While wheel taxying for take-off minor ground resonance occurred. The pilot lifted the helicopter off the ground and the resonance ceased. Severe main rotor vibration started and the pilot landed the helicopter. Ground resonance then began with one main rotor blade striking the cabin roof and severing the tailboom. The helicopter then turned through 180 degrees as it sustained further damage. Inflight main rotor imbalance was caused by the failure of a blade damper. The failure resulted from the seat displacement of a one-way valve by a portion of perforated plate. Blade imbalance then induced the failure of a main rotor spacer cable. Loss of control necessitated a landing and severe ground resonance induced by blade imbalance resulted.
