May 2005 Additions to the rec.aviation "Rogues Gallery"...

"Dave Butler" wrote

If the counterweights are involved, and the the torsional vibration issue
is a
propeller/cranskshaft phenomenon, do you change anything in the crankshaft
counterweights when you change the propeller?

Nope, you still don't quite have it.

Did you know that if you have two tuning forks than vibrate (resonate) the
same note, and you hit one to start it resonating, and you put the second
one up to it, the second one will start vibrating? Well, they do.

Same idea with the crank and the prop. The crank vibrates at one frequency,
and at a certain RPM, that is the frequency that it wants to vibrate at. If
the crank has no other things vibrating at the same frequency touching it,
it is stiff enough to not be a problem.

Now you add a prop that *does* vibrate at the same frequency as the crank,
and run it at that critical RPM, the prop starts its vibration, and the
crank is doing the same thing. Think back to the tuning forks, and now the
one excites the other, and it keeps on exciting each other, getting louder
(think more movement) and louder, until something breaks.

So if you put a different prop on, (3 blade) that does not vibrate like a
tuning fork at the same critical frequency as the crank, the crank still
vibrates at its' frequency, but the prop does not, so it does not help the
crank vibrate bigger. (louder) No problem. The restriction for the
combination is removed.
--
Jim in NC

Morgans wrote:
"Dave Butler" wrote


If the counterweights are involved, and the the torsional vibration issue

is a

propeller/cranskshaft phenomenon, do you change anything in the crankshaft
counterweights when you change the propeller?


Nope, you still don't quite have it.

I guess I didn't make my question very clear, since you don't seem to have
understood it, or didn't answer it.


Did you know that if you have two tuning forks than vibrate (resonate) the
same note, and you hit one to start it resonating, and you put the second
one up to it, the second one will start vibrating? Well, they do.

Yes, I know that.


Same idea with the crank and the prop. The crank vibrates at one frequency,
and at a certain RPM, that is the frequency that it wants to vibrate at. If
the crank has no other things vibrating at the same frequency touching it,
it is stiff enough to not be a problem.

I question this explanation. I think it's rather that the whole assembly, prop
plus crankshaft, has a resonant torsional vibration, not that the the prop and
crankshaft vibrate independently and reinforce each other. I stand ready to be
corrected, though.


Now you add a prop that *does* vibrate at the same frequency as the crank,
and run it at that critical RPM, the prop starts its vibration, and the
crank is doing the same thing. Think back to the tuning forks, and now the
one excites the other, and it keeps on exciting each other, getting louder
(think more movement) and louder, until something breaks.

See above.


So if you put a different prop on, (3 blade) that does not vibrate like a
tuning fork at the same critical frequency as the crank, the crank still
vibrates at its' frequency, but the prop does not, so it does not help the
crank vibrate bigger. (louder) No problem. The restriction for the
combination is removed.

I concur that whether your tuning fork analogy is right or not, changing the
prop changes the vibration characteristics.

Now, back to my original question (as I intended, at least): since the torsional
resonance that the crankshaft counterweight vibration dampers were designed to
damp is no longer present, do you remove or otherwise modify the crankshaft
counterweight vibration dampers?

Dave

Dave Butler wrote:
Now, back to my original question (as I intended, at least): since the
torsional resonance that the crankshaft counterweight vibration dampers
were designed to damp is no longer present, do you remove or otherwise
modify the crankshaft counterweight vibration dampers?

If you remove the dampers, aren't you further changing the vibration
characteristics and frequency?

john smith wrote:
Dave Butler wrote:

Now, back to my original question (as I intended, at least): since the
torsional resonance that the crankshaft counterweight vibration
dampers were designed to damp is no longer present, do you remove or
otherwise modify the crankshaft counterweight vibration dampers?


If you remove the dampers, aren't you further changing the vibration
characteristics and frequency?

Yes, I suppose so. Is that a good thing or a bad thing?

Dave

"Dave Butler" wrote

Now, back to my original question (as I intended, at least): since the
torsional
resonance that the crankshaft counterweight vibration dampers were
designed to
damp is no longer present, do you remove or otherwise modify the
crankshaft
counterweight vibration dampers?

Dave

Hum, that would be a substantial job, since if you mess with the
counterweights, you are changing the balance relationship between the
piston/rod assembly, and the crank. I think you would be adding another set
of vibrations into the engine. Not a job for the homebuilder, I think.

It seems to me that if you added mass into the engine/prop combination, by
using a heavier flywheel, or harmonic balancer, you would have a chance at
fixing the problem.
--
Jim in NC

Morgans wrote:
"Dave Butler" wrote


Now, back to my original question (as I intended, at least): since the

torsional

resonance that the crankshaft counterweight vibration dampers were

designed to

damp is no longer present, do you remove or otherwise modify the

crankshaft

counterweight vibration dampers?

Dave


Hum, that would be a substantial job, since if you mess with the
counterweights, you are changing the balance relationship between the
piston/rod assembly, and the crank. I think you would be adding another set
of vibrations into the engine. Not a job for the homebuilder, I think.

Sorry, I didn't realize this was crossposted to r.a.homebuilt.

My point was
- when you change propellers, you are already messing with the vibration
characteristics.
- there are specific engine parts (those dynamic crankshaft counterweights) that
were presumably designed to dampen vibration with the original propeller.
- someone asserted that the yellow-arc on the tachometer could be removed if one
installed a 3-bladed prop.
- someone else (nrp) noted that the yellow-arc was there to avoid a torsional
vibration mode that occurs with the specific propeller/crankshaft comibnation.
- the question was intended to address certified airplanes, and I wondered
whether the STC for the propeller change addressed any other changes besides
just bolting on a new propeller.
- nrp seems to know a lot about torsional vibration, and I was hoping he would
respond w.r.t. the effect of changing propellers and not "messing with" the
vibration dampers that were installed to ameliorate the vibration with the
original prop.
- it was just a curiosity question. Never mind. Thanks.


It seems to me that if you added mass into the engine/prop combination, by
using a heavier flywheel, or harmonic balancer, you would have a chance at
fixing the problem.