It is defined as the angle between the airfoil chord and its direction of motion relative to the air (resultant relative wind). Several factors may cause rotor blade angle of attack to change. Some are controlled by the pilot and some occur automatically due to the rotor system design. Pilots are able to adjust angle of attack by moving the cyclic and collective pitch controls. However, even when these controls are held stationary, the angle of attack constantly changes as the blade moves around the circumference of the rotor disk. Other factors affecting angle of attack, over which the pilot has little control, are blade flapping, blade flexing, and gusty wind or turbulent air conditions. Angle of attack is one of the primary factors that determines amount of lift and drag produced by an airfoil.
In the absence of induced flow and/or aircraft airspeed, angle of attack and angle of incidence are the same. Whenever relative wind is modified by induced flow or aircraft airspeed, then angle of attack is different than angle of incidence.
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