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

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.