Definition
An up-and-down hinged movement of a helicopter rotor blade that allows it to rise on the advancing side and lower on the retreating side as the rotor turns. This movement equalizes the lift produced across the rotor disc during forward flight, preventing the helicopter from rolling due to uneven lift between the two sides of the rotor system.
Plain English
As a helicopter flies forward, one side of the spinning rotor moves into the wind and the other side moves with it, so they don't make the same amount of lift. To keep the helicopter from rolling over, each blade is hinged so it can rise and fall freely as it goes around. The blade flaps up where lift is stronger and down where lift is weaker, evening things out.
Context Anchor
Seen in helicopter aerodynamics, especially when discussing rotor blade motion, control response, and lift differences as the helicopter moves forward.
Derivation
Flap comes from the Middle English flappen, meaning to strike or beat with a broad surface, like a bird beating its wings. The word captures the up-and-down motion of the blade as it rises and falls during each rotation.
Why Pilots Care
Proper blade flap prevents excessive stress on the rotor head and mast while maintaining stable lift across the disk in forward flight.
Grounding Statement
Picture one rotor blade rising slightly and another lowering slightly as they spin around the mast.
Intuition Check
Blade flap does not mean the blade is loose or damaged. Here, it means a normal up-and-down motion built into the rotor system.
Example Sentence 1
The instructor explained that blade flap allows the rotor system to compensate for the uneven lift produced during forward flight.
Example Sentence 2
The student practiced recognizing the limits of blade flap to avoid mast bumping during low-G maneuvers in the training helicopter.