Final answer:
The angle between the chord line of a rotor blade and its motion relative to the air is the angle of attack. This angle is critical in rotorcraft aerodynamics, affecting lift and efficiency.
Step-by-step explanation:
The angle between the chord line of a rotor blade and its direction of motion relative to the air is the angle of attack, which is a critical angle. In rotorcrafts, such as helicopters, the angle of attack is important because it affects the lift produced by the blades. The rotor blades can be thought of as long airplane wings that are mounted to allow rotation on an axis, thus allowing them to engage the wind at varying angles to control efficiency. A helicopter blade, spinning at a given rate such as 100 revolutions per minute, experiences a range of aerodynamics, including centripetal acceleration due to its rotation.