lead-lag hinge

lead-lag hinge

lead-lag hingeclick for a larger image
The hinge at the root of a helicopter rotor blade with its axis perpendicular to the plane of rotation. It allows the blade tip to move back and forth in its plane of rotation. Movement about the lead-lag is called drag. Drag dampers oppose this movement. Also called an alpha hinge or a hunting hinge.
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The primary objective of the dampers is to avoid rotor imbalance conditions that can lead to "ground resonance." Ground resonance happens in helicopters with lead-lag hinges. It starts when the blade motion causes an unbalanced centrifugal force that "excites" the natural frequency of the aircraft rocking on its landing gear.