Definition
The fore-and-aft movement of a helicopter rotor blade in the plane of rotation, where the blade moves slightly ahead of (lead) or behind (lag) its neutral position relative to the rotor hub. This motion is caused by the conservation of angular momentum as the blade flaps up and down, and by the periodic acceleration and deceleration of the blade as it travels around the rotor disc. Most fully articulated rotor systems incorporate a drag hinge (also called a lead-lag hinge) that allows this movement, along with dampers to control it.
Plain English
As a helicopter rotor blade spins, it doesn't just travel in a perfect circle. It speeds up and slows down slightly, swinging a little ahead or a little behind where you'd expect it to be. The rotor system is built to allow this small back-and-forth motion so the blade isn't stressed by it.
Context Anchor
Seen in helicopter rotor system discussions, especially when studying blade motion, rotor vibration, and rotor head components.
Derivation
Lead and lag are everyday English words: 'lead' meaning to be ahead, 'lag' meaning to fall behind. Here they describe a blade's position relative to where it would sit if it traveled at a perfectly steady speed around the hub.
Why Pilots Care
Lead-lag motion is what makes drag hinges and lead-lag dampers necessary on fully articulated rotors. If dampers fail or are out of adjustment, the rotor can develop ground resonance — a destructive vibration that can destroy the helicopter on the ground in seconds.
Grounding Statement
Picture one rotor blade shifting a little ahead or behind its usual spot as it travels around the hub.
Intuition Check
Lead does not mean an electrical wire here, and lag does not just mean being late. Here, lead and lag describe a rotor blade moving slightly forward or backward in its circular path.
Example Sentence 1
The fully articulated rotor head uses a drag hinge at each blade root to allow blade lead or lag during rotation.
Example Sentence 2
Proper blade lead or lag settings reduce vibration felt in the cockpit during hover.