Crankshaft Counterweights

[rv8180]

Does anyone have first hand knowledge of "de-tuning" the crankshaft counterweights by applying throttle to fast?

I assume this would manifest itself by a change in vibration?

How are these weights secured and "tuned"?

Is this peculiar to specific engines?

Wow!

I have an AEIO-390 with a countweighted crank.
There is no manual, specific to the 390. I was told to use the stuff for the IO-360 (200 hp) engine.

The Operator's Manual, that I have, warns against applying the throttle too fast because there is a danger of "de-tuning" the crankshaft counterweights.

[DanH]

<<"de-tuning" the crankshaft counterweights>>

Pendulums. Sometimes pendulum absorbers also serve as crankshaft counterweights, sometimes not. Your engine is in the "not" group. This little detail isn't important unless you wish to understand them.

<<I assume this would manifest itself by a change in vibration?>>

Maybe, maybe not. Assume your engine model has a 6th order pendulum. At 2500 RPM the vibration the damaged pendulum was intended to cancel would be at 250 hertz. Hard to discern much about something that happens 250 times per second using only feeble human senses.

<<How are these weights secured and "tuned"?>>

In this case they hang on rollers, but that ain't the only way it's done. You really need a book, because it is hard to understand the four or five different
kinds of pendulum absorbers without diagrams and equations. J.P. DenHartog's classic "Mechanical Vibrations" is good, cheap, and available. The most complete treatment I've ever seen is in "A Handbook On Torsional Vibration", E.J. Nestorides. It is out of print, but available at a large university library.

[deene]

The counterweights are attached to the crank so that they can move over a very small range. They are designed so that they tend to move to cancel higher order harmonics in the crank as in 6th order.

As the rpm is changed they move to cancel the harmonic. If the rpm is changed very quickly, they "detune" by being in the wrong position. Again, the movement is very small and is controlled by very precise pins and bushings (and as noted earlier, the harmonics are high order and small in amplitude). The system was added noteably to the 200 hp angle valve engines on Piper Arrows and Mooneys to get the smoothest ride possible.

The hard acro guys really bang the airframe and engine around and this bangs the pins and bushings. In RV style soft acro (rolls, loops, etc) I doubt you will see any problems as you won't be slamming the trottle or the airframe around.

I also have a IO390X and expect no problems with it in my RV8 due to the limited kinds of acro approved for the RV.

Deene Ogden
RV8QB, IO390, BA prop
EAA Tech Counselor/Flight Advisor
CFII, CFIG, MEI

[rv8180]

Thanks!
I can now see how de-tuning would be at least be possible under certain condtions and use.

Your inputs were very helpful.....thanks!

I can't believe Lycoming doesn't publish literature for this engine......after all.....it only cost $33K!

Thank God for BPA's (Barret Precision Aircraft) after sale support. They are top notch!