PHAK 5-28

Alpha (angle of attack) CL Direction of Relative Wind Direction of Relative Wind Direction of Relative Wind 2 1 3 Coefficient of Lift Curve Calibration Set Points: Ground (Zero Set Point) Optimum Alpha Angle (OAA) (1.3 x Vs) 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. 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: • Calibration techniques • Probes or vanes not being heated • The type of indicator itself • Flap setting • Wing contamination Pilots of general aviation airplanes equipped with AOA indicators should contact the manufacturer for specific limitations applicable to that installation. Basic Propeller Principles 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 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. 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