Definition
The back-and-forth movement of a helicopter rotor blade in the plane of rotation, where the blade leads forward of or lags behind its neutral position relative to the rotor hub. This in-plane motion is permitted by the drag hinge (also called the lead-lag hinge) on a fully articulated rotor system and occurs as the blade speeds up and slows down through each revolution due to the Coriolis effect and changes in aerodynamic drag.
Plain English
As a helicopter rotor blade spins, it doesn't travel at a perfectly constant speed around the circle. It speeds up slightly in some parts of the rotation and slows down in others. To prevent stress on the blade and the hub, the blade is allowed to swing a little forward and backward on a hinge. That forward and backward swinging is called drag.
Context Anchor
Seen when studying or inspecting helicopter rotor blades and their allowed movements.
Derivation
The word 'drag' here is borrowed from the everyday sense of something being pulled or trailing behind. When a rotor blade slows in its rotation, it appears to lag or 'drag' behind its expected position, which is where the name comes from. Note: this is unrelated to aerodynamic drag (the resistance force on an aircraft), even though the same word is used.
Why Pilots Care
Unmanaged drag forces can produce damaging blade lag or vibration, which is why drag hinges and dampers are built into many rotor heads.
Grounding Statement
Picture looking down at the rotor: drag is a blade tip shifting a little forward or backward along the circle it is already traveling.
Intuition Check
Do not read drag here as only the air force that slows an aircraft. In this helicopter-rotor context, drag means the blade’s forward-and-back movement within the rotor’s turning circle.
Example Sentence 1
The drag hinge on a fully articulated rotor system allows each blade to lead and lag as it rotates, relieving stresses caused by changing rotational speed.
Example Sentence 2
As the rotor sped up, the pilot felt a slight change in vibration caused by varying drag on each blade.