and produce forces that create the thrust to pull, or push,  the aircraft through the air. The engine furnishes the power  needed to rotate the propeller blades through the air at high  speeds, and the propeller transforms the rotary power of the  engine into forward thrust.  equipped with these devices. Like advanced automation,  such as autopilots and moving maps, the misunderstanding  or misuse of the equipment can have disastrous results. Some  items which may limit the effectiveness of an AOA indicator  are listed below:  A cross-section of a typical propeller blade is shown in  Figure 5-43.  This section or blade element is an airfoil  comparable to a cross-section of an aircraft wing. One  surface of the blade is cambered or curved, similar to the  upper surface of an aircraft wing, while the other surface is  flat like the bottom surface of a wing. The chord line is an  imaginary line drawn through the blade from its leading edge  to its trailing edge. As in a wing, the leading edge is the thick  edge of the blade that meets the air as the propeller rotates.  Blade angle, usually measured in degrees, is the angle  between the chord of the blade and the plane of rotation  and is measured at a specific point along the length of the  blade.  [Figure 5-44]  Because most propellers have a flat  blade “face,” the chord line is often drawn along the face  of the propeller blade. Pitch is not blade angle, but because  pitch is largely determined by blade angle, the two terms are  The aircraft propeller consists of two or more blades and a  central hub to which the blades are attached. Each blade of  an aircraft propeller is essentially a rotating wing. As a result  of their construction, the propeller blades are like airfoils  Pilots of general aviation airplanes equipped with AOA  indicators should contact the manufacturer for specific  limitations applicable to that installation.  5-28  Basic Propeller Principles  •  Calibration techniques  •  Probes or vanes not being heated  •  The type of indicator itself  •  Flap setting  •  Wing contamination  Coefficient of Lift Curve  Direction of  Alpha (angle of attack)  C$_{L }$ Direction of  Relative Wind  Direction of  Relative Wind  Relative Wind  2  1  3  Calibration Set Points:  Ground (Zero Set Point)  Optimum Alpha Angle (OAA)  (1.3 x V$_{s}$)  Cruise Alpha  (Va or Maneuvering Speed)  1  2  3  Direction of  Relative Wind  Figure 5-42.  An AOA indicator has several benefits when installed in general aviation aircraft. 