Ground resonance is a vibration that can be destructive to a helicopter.
Ground resonance is primarily a problem in a fully-articulated rotor system. In these systems, ground contact can jolt the aircraft and produce a shock wave that can resonate in the aircraft. This resonance can then destroy the aircraft in a short amount of time. Basically, the helicopter will shake itself apart. Ground resonance is the reason that many helicopter’s landing gear has some sort of damping feature, such as shocks or OLEO struts. Many helicopters also have damping features within the rotor system as well, such as the 300CB. Should ground resonance be suspected and rotor RPM allows, the pilot should immediately lift the helicopter off the ground, stopping the resonance. Recovery from ground resonance is one reason to keep rotor RPM at flight speed until the collective is completely down. If unable to lift off, an option is to immediately lower collective and shut down the rotor. Wheeled type helicopters are at increased risk as the wheels do not flex horizontally.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-11
Principles of Helicopter Flight, 2nd Edition, pg. 162
FM 3-04.203-2007 Fundamentals of Flight pg. 1-68