Lateral vibrations in helicopter rotor systems are quite common. The balancing of high-speed parts is very important. This type vibration is more dangerous than a lateral or vertical one because it causes crystallization of metal, which weakens it. This vibration must be corrected before the equipment can be operated. Ground Resonance Ground resonance is the most dangerous and destructive of the vibrations discussed here.
Ground resonance can destroy a helicopter in a matter of seconds.
It is present in helicopters with articulated rotor heads. Ground resonance occurs while the helicopter is on the ground with rotors turning it will not happen in flight. Ground resonance results when unbalanced forces in the rotor system cause the helicopter to rock on the landing gear at or near its natural frequency. Correcting this problem is difficult because the natural frequency of the helicopter changes as lift is applied to the rotors. With all parts working properly, the design of the helicopter landing gear, shock struts, and rotor blade lag dampeners will prevent the resonance building up to dangerous levels.