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The angle of attack (AOA) in the context of a rotor blade is defined as the angle between the chord line of the blade and the direction of motion relative to the air. This measurement is crucial because it affects the lift generated by the rotor blades. When the angle of attack increases, the lift increases up to a certain point known as the critical angle of attack, beyond which lift will begin to decrease due to flow separation and potential stall conditions.

Understanding AOA is essential for pilots and aviation personnel as it directly influences the performance characteristics of the rotorcraft. Correctly managing AOA helps in optimizing performance during various phases of flight, including takeoff, hovering, and maneuvers. This emphasizes the importance of maintaining an appropriate angle to ensure controlled and efficient flight operations.

In contrast, while the mean camber line plays a role in airfoil design and performance, it does not define the angle of attack in this context. Thus, focusing solely on the chord line and its relationship to airflow gives a clear and direct understanding of what AOA indicates for rotor blades.