Definition
The up-and-down movement of a helicopter rotor blade about a horizontal hinge (the flapping hinge) at or near the rotor hub, allowing each blade to rise and fall as it rotates. Flapping equalizes lift between the advancing and retreating blades during forward flight, preventing the rolling tendency that would otherwise occur from the asymmetrical airspeed across the rotor disc.
Plain English
It’s the way each rotor blade is allowed to hinge up and down as it spins around. This movement lets the blades balance out the lift they produce, so the helicopter doesn’t want to roll over when flying forward.
Context Anchor
Seen in helicopter rotor system descriptions, maintenance inspections, and explanations of how rotor blades handle changing lift as they go around.
Derivation
From the everyday sense of ‘flap’ — to move up and down, like a bird’s wing. Helicopter engineers borrowed the word because the blade’s motion looks like that flapping action, even though the movement is small and controlled by a hinge.
Why Pilots Care
Flapping lets the rotor disc tilt smoothly for directional control and reduces vibration and structural loads during forward flight.
Intuition Check
Flap does not mean the airplane wing flap used for takeoff and landing here. In this helicopter context, flap means the rotor blade’s up-and-down movement.
Example Sentence 1
The fully articulated rotor head allows each blade to flap independently, evening out the lift as the helicopter moves forward.
Example Sentence 2
Maintenance technicians inspect the flap hinges for wear during rotor head checks.