Angle of Attack

ANY Airfoil's Angle Of Attack or AOA (4) is an aerodynamic one.

It is: The angle between the airfoil chord line and its direction of motion relative to the air (the resulting Relative Wind).

Several factors will affect rotor blade AOA. Some are controlled by the pilot and some occur automatically due to the rotor system design. Pilots are able to adjust AOA by moving the cyclic and collective pitch controls. However, even when these controls are held stationary, the AOA constantly changes as the blade moves around the circumference of the rotor disk. Other factors affecting AOA, over which the pilot has little control, are:

Blade Flapping

Blade Flexing

Wind Gusts / Turbulence

AOA is one of the primary factors that determines amount of lift and drag produced by an airfoil.

Angle of attack should not be confused with the Angle Of Incidence.

Angle of Incidence (or AOI) is the angle between the blade chord line and the plane of rotation of the rotor system.
It is a mechanical angle rather than an aerodynamic angle:

In the absence of induced flow and/or aircraft airspeed, AOA and AOI are equal.
Whenever the relative wind is modified (by induced flow / aircraft airspeed), then AOA and AOI diverge becoming unequal.

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