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

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