Definition
The up-and-down movement of a helicopter rotor blade about a hinge (or flexible section) at its root, allowing each blade to rise and fall independently as it travels around the rotor disc. Flapping equalises the lift produced by the advancing blade (which sees a higher relative airspeed) and the retreating blade (which sees a lower relative airspeed), preventing the rotor from rolling the helicopter in forward flight.
Plain English
As a helicopter flies forward, one side of the spinning rotor moves into the wind faster than the other side. To keep the lift even, each blade is allowed to hinge up and down as it goes around. This rising and falling motion is called blade flapping.
Context Anchor
Used in helicopter aerodynamics, rotor system descriptions, and discussions of how a helicopter rotor handles forward flight.
Derivation
From the everyday meaning of flap — to move up and down like a bird's wing. The term is descriptive: the blade literally flaps as it rotates.
Why Pilots Care
Allows the rotor to remain balanced and stable in forward flight without excessive vibration or stress.
Grounding Statement
Picture one rotor blade rising slightly and another lowering slightly as they turn, so the rotor can keep the helicopter balanced as air loads change.
Intuition Check
Flapping does not mean uncontrolled shaking or a loose blade. In this context, it means a designed up-and-down movement of the rotor blade.
Example Sentence 1
Blade flapping allows the rotor system to compensate for the difference in airspeed between the advancing and retreating blades during forward flight.
Example Sentence 2
The pilot checks for normal blade flapping during preflight rotor spin-up.