lead-lag
In helicopters, the movement of main rotor blades in the plane of rotation due to the inward and outward shifting of their centres of gravity as they flap up and down.
lead-lag: blade motion tracking the rotor disk
In a helicopter main rotor, lead-lag is the fore-and-aft movement of each blade within the plane of rotation, occurring as the blade flaps up and down. As a blade cycles through its rotation, aerodynamic lift changes its angle of attack and vertical position; this flapping action shifts the blade's center of gravity inward (toward the mast) when it rises and outward when it falls. The resulting oscillation in the rotational plane is lead-lag motion, measured in degrees relative to the nominal blade position.
The mechanism is primarily inertial. When a blade flaps upward, centrifugal force distribution changes, and the blade's mass effectively moves closer to the rotor hub. As it flaps downward, mass shifts outward. Because the rotor is spinning, this radial shift couples with rotational velocity to produce tangential motion along the rotor disk. This is distinct from flapping (up-and-down motion in a vertical plane) and feathering (blade pitch change), though all three motions occur simultaneously in normal flight.
Damping and control
Uncontrolled lead-lag motion can excite dangerous resonances and create dynamic imbalances. Helicopters use lead-lag dampers, fluid-filled or friction devices mounted between the blade root and the pitch horn, to dissipate oscillatory energy. Elastomeric blade roots or articulated hub designs also permit lead-lag motion in a controlled manner, preventing rigid buildup of inertial loads. Without adequate damping, blade flutter and mast bending moments increase rapidly.
Lead-lag is most pronounced during hover and low-speed flight, where aerodynamic loads are least uniform across the rotor disk. Maneuvers that change collective or cyclic pitch abruptly can spike lead-lag amplitudes. Maintenance checks include inspection of lead-lag damper fluid level, evidence of seepage, and freedom of motion in the lead-lag hinge or bushing. Worn or degraded dampers allow excessive blade motion and accelerate fatigue cracking at the blade root and pitch horn attachment points.
The term reflects the phenomenon's history: blades in lead (ahead of their nominal azimuth) and lag (behind it) as they cycle through vertical position changes. Understanding lead-lag is essential for diagnosing rotor vibration complaints, interpreting blade tracking measurements, and evaluating structural health after rough operations or hard landings.