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Articulated rotor systems with drag hinges allow each individual blade to advance or lag in its rotation to compensate for stress on the blade caused by the acceleration and deceleration of the rotor hub. When the spacing of the blades becomes irregular, it shifts the rotor's center of gravity from the axis of rotation, which causes an oscillation. When the airframe begins to rock back and forth from the oscillation, the oscillations can reinforce each other and cause the rotor's center of gravity to spiral away from the axis of rotation to a point beyond the compensating ability of the damping system. ground resonance is usually precipitated by a hard landing or an asymmetrical ground contact, and is more likely to occur when components of the landing gear or damping system are improperly maintained, such as the drag hinge dampers, oleo struts, or wheel tire pressure. Under extreme conditions, the initial shock can cause violent oscillations that quickly build and result in catastrophic damage of the entire airframe.
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