There is one used in automotive racing.
It is manufactured by Comp cams/TCI, and called the "rattler"
for obvious reasons.
It mounts in place of the normal torsional damper on the nose
of the crankshaft, and is normal in appearance.
Packard used a similar device in their inline engines of the
late 20's to 30's.
Archie

Paul,
I had a quick look at the Rattler, but more importantly there are some good
descriptions of what Torsional Vibration/Resonance Frequency is and how it
affects the engine.

http:/www.tciauto.com/Products/TechInfo/rattler_tech.asp

I wonder if it works well on a single rotor engne?

My understanding of Torsional Frequency is 'event per second OR event per
RPM'. My question is what resonant frequency do we design for for both the 2
rotor and the single rotor.
George (down under)