A new helicopter design must undergo thorough testing to ensure the reliability of the design with respect to vibration.
A new design undergoing testing may encounter severe vibration while it is on the ground, preparing for takeoff. A similar problem may occur immediately after landing.
Many helicopters have fully-articulated rotor systems. The advancing blades flap upward and the retreating blades flap downward in order to maintain uniform lift during forward flight.
A helicopter with a fully-articulated rotor has some measure of an unbalanced rotational force, which causes an oscillating force. This force may excite one of the helicopter’s natural frequencies, as the rotor accelerates to its full operating speed prior to takeoff.
This is an example of resonant excitation.
Further information is given in the paper: helicop.pdf
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Ground Resonance Videos
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– Tom Irvine