angle of attack
of attack is an aerodynamic angle and is illustrated here:
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.
Angle of attack should not be
confused with angle of incidence (blade pitch angle). Angle of incidence 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, 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.