Errol Cavit wrote:
Anyone know how common ground resonance incidents are in helicopters in
general and SH-2s in particular?

Don't know much about the H-2, but on the H-60 there are a couple of
tiedown points just above and behind the cockpit doors. While the
rotors are turning you don't use these to chain the aircraft to the
ship, because the aircraft can get into ground resonsance that way.

I don't think ground resonsance incidents are very common. Common
causes might be a weak blade damper (like the shock absorber on your
car), combined with a hard landing that may "excite" one of the
blades to vibrate divergently. The way the aircraft is touching the
ground has a lot to do with it too- it will vibrate differently in
the air than on the ground when the wheels are on the deck or
depending on different tiedowns points being used. Possible
responses by the pilot could be immediately taking off again (to
break the connection to the ground) or immediately shutting down the
rotor system. The problem usually does not solve itself but instead
gets very bad in a matter of seconds.

That's it in a general terms. Each thing I said is not necessarily
true in every case of ground resonance.

If you want to understand it on a fundamental level, I think the
mathematical explanation is pretty advanced, way beyond my level
anyway