Counterbalanced crankshaft

[Romeo Victor]

Hello builders,

I run an 8 with an IO360A1B6, LSE ignition and a MT prop. Friend of mine has a non bounterbalanced crankshaft with the same ignition and the same prop. He encountered cracks on his prop while my prop worked fine without any damages.
I know about the problem with the composite propellers in combination with the ignition.
For my next project, RV7 with a carburated engine and a composite prop, I intend to install the LSE again. What I would like to know is:
1. "Which combination of composite propeller and engine works even fine?"
2. "Any chance to save the extra $ 5000,- for a counterbalanced crankshaft?"

Best Regards

Stephan

[DanH]

<<Any chance to save the extra $ 5000,- for a counterbalanced crankshaft?">>

As a first step consider the mechanics.

Lycoming crankshaft pendulums do not "counterbalance" anything but themselves. They are positioned at 90 and 270 degrees if you consider the crankthrow as 0-180, and thus have little to do with recip balance. A pendulum is tuned to a specific order and opposes torsional vibration of the crank at that order by acting as an infinite mass. Pendulums are not "dampers" as they do not remove energy from the system.

Yes, I know, the Lycoming books call them "counterweights". The four viscous dampers on your car are commonly called "shock absorbers", but that doesn't help understanding either <g>

Some engines do place a pendulum on the 0-180 axis, thus playing a dual role as both a torsional vibration device and a recip counterweight. May even be some Lyc crank designed that way; I've certainly not seen all of them.

Not poking at you Stephan; just polishing the pins. Too many of these discussions flail away without fundamentals, and bad nomenclature doesn't help.

To address your question, pendulums reduce or eliminate one or two orders of crankshaft torsional vibration that may have otherwise been available to excite propeller propeller blade vibration or stress the hub. Without pendulums (or even with them), the only way to know blade and hub stress for sure is to place strain gauges and observe live telemetry. Lacking measured stress values, you have only word of mouth regarding failures....which may or may not have had anything to do with crankshaft vibration.


Shot at 2007-08-18

[rvbuilder2002]

I think it is the lack of torsionally dampening counter weights that can be the cause of the propeller cracking problem though. Though I agree that without specialized testing we can't know for sure. "it worked ok on my airplane" only goes so far.

With strong power pulses of each cyl., it can start a torsional twisting resonance over the length of the crankshaft. This is the reason for the weights on some engines.

Some engines do ok without the weights because the crankshaft can tolerate the torsional resonance...but if you add a propeller that has a natural resonant freq. that matches the one in the crankshaft...you are likely to have trouble.

For many years, different propellers have had a yellow arc range at a certain RPM on certificated aircraft. This was common on many airplanes with fixed pitch props (lower mass and stiffness than constant speed) on uncounter weighted 180 HP lycomings. Harzell props have these restricted ranges on some engie prop combinations.
Years ago, after Sensenich introduced the fixed pitch prop for 150/160HP Lycoming powered RV's, they said it was highly unlikely that they would be able to develop one for the 180HP Lyc because of this exact problem. They gave it a try anyway and fortunately for many of us, they were successful.