Ground resonance
It occurs when the landing gear is prevented from freely moving about on the horizontal plane, typically when the aircraft is on the ground.
When the spacing of the blades becomes irregular, it shifts the rotor's center of mass from the axis of rotation, which causes an oscillation.
Under extreme conditions, the initial shock can cause violent oscillations that quickly build and result in catastrophic damage of the entire airframe.
In some cases, destruction occurs, e.g. body panels, fuel tanks, and engines are torn away, even at normal rotor speed.
Proper maintenance of the helicopter's damping system components can prevent ground resonance from taking hold.
Illustration of the shift in the location of the center of mass of a helicopter rotor caused by the individual blades' rotation in their respective vertical joints.