. I replied with BULL**** back then, and I'll do the
same, now. To say that nobody except Rotax and Powersport have tested their
drives is bull****, and if I were a drive manufacture that had, I would sue
you for slander.

Go back into your hole, where you have been for the past 10 or so months.
We don't need know-it-all like you, spouting off.
--
Jim in NC

P.S. Go ahead, be true to form, and make some personal remarks about me
now. I can take it. I consider the source, and take it for what it is
worth. Zero.

[Fly]

Hi Dan,
Yes, I'm still busy. Interesting example of the Suzuki.
I lost your email in my last CPU crash. And occasionally have wondered what
you was up to.
I'll write you later.


Btw, I'd to take this opportunity make an announcement to others in this
thread.

I've hung out on RAH since ohh maybe 1998-99 and I still check in a couple
times per week.
Anymore though, when I open the cellar door I hear too much scurrying to
want to come down the steps.

Last time I tried to contribute a little benefit of my experience about
balancing and vibration to a thread, some people quickly turned it personal,
inferred me and my friends were crooks and pieces of ****, or P.O.S. I think
was their term. Others here supported them.
In fact some same people contributed to this recent thread!
They know who they are. .
F Amateurs. Go Get a job.

Kent Felkins





"Dan Horton" > wrote in message
oups.com...
>
> Hi Kent,
> Long time no talk. You still doing balance work?
>
> Seemed to be a whirl mode much like that described for a radial with
> a too-loose front propshaft bearing. This was with the 3 cyl Suzuki,
> an engine with a natural wobble, again much like the radial. The PSRU
> was a cantilever upper axle type, so just a little excess freeplay in
> the bearing setup was enough to set it whirling at high power. The
> torsional amplitude pulsed at about 2 hertz on the o-scope display.
> Took awhile to realize what we were had. You could hear it in the prop
> noise and see the whirl at night if you lit the prop disk with a flood.
> Didn't explore it much as we had other stuff on the front burner.
> Just got rid of the freeplay.
>
> Dan
>


*** Posted via a free Usenet account from http://www.teranews.com ***

[Dan Horton]

Gordon Arnaut wrote:
<<However, the prop is a spring mass that acts to amplify the
excitations that comes from other sources...>>

No. Blade root bending within the plane of rotation can be modeled
as equivalent to a shaft stiffness. The prop itself has no special
ability to excite anything.

<<And yes, it is the inertia produced by the centrifugal force, not the
centrifugal force itself ....>>

Centrifugal force has nothing to do with it, period.

<<However I do not agree that the problem frequency will necessarily
have to fall within the operating range. Stiffness of the shaft will be
largely a function of its slenderness ratio, so using a material with a
high modulus, perhaps carbon, and a large diameter, could produce a
shaft that is light yet stiff enough to do the trick.>>

Overall system stiffness is cumulative. Every shaft or shaft
equivalent (crank twist, belt or chain elongation, flexible structure
between sprockets, blade root bending, whatever) contributes so that
the overall system is somewhat less than infinitely stiff. Even with a
hell-for-stiff carbon shaft I don't think you can push F1 up above the
operating range. You'll need an F1 above 220 hz to work with a 5500
RPM 4-cyl, or 300 hz for a 6-cyl with a 5000 RPM operating range. The
current average for short "hard" systems (think Blanton style) is about
50 hz. Good luck.

Dan

[Gordon Arnaut]

Dan,

The prop can indeed contribute excitations, if it is first excited to go
into resonance itself. As the prop oscillations grow in amplitude it will
overstress other components that are not as flexible, such as the crank or
gearbox.

This can happen to a non-counterweighted Lycoming with certain CS props if
you operate it continuously between 2050 and 2250 rpm, or thereabouts.

What happens is the prop has a resonant frequency at that rpm range, which
means it will go into resonance and its oscillations will continue to grow.
But long before the prop breaks, the crank will break, because it is far
less flexible.

Without the prop, the engine has no problem running all day at that rpm, so
it is the prop that is contributing the excitations. Further proof is that
the lighter MT prop is STC'd to eliminate the placarded operating
limitation.

So the prop does indeed contribute to the excitation side of the equation,
if only in a roundabout way.

Of course the full explanation is more complex because when you combine a
prop that has its particular resonant frequency and and and engine with a
different set of resonant frequencies, what you get is a whole new set of
resonant frequencies. By bolting these two items together, you now have a
new system with its own vibrational characteristics.

And if you add a gearbox too, then you get a whole new set of resonant
frequency, because now you have a new system again. And here again the
g4earbox itself can now also contribute to the excitation side of equation
in the same way as the prop -- by going into resonance and either breaking
itself or something else in the chain.

An interesting approach has been to use a spring-loaded clutch disk between
the crank and the gearbox -- this is used in the Ross gearbox for example.
Most people assume that this works because the springs compress to
"damp"some of the torsional vibration, but that's not how it works.

It works because the springs have a preload of a certain force and will
compress only when torsional oscillations reach a certain amplitude. What
happens then is not some kind of damping, but the fact that as soon the
springs are compressed the system now instantly has more springiness, which
means its resonant frequencies are now completely different from when the
springs were locked up solidly. You have in essence a drivetrain system with
variable resonant characteristics.

This means that as soon as the springs kick in, the system is no longer in
resonance because its resonant frequencies are now quite a bit lower, due to
the added flexibility of the entire system. So the oscillations stop. You
yourself alluded to this in your remarks about overall system stiffness.

This is illustrative of how the problem needs to be approached from a
drivetrain system perspective -- because each piece that you attach to an
engine will change the system as a whole. It also shows that each piece can
contribute excitations if any constituent piece is allowed to go into
resonance.









"Dan Horton" > wrote in message
ups.com...
> Gordon Arnaut wrote:
> <<However, the prop is a spring mass that acts to amplify the
> excitations that comes from other sources...>>
>
> No. Blade root bending within the plane of rotation can be modeled
> as equivalent to a shaft stiffness. The prop itself has no special
> ability to excite anything.
>
> <<And yes, it is the inertia produced by the centrifugal force, not the
> centrifugal force itself ....>>
>
> Centrifugal force has nothing to do with it, period.
>
> <<However I do not agree that the problem frequency will necessarily
> have to fall within the operating range. Stiffness of the shaft will be
> largely a function of its slenderness ratio, so using a material with a
> high modulus, perhaps carbon, and a large diameter, could produce a
> shaft that is light yet stiff enough to do the trick.>>
>
> Overall system stiffness is cumulative. Every shaft or shaft
> equivalent (crank twist, belt or chain elongation, flexible structure
> between sprockets, blade root bending, whatever) contributes so that
> the overall system is somewhat less than infinitely stiff. Even with a
> hell-for-stiff carbon shaft I don't think you can push F1 up above the
> operating range. You'll need an F1 above 220 hz to work with a 5500
> RPM 4-cyl, or 300 hz for a 6-cyl with a 5000 RPM operating range. The
> current average for short "hard" systems (think Blanton style) is about
> 50 hz. Good luck.
>
> Dan
>

[Charles Vincent]

Dan Horton wrote:
> Gordon Arnaut wrote:
> <<However, the prop is a spring mass that acts to amplify the
> excitations that comes from other sources...>>
>
> No. Blade root bending within the plane of rotation can be modeled
> as equivalent to a shaft stiffness. The prop itself has no special
> ability to excite anything.
>
> <<And yes, it is the inertia produced by the centrifugal force, not the
> centrifugal force itself ....>>
>
> Centrifugal force has nothing to do with it, period.
>

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.

Centrifugal force is a result of the inertia, not the other way around.
At least in this reality.

Not only does the crank/rod/piston system have multiple resonant
torsional frequencies, they move during operation.

While we're at it, it sure would be handy to see a list of V8
crankshafts with a critical lower than the idle rpm gas excitation
forces, since that is apparently often the case.


Charles

[Morgans]

"Gordon Arnaut" > wrote

> This means that as soon as the springs kick in, the system is no longer in
> resonance because its resonant frequencies are now quite a bit lower, due
> to the added flexibility of the entire system. So the oscillations stop.
> You yourself alluded to this in your remarks about overall system
> stiffness.

For pete's sake, give it a rest. Your incomplete knowledge is showing in
spades.
--
Jim in NC

[Gordon Arnaut]

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.






"Charles Vincent" > wrote in message
et...
> Dan Horton wrote:
>> Gordon Arnaut wrote:
>> <<However, the prop is a spring mass that acts to amplify the
>> excitations that comes from other sources...>>
>>
>> No. Blade root bending within the plane of rotation can be modeled
>> as equivalent to a shaft stiffness. The prop itself has no special
>> ability to excite anything.
>>
>> <<And yes, it is the inertia produced by the centrifugal force, not the
>> centrifugal force itself ....>>
>>
>> Centrifugal force has nothing to do with it, period.
>>
>
> 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.
>
> Centrifugal force is a result of the inertia, not the other way around. At
> least in this reality.
>
> Not only does the crank/rod/piston system have multiple resonant torsional
> frequencies, they move during operation.
>
> While we're at it, it sure would be handy to see a list of V8 crankshafts
> with a critical lower than the idle rpm gas excitation forces, since that
> is apparently often the case.
>
>
> Charles
>
>
>
>
>

[Dan Horton]

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

[Charles Vincent]

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

[Charles Vincent]

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
>

[Charles Vincent]

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.
>
>

[Morgans]

"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

[Dan Horton]

<<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

[Mark Hickey]

"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/index.cfm?fa=p&pid=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

[Gordon Arnaut]

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
>

[Gordon Arnaut]

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

[Charles Vincent]

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

[Charles Vincent]

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
>
>
>

[Charles Vincent]

Gordon, you can run down the rabbit hole of relativity with someone
else. The fact is your statement that "it is the inertia produced by
the centrifugal force," is wrong. Inertia is not the result of
centrifugal force, it is the reverse. You could even argue semi
successfully that centrifugal force was just another name for inertia,
but you can't say inertia is produced by the centrifugal force and be
correct.

Charles

Gordon Arnaut wrote:
> Charles,
>
> I guess the concept of relativity is just a "convenience" for doing the math
> too right?
>
> Please do not descend any further into absurdities. Even Newton was well
> aware of the relativity of motion long before Einstein came along, which is
> why his law of inertias takes into account frames of reference.
>
> Please quit before you bury yourself in a sinkhole.
>
> Regards,
>
> Gordon.
>
>

[Bill Daniels]

Just a thought here.

If the question is about a longish shaft that may support 'windup' torsional
vibrations, there are small torque sensors that can be clamped to a rotating
shaft that transmit their data wirelessly to a handheld display unit. Some
of these should have enough bandwidth to show the amplitude of any torsional
vibrations. This is a testing issue but it may allow the tuning of a
redrive and disclose any RPM bands that should be avoided.

bildan

"Mark Hickey" > wrote in message
...
> "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/index.cfm?fa=p&pid=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 Horton]

Gordon Arnaut wrote:
<<So multiply the spring rate that you measured by the number of
springs.>>

The quoted data is for all springs combined, not that it would make
any difference.

<<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.>>

No change in operating principle, just torque capacity before
bottoming.

<<In any case, what's your point?>>

That your assertion ("It works because the springs have a preload
of a certain force and will compress only when torsional oscillations
reach a certain amplitude.") is bull****. If your assertion were true,
the initial part of the graph would be a vertical line.

Dan

[Dan Horton]

<<Up until that point the springs are not compressed and the coupling
is in effect a solid coupling.>>

Good God, he still doesn't get it.

Gordon, plot the supplied data and study at it carefully. If your
assertion were true, the plot would not leave the Y axis until reaching
some elevated torque value. Note that the real plot begins at 0-0.

Dan

[Dan Horton]

Bill writes:
<<small torque sensors that can be clamped to a rotating shaft that
transmit their data wirelessly>>

Yes, for torsion the classic setup is a wheatstone bridge strain
guage array feeding a telemetry transmitter. I shopped new digital
transmitters a few years ago but didn't buy, hoping for better prices
and bandwidth later. We'll see. In 99/early 2000 we used a wheatstone
bridge and a borrowed analog transmitter from Wireless Data. The
bridge was still on the propshaft at S&F that year. I was there all
week and nobody ever asked why the propshaft was wrapped in strapping
tape <g>.

Dan

[Dan Horton]

Hello Mark,
<<was intrigued to find one of these in front of the first drive
shaft>>

Appears to be a Goetz brand soft element, which is what Rotax
installs in the C and E box. Lord makes a similar unit called a
Dynaflex LCD (or something like that). I like the Lovejoy Centaflex.
One of the bolt sets is aligned radially, thus no need for the
installation band, plus the machine you're designing only needs one
flange instead of two. The radial bolt set can connect directly into
the driving or driven shaft.

All of the above come in carefully graduated spring rates. They are
generally used to lower natural frequency. You select one based on
your need for a particular torsional stiffness, and then make sure it
also meets criteria for torque capacity, etc.

The successful Suzuki drive used a Shore 50 Centaflex CF12.
Running in parallel with a viscous disk damper, maximum measured
vibratory torque during steady-state resonant operation was about 115
ft-lbs at 1500. Without damping maximum amplitude was about 180
ft-lbs. We did not install telemetry on the previous bad drive (a hard
system with no soft element) but we did model it. Predicted vibratory
amplitude was around 10 times the above, at a critical RPM of 2200.
Having flown it, I had no trouble accepting the model results. It
sucked <g>

Dan

[Gordon Arnaut]

Dan,

You're right about one thing. I don't get what your objection is.

Are you saying the springs immediately begin to compress at the first sign
of torque? Hence the plot beginning at 0-0?

I don't see how this is possible unless the springs were installed without
any preload at all. My understanding is that the springs in a clutch disk
are under preload, so the torque has to rise to a certain level before they
will compress. Until that point it is a solid coupling.

If the springs had no preload, it would never be a solid coupling. It would
contantly be compressing and decompressing. How could that kind of clutch
even be usable in a car? It would be lurching all the time.

Also glad you mentioned the rubber torsional coupling brought up by the
poster in reference to the Alfa driveshaft.

The Rotax boxes you mention use this for the same reason the Ross box uses
the clutch disk with the springs. But as I was trying to point out, the
springs are not used to actually isolate the vibrations but to introduce
variable stiffness into the system.

Regards,

Gordon.







"Dan Horton" > wrote in message
oups.com...
> <<Up until that point the springs are not compressed and the coupling
> is in effect a solid coupling.>>
>
> Good God, he still doesn't get it.
>
> Gordon, plot the supplied data and study at it carefully. If your
> assertion were true, the plot would not leave the Y axis until reaching
> some elevated torque value. Note that the real plot begins at 0-0.
>
> Dan
>

[Peter Dohm]

> Charles,
>
> I did in fact say that the inertia is produced by the centrifugal force,
> which is just as wrong as saying the centrifugal force is produced by the
> inertia.
>
> That is ridiculous, yet you cling to it. At least I have the good sense to
> go back and see that I did make an error in phrasing. I thought I had said
> that inertia is proportional to the centrifugal force, which is correct.
The
> two are irrevokably linked.
>
> But one does not cause the other. The cause of both is the energy that
> produces the rotation to begin with and the centripetal force that keeps
the
> rotating object from flinging off into space.
>
> If you have no centripetal force, you have no centrifugal force. You also
> have no rotational inertia.
>
> And there is nothing rabbit-holish about inertial reference frames. It
> applies fully to this discussion. Much of our understanding of modern
> physics has been built on the underpinnings of Newtonian relativity.
> Inertial reference frames are an important and very real concept, and we
can
> thank Newton and Galileo before him for making us aware of the importance
of
> those reference frames in understanding the physics of motion.
>
> Amazing how you can be so cavalier with your wording and then lecture me
on
> the meaning of semantics.
>
> Regards,
>
> Gordon.
>
Come on. Lighten up and count to ten before you pound your keyboard.

Inertia exists are a property of mass, which is a property of matter. You
can have all the inertia that you please with out centrifugal, or
centripetal, force--provided that the motion of the mass is linear rather
than curved.... Inertia is the source, while centrifugal force is one of
the possible products.

There are even those who claim that centripetal and/or centrifugal forces
are fictions used for illustrative purposes. I have no way of knowing
whether some such remarks were serious, intended for a liers club meeting,
or just what happens when physicists are bored and alcohol is involved.

Peter

[Morgans]

"Dan Horton" > wrote in message
oups.com...
> Bill writes:
> <<small torque sensors that can be clamped to a rotating shaft that
> transmit their data wirelessly>>
>
> Yes, for torsion the classic setup is a wheatstone bridge strain
> guage array feeding a telemetry transmitter. I shopped new digital
> transmitters a few years ago but didn't buy, hoping for better prices
> and bandwidth later. We'll see. In 99/early 2000 we used a wheatstone
> bridge and a borrowed analog transmitter from Wireless Data. The
> bridge was still on the propshaft at S&F that year. I was there all
> week and nobody ever asked why the propshaft was wrapped in strapping
> tape <g>.

What was this rig-up on?

What is it you do, Dan? It sounds interesting.

It is refreshing to hear from someone who knows his stuff, unlike some other
poster, as of late! <g>
--
Jim in NC

[Morgans]

"Dan Horton" > wrote

> The successful Suzuki drive used a Shore 50 Centaflex CF12.
> Running in parallel with a viscous disk damper, maximum measured
> vibratory torque during steady-state resonant operation was about 115
> ft-lbs at 1500.

So, do you have a marketable Susuki flying? What association are you with
the project?

The Suzuki looks like it is about the right setup for my *future* needs.
Note the future. I'm talking around 5 years down the road. It still is fun
to figure and plan, or is it scheme? <g>
--
Jim in NC

[Charles Vincent]

My original remarks and the quote was in response to the following
statement you made:

Gordon Arnaut wrote: "Yet another source of excitation in airplanes is
the spring effect of the prop, where the blade tips whipsaw back and
forth as they undergo acceleration and deceleration due to the torque
spikes of cylinder firing."

I had assumed that you were refering to the potential of the blades to
flex parallel to the axis of the hub due to variations of blade loading
created by the periodic variations of torque. I made that assumption
since you were describing the prop as the source of the excitation and
yet still made specific mention of the engine torque variations. I had
once thought axial loading and flexure might feed back into the system
at one time myself, but Ker states that it does not( except in
extraodrinary cases), which I took note of in my studies -- thus the
quote. If you were not refering to axial vibrations, then your
statement is silly since you identify the prop ( or its spring effect)
as the source of excitation and yet you end the statement with the
actual excitation source ( torque spikes ). Alter the frequency of the
"torque spikes" and the system returns to normal.

Dan is the one that brought up blade interactions with disturbed
airflow. And I quoted the passage regarding issues of a non axial
airstream (which is not the same as disturbed airflow).

Even via he prop, Wilson is relatively proscriptive:

Wilson wrote: "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 phrase "of aerodynamic origin" when not cropped out is the
significant part. You may want to claim the prop as a source since it
is complicit in the translation of the vibrations, but the same could be
said of the crank, so whats the point? In any event you state:

Gordon Arnaut wrote: "Yet another source of excitation in airplanes is
the spring effect of the prop"

The Wilson quote mentions nothing of spring effectof the prop as a
source of excitations and in fact if you think about it you will see (
or at least most people would) that the vibrations "of aerodynamic
origin" do not require any springiness of the prop blade to impinge upon
the system.


And I didn't even bother to comment on this gem of yours:

Gordon Arnaut wrote: "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."

Which is just plain wrong. In the industry I work in there are machines
with moments of inertia measured in tons and they are still subject to
torsional vibration issues. Upping the moment of inertia just alters
the resonant frequency ranges. Without damping of some sort, excitation
in the critical range will still drive the system into resonance. If
you meant to say "if you have a flywheel with enough inertia, it will
lower the resonant frequency of the system to a range conducive to safe
operation" that would be a different thing.



Charles


Gordon Arnaut wrote:
> Charles,
>
> What exactly are you saying? Once again, I find myself scratching my head
> trying to fathom your actual point. You are nothing if not a master of
> obfuscation.
>
> I said very plainly in my original post that the prop is a source of
> excitations, as is the cylinder firing of the engine, as well as imbalance
> in the system.
>
> I don't know who said what, but your post contained this in quotation marks:
> "In aero-engine/airscrew systems there are, in general, two series of
> excitations. The airscrew is one source..."
>
> So whoever said that, whether it is you or Wilson, it is quite plain and
> quite true. The Prop IS a source of excitations, whether they are of
> aerodynamic origin or whether they are due to resonance.
>
> If you disagree with this, please say so plainly, otherwise do not try to
> muddy the waters further -- it is only doing a disservice to the discussion.
>
> Regards,
>
> Gordon.
>
>
>
>
>
>
>
>
> "Charles Vincent" > wrote in message
> et...
>
>>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
>>>
>>>
>

[Dan Horton]

Gordon Arnaut wrote:
<<Are you saying the springs immediately begin to compress at the first
sign of torque? Hence the plot beginning at 0-0?>>

Very good Gordon. That is indeed what a 0-0 datapoint indicates.

<<I don't see how this is possible unless the springs were installed
without any preload at all.>>

You find out all kinds of interesting things when you actually
measure and think. Steady reliance on Google searches results in a lot
of GIGO <g>

<<My understanding is that the springs in a clutch disk are under
preload, so the torque has to rise to a certain level before they will
compress.>>

Some clutch disks are indeed that way. No need to start at 0-0
when designing a clutch for an engine that makes, say, 150 or 200
ft-lbs of torque at idle. They only need to be soft enough to set
system F1 well below idle speed.

<<If the springs had no preload, it would never be a solid coupling.>>

Very good Gordon! And even if they do have preload, as described
above, it still isn't intended to be a solid coupling when in
operation.

Again consider the lessons found in the Subaru clutch. The range
of torque capacity is 0 to 162 ft-lbs. A late 1980's EA81 was rated
73hp @4800 and 94 ft-lbs torque @ 2400. Don't know about idle speed
torque (anybody have a chart?), but let's guess 40 ft-lbs. So, we have
40 ft-lbs as we ease away from a stop, 94 ft-lbs in economy cruise, and
80 ft-lbs when pushing hard. Read carefully Gordon. All these numbers
are well within the range of 0 to 162. Actually they are all within
the single 1547 ft-lbs/rad spring rate found between 3.5 degrees and 6
degrees. Clearly engine torque has the springs in play at all times.

<<It would contantly be compressing and decompressing.>>

Give that man a gold star! Yes Gordon, overall angular
displacement of the clutch center varies with throttle position. At
the hertz level, angular displacement oscillates at the exciting
frequency.

<<How could that kind of clutch even be usable in a car? It would be
lurching all the time.>>

Think again.

Dan

[Dan Horton]

Jim asks:
<<What was this rig-up on? What is it you do, Dan? It sounds
interesting. So, do you have a marketable Suzuki flying? What
association are you with the project?>>

Scale JN-4C. In the real world I'm an automobile dealer, and it is
not very interesting. However, it is a hell of a lot more profitable
than getting in the PSRU business, so the only Suzuki you can buy from
me comes with 4 tires and a horn.

The JN-4C project had several goals. A friend wanted to proof
custom software written to model torsional vibration in a complex
aircraft drive system (a pusher with with long shafts, might I add). I
wanted a new and improved PSRU. So, we modeled the old drive and then
altered the model for optimum predicted results. Then I designed a
drive to match the model inputs, built it, and ran it with telemetry to
check the accuracy of the predictions. Along the way we developed a
damper and tested it, ran two different props while I had the
telemetry, played with strobing linear vibration, and a whole bunch of
other stuff. End result was proven software and a pretty good PSRU,
plus an education.

Dan

[George A. Graham]

Really wonderful stuff you've been posting here Dan.
Thank you very much for answering so many silly questions!

George Graham
RX-7 EZ with RD-1A Redrive.


On 15 Apr 2006, Dan Horton wrote:

> Jim asks:
> <<What was this rig-up on? What is it you do, Dan? It sounds
> interesting. So, do you have a marketable Suzuki flying? What
> association are you with the project?>>
>
> Scale JN-4C. In the real world I'm an automobile dealer, and it is
> not very interesting. However, it is a hell of a lot more profitable
> than getting in the PSRU business, so the only Suzuki you can buy from
> me comes with 4 tires and a horn.
>
> The JN-4C project had several goals. A friend wanted to proof
> custom software written to model torsional vibration in a complex
> aircraft drive system (a pusher with with long shafts, might I add). I
> wanted a new and improved PSRU. So, we modeled the old drive and then
> altered the model for optimum predicted results. Then I designed a
> drive to match the model inputs, built it, and ran it with telemetry to
> check the accuracy of the predictions. Along the way we developed a
> damper and tested it, ran two different props while I had the
> telemetry, played with strobing linear vibration, and a whole bunch of
> other stuff. End result was proven software and a pretty good PSRU,
> plus an education.
>
> Dan
>
>
>

George Graham
RX-7 Powered Graham-EZ, N4449E
Homepage <http://bfn.org/~ca266>

[Morgans]

"Dan Horton" > wrote

> End result was proven software and a pretty good PSRU,
> plus an education.

Have you considered releasing some plans for the PSRU?

How about your JN project? Are there pictures anywhere?

Congratulations on your achievements. You should write a blog about it.
I'm sure it would be an interesting read.
--
Jim in NC

[stol]

>Today's multi-v belts -- the kind used to drive engine accessories on newer
>cars -- are highly efficient and can handle huge amounts of power, up to
>1000 hp.


>Best of luck to ADK with his project.


>Regards,


>Gordon.



"cavelamb" > wrote in message


Hmmmmm. If you are stating that a "serpentine belt", one that is a
about 1 inch wide and is used in most current vehicles will transmit
1000HP you might need to get another very stiff drink.
!!!!!!!!!!!!!!!!!!!!!!!!!!! <G>

Ben

[stol]

>In the real world I'm an automobile dealer, and it is
not very interesting. However, it is a hell of a lot more profitable
than getting in the PSRU business, so the only Suzuki you can buy from
me comes with 4 tires and a horn.


The JN-4C project had several goals. A friend wanted to proof
custom software written to model torsional vibration in a complex
aircraft drive system (a pusher with with long shafts, might I add). I

wanted a new and improved PSRU. So, we modeled the old drive and then
altered the model for optimum predicted results. Then I designed a
drive to match the model inputs, built it, and ran it with telemetry to

check the accuracy of the predictions. Along the way we developed a
damper and tested it, ran two different props while I had the
telemetry, played with strobing linear vibration, and a whole bunch of
other stuff. End result was proven software and a pretty good PSRU,
plus an education.


Dan<<<


///////////////////////////////////////////////
Ya know, I really like this guy. This is "experimental" aviation at its
finest............................................ .
Only in America !!!!!!!!!!!!!!!!

Ben.

[Peter Dohm]

> What was this rig-up on?
>
> What is it you do, Dan? It sounds interesting.
>
> It is refreshing to hear from someone who knows his stuff, unlike some
other
> poster, as of late! <g>
> --
> Jim in NC
>
I can certainly second all of that--and add that I have deservedly stood
corrected for blindly reciting a few things I heard of read on an earlier
and vaguely related thread.

Pete

[Ernest Christley]

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
>

Gordon, if you preload the springs, what will protect your transmission
when you rev the engine and pop the clutch?

--
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)."

[Ernest Christley]

Gordon Arnaut wrote:
> Dan,
>
> I wasn't talking about bottoming the springs, just compressing them in order
> to introduce flexibility into the coupling and bring about a change in the
> critical frequency of the system as a whole.
>
> And this is exactly what happens. The springs do compress at a certain point
> when the twisting oscialltions overcome the spring pressure and rotational
> inertia. Up until that point the springs are not compressed and the coupling
> is in effect a solid coupling.
>
> As soon as the springs are compressed we have a flexible coupling that
> changes the critical frquencies of the system.

Most clutches I've ever seen, you can easily twist the springs with no
problem. Some will even rattle just slightly.

Could you tell us what protects the transmission gears when someone revs
the engine and pops the clutch? How could this component do it's job if
it were already preloaded?

--
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)."

[Ernest Christley]

stol wrote:
>>Today's multi-v belts -- the kind used to drive engine accessories on newer
>>cars -- are highly efficient and can handle huge amounts of power, up to
>>1000 hp.
>
>
>
>>Best of luck to ADK with his project.
>
>
>
>>Regards,
>
>
>
>>Gordon.
>
>
>
>
> "cavelamb" > wrote in message
>
>
> Hmmmmm. If you are stating that a "serpentine belt", one that is a
> about 1 inch wide and is used in most current vehicles will transmit
> 1000HP you might need to get another very stiff drink.
> !!!!!!!!!!!!!!!!!!!!!!!!!!! <G>
>
> Ben
>

Ben, no problem. Just have 3ft wide pulleys and run it at 10,000rpm.
Airplanes are all about compromises, right? 8*)

--
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)."

[Dan Horton]

Hi George,
<<Thank you very much for answering so many silly questions!>>

No problem, you're welcome.

I apologize for my crappy memory, but didn't we have a talk some
years ago after you broke your transmission by running on one rotor?

Dan

[Dan Horton]

Jim,
<<Have you considered releasing some plans for the PSRU?>>

No.

<<How about your JN project? Are there pictures anywhere?>>

See Oct 98 Experimenter or Sept 04 Sport Aviation for cover shots
and articles (with two different paint schemes). EAA used it to
illustrate a lot of Sport Pilot articles. Dan Simonsen put it in one
of his fancy bookstore calendars. The best one was when a New York
financial magazine flew a photographer and his bigwig subject all the
way to Wetumpka, Alabama. They paid me $250 to dress the guy in my
leather jacket and get his picture taken standing by the airplane, all
for an article called "The Barnstormer of Wall Street". Compared to
New York prices, I'm sure they felt they were fleecing the rube. I
didn't mind. In Wetumpka, $250 was two month's hangar rent <g>

Dan

[George A. Graham]

On 15 Apr 2006, Dan Horton wrote:

Yes Dan, and it eventually failed inflight as well, after almost 400
hours,and without warning. Fortunately, I was a mile high, and found a
nice road for landing. I have since changed to a Tracy Crook design
planetary PSRU- so far so good.

Great to hear from you again!

George, Sarasota Florida


>
> I apologize for my crappy memory, but didn't we have a talk some
> years ago after you broke your transmission by running on one rotor?
>
> Dan
>
>
>

George Graham
RX-7 Powered Graham-EZ, N4449E
Homepage <http://bfn.org/~ca266>

[Charles Vincent]

Dan Horton wrote:
> Gordon Arnaut wrote:
>
> <<My understanding is that the springs in a clutch disk are under
> preload, so the torque has to rise to a certain level before they will
> compress.>>
>
> Some clutch disks are indeed that way. No need to start at 0-0
> when designing a clutch for an engine that makes, say, 150 or 200
> ft-lbs of torque at idle. They only need to be soft enough to set
> system F1 well below idle speed.
>

Dan, based on my studies, I don't think they are there to drop total
system resonance below idle speed. I think they serve another purpose.
I am pretty sure that the frequency of the driveline system is already
low. The engine/transmission is rubber mounted usually and the rear
axle is spring mounted and can rotate about the driveshaft axis. The
spring rate and compliance of both these elements factor into the
torsional stiffness of the system as a whole(i.e. total degrees of
deflection from the driveshaft perspective between engine and axle as a
result of torque inputs). I am pretty sure those elements will drop the
system frequency more than any clutch spring damper could hope to. Add
the fact that I think you would have to add the mass of the whole axle
assembly and its moment of inertia as well. Just my undertanding, but
reading various SAE papers it appears that the frequency of the
drivetrain is low enough that the problem they face is really low
frequency inputs like drivers jumping on and off the trhottle, which for
some reason they call tip-in and tip-out. The driveline reactions are
called shunt and shuffle and resonate evidently in the low hz range.

Regarding the torsional damper in the clutch , from Malloy's "Automobile
Engineers Reference" in the section "Clutches and Fluid Drives" there
are several passages that I believe are relevant and directly on topic.
In the "Requirements of Clutches" there is a paragraph on torsional
damping:

"Modern clutches, notwithstanding their own peculiar difficulties, are
also expected to incorporate some torsional-damping devices to eliminate
noise arising in the transmission. Suitable dampers could be placed in
many positions along the transmission, but it is generally considered
convienient for a variety of reasons to incorporate this feature within
the clutch itself"

and later in a few paragraphs titled "Transmission Noises" :

--Begin quote from Malloy ----

" The other major subject of complaint in a transmission system which
involves the clutch, although in this case as a means of providing a
cure, is that of transmission rattle.
The whole of the transmission can be regarded as a series of spring
mass systems, the spring element being provided by the torsional
deflection of various shafts, including those in the gearbox, and the
mass element by the inertia of the various gear wheels, etc.
Each of these units will have a natural frequency of vibration, and
if one or more should be excited by a disturbing force of an appropriate
frequency, then it may be set in vibration, and should the amplitude be
sufficient to take up the clearance between mating parts, noise may
ensue and some kind of torsional damping will be necessary. Two types
of damper are commonly in use, one being a seperate unit often attached
to the crankshaft, the other being a spring-cum friction unit in the
clutch driven plate. This latter unit can be tuned by the fitting of
different springs and varying the amount of friction. "

---end quote from Malloy ---

> Again consider the lessons found in the Subaru clutch. The range
> of torque capacity is 0 to 162 ft-lbs. A late 1980's EA81 was rated
> 73hp @4800 and 94 ft-lbs torque @ 2400. Don't know about idle speed
> torque (anybody have a chart?), but let's guess 40 ft-lbs. So, we have
> 40 ft-lbs as we ease away from a stop, 94 ft-lbs in economy cruise, and
> 80 ft-lbs when pushing hard. Read carefully Gordon. All these numbers
> are well within the range of 0 to 162. Actually they are all within
> the single 1547 ft-lbs/rad spring rate found between 3.5 degrees and 6
> degrees. Clearly engine torque has the springs in play at all times.
>

This would certainly be consistent with them damping transmission noise.
Regarding the rubber elements someone mentioned in their driveline,
"Automobile Engineers Reference" makes mention of these as well, saying
that they can provide similar damping to the clutch damper, and have the
added benefits of in some cases replacing the U-joint and the sliding
member in the driveshaft which is a real pain from an engineering
perspective.

To wrap it up, I am not claiming what these devices (clutch dampers) do
or don't do, just sharing what I have learned from my own reading. In
no automotive engineering text have I found them described as detuners
and your own empirical evidence suggests that in fact they are not.

Charles



Charles

[Charles Vincent]

Charles Vincent wrote:


I also forgot this quote from another source that might explain why the
spring rate is what you observe....

"In the case of idling noises the problem lies in the zero torque region
of the torsion characteristic of the clutch disk assembly. The problem
is alleviated if the torsional rigidity is low. Conversely, it is
necessary for the torsion characteristic of the clutch disk assembly to
be as rigid as possible to suppress the longitudinal vibrations caused
by tip-in and tip-out."

[Peter Dohm]

"Charles Vincent" > wrote in message
et...
> Dan Horton wrote:
> > Gordon Arnaut wrote:
> >
> > <<My understanding is that the springs in a clutch disk are under
> > preload, so the torque has to rise to a certain level before they will
> > compress.>>
> >
> > Some clutch disks are indeed that way. No need to start at 0-0
> > when designing a clutch for an engine that makes, say, 150 or 200
> > ft-lbs of torque at idle. They only need to be soft enough to set
> > system F1 well below idle speed.
> >
>
> Dan, based on my studies, I don't think they are there to drop total
> system resonance below idle speed. I think they serve another purpose.
> I am pretty sure that the frequency of the driveline system is already
> low. The engine/transmission is rubber mounted usually and the rear
> axle is spring mounted and can rotate about the driveshaft axis. The
> spring rate and compliance of both these elements factor into the
> torsional stiffness of the system as a whole(i.e. total degrees of
> deflection from the driveshaft perspective between engine and axle as a
> result of torque inputs). I am pretty sure those elements will drop the
> system frequency more than any clutch spring damper could hope to. Add
> the fact that I think you would have to add the mass of the whole axle
> assembly and its moment of inertia as well. Just my undertanding, but
> reading various SAE papers it appears that the frequency of the
> drivetrain is low enough that the problem they face is really low
> frequency inputs like drivers jumping on and off the trhottle, which for
> some reason they call tip-in and tip-out. The driveline reactions are
> called shunt and shuffle and resonate evidently in the low hz range.
>
> Regarding the torsional damper in the clutch , from Malloy's "Automobile
> Engineers Reference" in the section "Clutches and Fluid Drives" there
> are several passages that I believe are relevant and directly on topic.
> In the "Requirements of Clutches" there is a paragraph on torsional
> damping:
>
> "Modern clutches, notwithstanding their own peculiar difficulties, are
> also expected to incorporate some torsional-damping devices to eliminate
> noise arising in the transmission. Suitable dampers could be placed in
> many positions along the transmission, but it is generally considered
> convienient for a variety of reasons to incorporate this feature within
> the clutch itself"
>
> and later in a few paragraphs titled "Transmission Noises" :
>
> --Begin quote from Malloy ----
>
> " The other major subject of complaint in a transmission system which
> involves the clutch, although in this case as a means of providing a
> cure, is that of transmission rattle.
> The whole of the transmission can be regarded as a series of spring
> mass systems, the spring element being provided by the torsional
> deflection of various shafts, including those in the gearbox, and the
> mass element by the inertia of the various gear wheels, etc.
> Each of these units will have a natural frequency of vibration, and
> if one or more should be excited by a disturbing force of an appropriate
> frequency, then it may be set in vibration, and should the amplitude be
> sufficient to take up the clearance between mating parts, noise may
> ensue and some kind of torsional damping will be necessary. Two types
> of damper are commonly in use, one being a seperate unit often attached
> to the crankshaft, the other being a spring-cum friction unit in the
> clutch driven plate. This latter unit can be tuned by the fitting of
> different springs and varying the amount of friction. "
>
> ---end quote from Malloy ---
>
> > Again consider the lessons found in the Subaru clutch. The range
> > of torque capacity is 0 to 162 ft-lbs. A late 1980's EA81 was rated
> > 73hp @4800 and 94 ft-lbs torque @ 2400. Don't know about idle speed
> > torque (anybody have a chart?), but let's guess 40 ft-lbs. So, we have
> > 40 ft-lbs as we ease away from a stop, 94 ft-lbs in economy cruise, and
> > 80 ft-lbs when pushing hard. Read carefully Gordon. All these numbers
> > are well within the range of 0 to 162. Actually they are all within
> > the single 1547 ft-lbs/rad spring rate found between 3.5 degrees and 6
> > degrees. Clearly engine torque has the springs in play at all times.
> >
>
> This would certainly be consistent with them damping transmission noise.
> Regarding the rubber elements someone mentioned in their driveline,
> "Automobile Engineers Reference" makes mention of these as well, saying
> that they can provide similar damping to the clutch damper, and have the
> added benefits of in some cases replacing the U-joint and the sliding
> member in the driveshaft which is a real pain from an engineering
> perspective.
>
> To wrap it up, I am not claiming what these devices (clutch dampers) do
> or don't do, just sharing what I have learned from my own reading. In
> no automotive engineering text have I found them described as detuners
> and your own empirical evidence suggests that in fact they are not.
>
> Charles
>
>
>
> Charles

Another way to look at this might just be to say that the clutch springs
isolate (or decouple) the engine assembly from the driveline assembly (which
could be the psru and prop, the automobile's driveline, or some other
equipment).

On that basis; it would be reasonable to hypothesize that, so long as the
complete engine and clutch assembly is used without and modification, the
engine can be treated as a "black box" unit.

That would drastically reduce the work necessary to design and test the
psru.

Additionally; any theoretical resonance between the engine and psru in the
idle range would be mitigated by the progressive rate, and somewhat
uni-directional, nature of the clutch springs.

I am deferring to Dan for further observations.

Peter

P.S.: This really does not fully address the issue of the pusher
configuration which originated in an earlier thread. The disturbed air from
the wings, tail, andor fuselage could still cause a resonance in the prop
and/or psru which could destroy one or both.

[Dan Horton]

<<Dan, based on my studies, I don't think they (clutch springs) are
there to drop total
system resonance below idle speed......I am pretty sure that the
frequency of the driveline system is already low.>>

Good argument. Consider me corrected, with a caveat. I think
you're right about driveline frequency already being low, even if there
were no clutch. The caveat? The clutch springs are one of the
stiffnesses in that system, and contribute to that low frequency. Take
them out, frequency goes up. Design them in, frequency goes down.

This clutch data is for a FWD car. No driveshaft, no long skinny
billet axles, no axle housing on a flexible mount. The clutch spring
rate of 1547 ft-lbs/rad is very soft. (For comparison, the little
rubber Centaflex CF12 I used in the Suzuki drive was 1991 ft-lbs/rad.)


System stiffnesses are additive like resistors;

1 / (1/K1 + 1/K2 + 1/K3 + 1Kx...) = K combined

Just for fun let's assume a simplified system with either two or
three connecting stiffnesses. Assume the drive axles are very soft
(1500 ft-lbs/rad) and the transmission shafts are equally soft. With
no clutch springs in the system, combined stiffness would be 750
ft-lbs/rad. Add a set of 1500 ft-lb/rad clutch springs, and overall
stiffness goes to 500 ft-lbs/rad.

Now assume drive axles and transmission shafts at 5000 ft-lbs/rad.
Without the clutch springs, stiffness is 2500. With the clutch, 938.

The point? Even in the company of other soft elements, the
addition of another makes a significant difference. If the addition is
a lot softer than the other elements, it makes a huge difference.

<<transmission rattle.....some kind of torsional damping will be
necessary. Two types
of damper are commonly in use, one being a seperate unit often attached
to the crankshaft, the other being a spring-cum friction unit in the
clutch driven plate. This latter unit can be tuned by the fitting of
different springs and varying the amount of friction. ">>

Lemme tell a little story. After testing a viscous disk damper
running in parallel with a soft element and finding out how well it
worked, I got the idea that perhaps I should obtain a patent. Before
spending money on an attorney, I did some searches in the patent
database. Turned out that Eaton had already patented a clutch for HD
truck (big rig) applications with a serious viscous damper in parallel
with the clutch springs. There were lots of friction damped clutches
too. I didn't pursue the patent on my "invention".

Note the use of the term "damper" in the quoted text. Are you sure
the text wasn't speaking of something a bit larger than our light duty
clutches? Not much sign of a frictional damper in the Subaru clutch.

>>.. Actually they are all within the single 1547 ft-lbs/rad spring rate found between 3.5 degrees and 6 degrees.<<
<<This would certainly be consistent with them damping transmission
noise.>>

I'm guessing that it is not the 1547 ft-lbs/rad spring rate. I
suspect that eliminating transmission noise (with selector in neutral,
clutch engaged, mainshaft spinning) is the purpose of the 654
ft-lbs/rad spring rate found at less than 3.5 degrees displacement.
There is no other logical explanation for the dual rate, since the
lowest torque output from the engine is more than 40 ft-lbs. Remember,
with the transmission engaged you have a system that includes
driveshafts, axles, etc. With the transmission in neutral you have an
entirely different truncated system; crank, flywheel, clutch and the
tranny mainshaft. If somebody here has a late 80's Subaru mainshaft
with gears, we could do a bifilar, get an inertia, and calculate
natural frequency for the truncated system.

I'm thinking out loud here, nothing more. Before today I've put
very little thought into automotive drivelines.

<<Regarding the rubber elements someone mentioned in their driveline,
"Automobile Engineers Reference" makes mention of these as well, saying
that they can provide similar damping to the clutch damper>>

Rubber elements do have a damping value, although it is very, very
small. We got the actual value from Lovejoy when we were doing the
modeling, but logic alone tells you it ain't much. If it had much
damping value, it would melt <g>

Dan

[Dan Horton]

<<Dan, based on my studies, I don't think they (clutch springs) are
there to drop total
system resonance below idle speed......I am pretty sure that the
frequency of the driveline system is already low.>>

Good argument. Consider me corrected, with a caveat. I think
you're right about driveline frequency already being low, even if there
were no clutch. The caveat? The clutch springs are one of the
stiffnesses in that system, and contribute to that low frequency. Take
them out, frequency goes up. Design them in, frequency goes down.

This clutch data is for a FWD car. No driveshaft, no long skinny
billet axles, no axle housing on a flexible mount. The clutch spring
rate of 1547 ft-lbs/rad is very soft. (For comparison, the little
rubber Centaflex CF12 I used in the Suzuki drive was 1991 ft-lbs/rad.)


System stiffnesses are additive like resistors;

1 / (1/K1 + 1/K2 + 1/K3 + 1Kx...) = K combined

Just for fun let's assume a simplified system with either two or
three connecting stiffnesses. Assume the drive axles are very soft
(1500 ft-lbs/rad) and the transmission shafts are equally soft. With
no clutch springs in the system, combined stiffness would be 750
ft-lbs/rad. Add a set of 1500 ft-lb/rad clutch springs, and overall
stiffness goes to 500 ft-lbs/rad.

Now assume drive axles and transmission shafts at 5000 ft-lbs/rad.
Without the clutch springs, stiffness is 2500. With the clutch, 938.

The point? Even in the company of other soft elements, the
addition of another makes a significant difference. If the addition is
a lot softer than the other elements, it makes a huge difference.

<<transmission rattle.....some kind of torsional damping will be
necessary. Two types
of damper are commonly in use, one being a seperate unit often attached
to the crankshaft, the other being a spring-cum friction unit in the
clutch driven plate. This latter unit can be tuned by the fitting of
different springs and varying the amount of friction. ">>

Lemme tell a little story. After testing a viscous disk damper
running in parallel with a soft element and finding out how well it
worked, I got the idea that perhaps I should obtain a patent. Before
spending money on an attorney, I did some searches in the patent
database. Turned out that Eaton had already patented a clutch for HD
truck (big rig) applications with a serious viscous damper in parallel
with the clutch springs. There were lots of friction damped clutches
too. I didn't pursue the patent on my "invention".

Note the use of the term "damper" in the quoted text. Are you sure
the text wasn't speaking of something a bit larger than our light duty
clutches? Not much sign of a frictional damper in the Subaru clutch.

>>.. Actually they are all within the single 1547 ft-lbs/rad spring rate found between 3.5 degrees and 6 degrees.<<
<<This would certainly be consistent with them damping transmission
noise.>>

I'm guessing that it is not the 1547 ft-lbs/rad spring rate. I
suspect that eliminating transmission noise (with selector in neutral,
clutch engaged, mainshaft spinning) is the purpose of the 654
ft-lbs/rad spring rate found at less than 3.5 degrees displacement.
There is no other logical explanation for the dual rate, since the
lowest torque output from the engine is more than 40 ft-lbs. Remember,
with the transmission engaged you have a system that includes
driveshafts, axles, etc. With the transmission in neutral you have an
entirely different truncated system; crank, flywheel, clutch and the
tranny mainshaft. If somebody here has a late 80's Subaru mainshaft
with gears, we could do a bifilar, get an inertia, and calculate
natural frequency for the truncated system.

I'm thinking out loud here, nothing more. Before today I've put
very little thought into automotive drivelines.

<<Regarding the rubber elements someone mentioned in their driveline,
"Automobile Engineers Reference" makes mention of these as well, saying
that they can provide similar damping to the clutch damper>>

Rubber elements do have a damping value, although it is very, very
small. We got the actual value from Lovejoy when we were doing the
modeling, but logic alone tells you it ain't much. If it had much
damping value, it would melt <g>

Dan

[Dan Horton]

Charles,
I said: << I suspect that eliminating transmission noise (with selector
in neutral, clutch engaged, mainshaft spinning) is the purpose of the
654 ft-lbs/rad spring rate found at less than 3.5 degrees
displacement.>>

Got curious and ran numbers for a simple two element model. I used
what I think are reasonable guesses for the inertias, 0.07 slugs-ft^2
(crank, flywheel, and most of the clutch assembly), and 0.01 slug-ft^2
(transmission mainshaft and gearset). The connecting stiffness is of
course 654 ft-lbs/rad.

No joy. The above yields an F1 of 43.5 hz. That would make the
mainshaft rattle like hell at 1305 engine RPM with the selector in
neutral, so my guess about the purpose of the 654 spring rate does not
appear to be true. 654 isn't soft enough.

You got an idea about the 654 rate?

Dan

[Charles Vincent]

Dan Horton wrote:
> <<Dan, based on my studies, I don't think they (clutch springs) are
> there to drop total
> system resonance below idle speed......I am pretty sure that the
> frequency of the driveline system is already low.>>
>
> Good argument. Consider me corrected, with a caveat. I think
> you're right about driveline frequency already being low, even if there
> were no clutch. The caveat? The clutch springs are one of the
> stiffnesses in that system, and contribute to that low frequency. Take
> them out, frequency goes up. Design them in, frequency goes down.

I wasn't disputing the effect you had described, it would be hard to
disagree. My point was more along the lines of --- "are you sure that
is why they are there?" Crankshaft counterweights also lower the
system frequency, but thats not why they are on the crank. It is also
why I am tentative in this. I don't know for a fact, so I may
speculate, but will label it as speculation. I had researched it
before because I had heard the story about them being detuners which I
never found any support for in the literature and your numbers put to
bed. The only things I have found attributed to them in engineering
texts ( not websites or enthusiast pubs like Hot Rod) was shock loading
and the excerpts I quoted before.

> Note the use of the term "damper" in the quoted text. Are you sure
> the text wasn't speaking of something a bit larger than our light duty
> clutches? Not much sign of a frictional damper in the Subaru clutch.
>

I am pretty sure, given that the image next to the text was the classic
clutch disc we are talking about. For further insight look up patent
2,674,863 It talks about the limitations of the friction mechanism in
standard clutch disc dampers. I have found though that in automotive
practice they seem to take a lot of liberties with the terms dampers and
absorbers. Look at the term "shock absorber". And they call the
detuner a balancer. I don't have a clutch disc around here at the
moment to look at. I have one in a storage building across town, so
may go look at it in more detail.


> <<Regarding the rubber elements someone mentioned in their driveline,
> "Automobile Engineers Reference" makes mention of these as well, saying
> that they can provide similar damping to the clutch damper>>
>
> Rubber elements do have a damping value, although it is very, very
> small. We got the actual value from Lovejoy when we were doing the
> modeling, but logic alone tells you it ain't much. If it had much
> damping value, it would melt <g>

If you stick one in the driveline of an auto it cant help but get hit
with torsional vibrations and they can be found there. We are not
talking about damping at system resonance. The damping is dependent on
the hysteresis of the ruber which obviously creates heat, but it is
exposed to ample cooling air in a drive shaft if the amplitudes and
frequencies are mild I would expect. The rubber is going to heat up if
you are using them to correct for axial misalignment as well, since you
will be doing the same thing to it, alternately stretching and
compressing it. In any event, not my suggestion, just right out of the
manual "Autombile Engineers Reference Book" by Molloy (the book does not
use these terms, but the pictures make it obvious that the Layrub is a
rubber in compression unit and the Rotoflex is a rubber in shear.):

"A normal rear axle with a hotchkiss drive has probably adequate overall
flexibility in the drive shaft aft of the gearbox, and any flexibility
provided by such couplings as the Layrub and Rotoflex is probably
desirable only for local effect, i.e. to reduce gearbox chatter in some
cases. There does, however, in some cases seem to be a marked and very
welcome quieting effect in the car from their use and this may arise
from their effect in reducing the transmission of road noise. Their
torsional flexibility also becomes more desireable overall where more
positive and therefore more rigid drive and braking torque resisting
means are provided on the axle, and even more when independent rear
suspensions or a De Dion axle is used and the flexibility of the
half-shafts is largely subtracted too. This aspect, torsional
flexibility, can therefore be of increasing importance in the future"

and

"In both these cases, the axial flexibility is sufficient to make it
possible to dispense witha sliding joint in the propellor shaft..."


The comment about local effect is interesting and goes back to the
original discussion about the springs in the clutch plate.


Charles

[Charles Vincent]

Dan Horton wrote:
> Charles,
> I said: << I suspect that eliminating transmission noise (with selector
> in neutral, clutch engaged, mainshaft spinning) is the purpose of the
> 654 ft-lbs/rad spring rate found at less than 3.5 degrees
> displacement.>>
>
> Got curious and ran numbers for a simple two element model. I used
> what I think are reasonable guesses for the inertias, 0.07 slugs-ft^2
> (crank, flywheel, and most of the clutch assembly), and 0.01 slug-ft^2
> (transmission mainshaft and gearset). The connecting stiffness is of
> course 654 ft-lbs/rad.
>
> No joy. The above yields an F1 of 43.5 hz. That would make the
> mainshaft rattle like hell at 1305 engine RPM with the selector in
> neutral, so my guess about the purpose of the 654 spring rate does not
> appear to be true. 654 isn't soft enough.
>
> You got an idea about the 654 rate?
>
> Dan
>

Two things. To get that spring rate(654), there would have to be
enough resistance in the transmission to absorb that level of torque. I
think that would be a pretty inefficient transmission. I doubt it. I
expect that in neutral, clutch engaged , idling you would have to be at
the lower limit of the spring since by definition you are at the lower
limit of torque, so look at the rates there. Once the shaft was
acclerated up to speed, the only torque that could be transmitted
through the springs would be the result of friction.

Second thing, dyno charts show the maximum torque the engine can provide
at a given rpm, doesn't mean that the engine has to produce that torque
at that rpm ( you do have a throttle right? the engine speed isn't
controlled solely by load). Every car is different, but I have heard
quotes of 25-30hp to cruise on the highway, so your 40ft lbs of torque
is almost enough at 3600 rpm for highway cruising, which means you could
see much lower torque values in high gear crusing at thirty miles an
hour. Finally, the torque on the dyno chart is the mean torque, not an
absolute ( the torque variations are how we got here in this
conversation) so I would expect it to have a range. But a definative
answer I can't give you, just my very unqualified thoughts. By the way,
I think I figured out why the auto engineers call it tip in and tip out.
Must be slang for TP in and TP out, the TP being throttle position.

Charles

[Charles Vincent]

After sleeping on it, I will bet that the lower rate is there for tip in
tip out discussed from cruise and or initial engagement. I do remember
reading about using multiple spring rates for overdrive vs non overdrive
states, but don't recall where I read that. Realized after I posted
that the rate you quoted was probably from 0-3.5 not starting at 3.5

Charles

[Peter Dohm]

"Dan Horton" > wrote in message
oups.com...
> <<To get that spring rate(654), there would have to be enough
> resistance in the transmission to absorb that level of torque.>>
>
> Uh oh, terms confusion. That's per radian. Torque vs deflection
> is 0 to 40 ft-lbs across 3.5 degrees. Convert that to ft-lbs/rad (or
> an equivelent term like N m/rad) to do a natural frequency calculation.
> (40/3.5) = 11.428571 ft-lbs per degree. Multiply by 57.29578 = 654
> ft-lbs per radian.
>
> <<Second thing, dyno charts show the maximum torque the engine can
> provide
> at a given rpm, doesn't mean that the engine has to produce that torque
>
> at that rpm>>
>
> Good point! You're right, duh on me. Entirely possible to be
> cruising part throttle at torque levels that do not deflect the clutch
> center beyond the first 3.5 degrees and it's softer rate.
>
> Uhhh, we're getting way sidetracked from a PSRU topic.

Well, we may be getting way sidetracked from a PSRU topic, and then again we
may not be.

I think that the clutch springs may also be a way of providing a more
constant torque, and eliminating the torque reversals that would otherwise
be inherent in a 3 or 4 cylinder engine. That would provide far less
excitation to the prop and be a lot easier on the PSRU. It may also be the
original reason for their use in a car.

The greater question, by my way of reasoning, would be how much additional
mass (flywheel) or damping (viscous/hydraulic disk) on the PSRU side of the
clutch/spring assembly would provide enough additional smoothing to justify
the added weight. I doubt that the decision is really "open and shut" since
the PSRU, prop, and prop shaft would not need to be as strong.

Peter
>
> Dan
>

[Richard Riley]

Dan Horton wrote:
> The JN-4C project had several goals. A friend wanted to proof
> custom software written to model torsional vibration in a complex
> aircraft drive system (a pusher with with long shafts, might I add).

Makes me sweat just thinking about it. What was the pusher - an Imp?

[cavelamb]

Peter Dohm wrote:
> "Dan Horton" > wrote in message
> oups.com...
>
>><<To get that spring rate(654), there would have to be enough
>>resistance in the transmission to absorb that level of torque.>>
>>
>> Uh oh, terms confusion. That's per radian. Torque vs deflection
>>is 0 to 40 ft-lbs across 3.5 degrees. Convert that to ft-lbs/rad (or
>>an equivelent term like N m/rad) to do a natural frequency calculation.
>> (40/3.5) = 11.428571 ft-lbs per degree. Multiply by 57.29578 = 654
>>ft-lbs per radian.
>>
>><<Second thing, dyno charts show the maximum torque the engine can
>>provide
>>at a given rpm, doesn't mean that the engine has to produce that torque
>>
>>at that rpm>>
>>
>> Good point! You're right, duh on me. Entirely possible to be
>>cruising part throttle at torque levels that do not deflect the clutch
>>center beyond the first 3.5 degrees and it's softer rate.
>>
>> Uhhh, we're getting way sidetracked from a PSRU topic.
>
>
> Well, we may be getting way sidetracked from a PSRU topic, and then again we
> may not be.
>
> I think that the clutch springs may also be a way of providing a more
> constant torque, and eliminating the torque reversals that would otherwise
> be inherent in a 3 or 4 cylinder engine. That would provide far less
> excitation to the prop and be a lot easier on the PSRU. It may also be the
> original reason for their use in a car.
>
> The greater question, by my way of reasoning, would be how much additional
> mass (flywheel) or damping (viscous/hydraulic disk) on the PSRU side of the
> clutch/spring assembly would provide enough additional smoothing to justify
> the added weight. I doubt that the decision is really "open and shut" since
> the PSRU, prop, and prop shaft would not need to be as strong.
>
> Peter
>
>>Dan
>>
>
>
>


I believe that brings the thread full circle.

Text is a hard medium.
A photograph with circles and arrows and a paragraph on the back comes to mind.
But in text, it's sometimes hard (spelled a lot of work) to be really articulate.

Well...
If someone needs a clue, this thread ought to give them plenty to think about.


Richard

Somewhere along the way, something strange clicked in somewhere and I think I
better understand how microwave ovens work now - and we didn't even go there!

[Peter Dohm]

-----------------snip---------
>
> I believe that brings the thread full circle.
>
-----------------snip----------

You're right, and discovered just now that I had missed Dan Horton's post
dated 4/15/06 at 7:16pm where he stated in part:

" The JN-4C project had several goals. A friend wanted to proof
custom software written to model torsional vibration in a complex
aircraft drive system (a pusher with with long shafts, might I add). I
wanted a new and improved PSRU. So, we modeled the old drive and then
altered the model for optimum predicted results. Then I designed a
drive to match the model inputs, built it, and ran it with telemetry to
check the accuracy of the predictions. Along the way we developed a
damper and tested it, ran two different props while I had the
telemetry, played with strobing linear vibration, and a whole bunch of
other stuff. End result was proven software and a pretty good PSRU,
plus an education."

So it appears that there is a solution, which may or may not be for sale or
rent...

Peter

[Dan Horton]

Peter Dohm wrote:
<<I think that the clutch springs may also be a way.......>>

Classic homebuilder's disease.

Why adapt a component (with unknown properties, clunky packaging,
and a least one severe drawback) when you can purchase products
engineered for the task? Consider the Goetz, Lord, or Lovejoy soft
couplers. Smaller, lighter, and (the really important part), they come
with complete engineering information.

A quick glance at the Centaflex application data shows 16 different
physical sizes with 34 different torsional spring rates, all with
complete data including nominal torque, max torque, allowable
misalignments, nominal and max twist in degrees, weight and mass moment
of inertia. You also get multiple mounting methods.

If nothing else, it is rather nice to be able to model for
predicted results, then select the required torsional stiffness from a
list.

<<The greater question, by my way of reasoning, would be how much
additional mass (flywheel)......>>

You really want to say "inertia", not mass. There are two good
reasons. First, using accepted, correct terms greatly improves
discussion. The subject is complex enough without everyone inventing
or misusing terms on the fly, and drilling yourself on correct terms
will help you with correct thinking. Second, as a practical matter, it
possible to build two flywheels of equal mass moment of inertia, but
unequal mass.

<<or damping (viscous/hydraulic disk) on the PSRU side of the
clutch/spring assembly would provide enough additional smoothing to
justify the added weight.>>

In the case of the Suzuki drive, the experimental damper added about
5 lbs and lowered steady-throttle resonant vibratory torque from 180
ft-lbs to 115 ft-lbs. Some portion of that 5 lbs also served as
flywheel inertia, a nice design bonus. A torsional damper does not
operate "on the PSRU side of the spring". It operates in parallel with
the spring, or to be more precise, in parallel with a connecting
torsional stiffness.

Dan

[Peter Dohm]

> <<I think that the clutch springs may also be a way.......>>
>
> Classic homebuilder's disease.
>
> Why adapt a component (with unknown properties, clunky packaging,
> and a least one severe drawback) when you can purchase products
> engineered for the task? Consider the Goetz, Lord, or Lovejoy soft
> couplers. Smaller, lighter, and (the really important part), they come
> with complete engineering information.
>
> A quick glance at the Centaflex application data shows 16 different
> physical sizes with 34 different torsional spring rates, all with
> complete data including nominal torque, max torque, allowable
> misalignments, nominal and max twist in degrees, weight and mass moment
> of inertia. You also get multiple mounting methods.
>
> If nothing else, it is rather nice to be able to model for
> predicted results, then select the required torsional stiffness from a
> list.
>

Yes, it is, and thanks for a pretty good initial list.

> <<The greater question, by my way of reasoning, would be how much
> additional mass (flywheel)......>>
>
> You really want to say "inertia", not mass. There are two good
> reasons. First, using accepted, correct terms greatly improves
> discussion. The subject is complex enough without everyone inventing
> or misusing terms on the fly, and drilling yourself on correct terms
> will help you with correct thinking. Second, as a practical matter, it
> possible to build two flywheels of equal mass moment of inertia, but
> unequal mass.
>
> <<or damping (viscous/hydraulic disk) on the PSRU side of the
> clutch/spring assembly would provide enough additional smoothing to
> justify the added weight.>>
>
> In the case of the Suzuki drive, the experimental damper added about
> 5 lbs and lowered steady-throttle resonant vibratory torque from 180
> ft-lbs to 115 ft-lbs. Some portion of that 5 lbs also served as
> flywheel inertia, a nice design bonus. A torsional damper does not
> operate "on the PSRU side of the spring". It operates in parallel with
> the spring, or to be more precise, in parallel with a connecting
> torsional stiffness.
>
>
> Dan
>
This is mostly a nomenclature issue. In most areas of electronics, where I
worked until a few years ago, we would have regarded the flywheel portion as
capacitance (and therefore the flywheels are parallel) while the springs and
dampener would be modeled as inductance and a small resistance (in series
with the capacitance of the flywheels).

In short, you appear to have designed/developed a pretty good solution which
can also be scaled for more or less power. I will add your list of
manufacturers and nomenclature to my notes for future use.

In the meantime, is the system or software you designed/developed available
for sale/rent to the guys who are currently trying to build flyable
aircraft? If so, from whom?

Peter

[Peter Dohm]

I may have started typing, and pressed send a little too quickly, so let me
flesh the question out a little more...

Here is a simplified electrical equivalent circuit, in an effort to provide
a little more precision that words alone.

(Note: This will display correctly only if your newsreader is displaying
fixed width font--probably Courier New--so I will also add a version spaced
to display in a readable fashion using Arial)

___________ ________
| DC | | DC |
| Generator |---------\/\/\/\---OOOOO-----------------| Motor |
| (Engine) | | R1 L1 | | | (Prop) |
|___________| | | \ |________|
| | /
=C1 =C2 \R2
| | /
| | |

Where C1 is the engine flywheel
C2 is the flywheel mass of the hydraulic disk dampener
L1 is the dampening effect of the disk dampener
R1 is the heat energy loss of the disk dampener
R2 is the friction loss of the PSRU and other bearings



Note that a true electrical equivalent is extremely difficult to draw, even
when one does not consider the limitations of text as graphics. However, in
my opinion; the system would be much more effectively damped, as seen by the
propeler, in the case that the hydraulic disk dampener is placed after the
soft coupling--and this would be more important in the case of the pusher
with a long driveshaft, where propeller excitation by the disturbed
slipstream is a practical issue.

So, what did you actually do?



(Note: The following is re-spaced to be readable on Outlook Express. It is
not very good, but you can sort-of make it out.)
___________
________
| DC |
| DC |
| Generator |---------\/\/\/\---OOOOO-----------------| Motor |
| (Engine) | | R1 L1 | |
| (Prop) |
|___________| | | \
|________|
| |
/
=C1 =C2 \R2
| |
/
| |
|

Where C1 is the engine flywheel
C2 is the flywheel mass of the hydraulic disk dampener
L1 is the dampening effect of the disk dampener
R1 is the heat energy loss of the disk dampener
R2 is the friction loss of the PSRU and other bearings



Peter

[Dan Horton]

<<Here is a simplified electrical equivalent circuit, in an effort to
provide a little more precision that words alone.>>

ä½*一定是開玩笑

Dan

[Peter Dohm]

"Dan Horton" > wrote in message
oups.com...
> <<Here is a simplified electrical equivalent circuit, in an effort to
> provide a little more precision that words alone.>>
>
> ???????
>
> Dan
>
OK, the display was worse than I thought. But gi'me a break.

I am still a little curious whether you placed the hydraulic disk (or
substitute) before or after the soft coupling.

I'll stifle my tendency to expound and leave it at that, because we've
pretty much beaten the subject to death.

Peter

[Dan Horton]

<<still a little curious whether you placed the hydraulic disk (or
substitute) before or after the soft coupling.>>

Neither.

As I wrote previously, a torsional damper operates in parallel with a
torsional stiffness, connecting the same two inertias.

Dan

[Richard Riley]

Dan, if I might ask, what was the long drive shaft pusher for?

[Dan Horton]

Richard,
<<what was the long drive shaft pusher?>>

A roadable aircraft project.

Dan