More vertical left vector Lift Induced flow velocity= 45 ft/sec Axis of rotation α = 10° Reduced blade pitch angle Angle of attack is the same  in- or out-of-ground effect Pitch angle = 14° Induced flow velocity = 45 ft/sec Reduced blade-tip vortexes MAX VELOCITY = 90 ft/sec Center of pressure Tip-path plane Chord line Angle of incidence Pitch Angle Angle of Attack Reference Plane Axis of Rotation Chord Line Relative Wind Figure 2-25.  The AOA is the angle between the airfoil chord line  and resultant relative wind. Figure 2-24.  Angle of incidence.  Figure 2-23.  Out of ground effect (OGE). Angle of Attack a rapid decrease in lift (refer to the Low Rotor RPM and  Rotor Stall section of Chapter 11, Helicopter Emergencies  and Hazards). AOA is the angle between the airfoil chord line and resultant  relative wind.  [Figure 2-25]  It is an aerodynamic angle and  not easy to measure. It can change with no change in the blade  pitch angle (angle of incidence, discussed earlier).  When the AOA is increased, air flowing over the airfoil is  diverted over a greater distance, resulting in an increase of  air velocity and more lift. As the AOA is increased further,  it becomes more difficult for air to flow smoothly across the  top of the airfoil. At this point, the airflow begins to separate  from the airfoil and enters a burbling or turbulent pattern.  The turbulence results in a large increase in drag and loss of  lift in the area where it is taking place. Increasing the AOA  increases lift until the critical angle of attack is reached. Any  increase in the AOA beyond this point produces a stall and  Several factors may change the rotor blade AOA. The pilot  has little direct control over AOA except indirectly through  the flight control input. Collective and cyclic feathering  help to make these changes. Feathering is the rotation of the  blade about its longitudinal axis by collective/cyclic inputs  causing changes in blade pitch angle. Collective feathering  changes angle of incidence equally and in the same direction  on all rotor blades simultaneously. This action changes AOA,  which changes coefficient of lift (CL), and affects overall  lift of the rotor disk. 2-13