6" 6" 6" 42" 6" 6" 6" 6" 6" 6" 42" Station 18" Station 12" Station Blade shank Blade butt Center of hub C a m b e r o f  u p e r s u r f a c e C am ber  of  l o w e r s u r f a c e Leading edge Chord line Mean camber line Trailing edge Centrifugal force A Torque bending force B Thrust bending  force C Aerodynamic  twisting force D Centrifugal  twisting force E Center of rotation Center of pressure Figure 7-7. Cross-section of a propeller blade. Figure 7-8. Forces acting on a rotating propeller.  Figure 7-6.  Typical propeller blade elements.  A cross-section of a typical propeller blade is shown in  Figure 7-7 . This section or blade element is an airfoil  comparable to a cross-section of an aircraft wing. The blade  back is the cambered or curved side of the blade, similar to  the upper surface of an aircraft wing. The blade face is the  flat side of the propeller blade. The chord line is an imaginary  line drawn through the blade from the leading edge to the  trailing edge. The leading edge is the thick edge of the blade  that meets the air as the propeller rotates. A rotating propeller is acted upon by centrifugal twisting,  aerodynamic twisting, torque bending, and thrust bending  forces. The principal forces acting on a rotating propeller  are illustrated in  Figure 7-8 . Centrifugal force is a physical force that tends to  throw the rotating propeller blades away from the hub.  [Figure 7-8A]  This is the most dominant force on the  propeller. Torque bending force, in the form of air resistance,  tends to bend the propeller blades in the direction opposite  that of rotation.  [Figure 7-8B]  Thrust bending force is the  thrust load that tends to bend propeller blades forward as the  aircraft is pulled through the air.  [Figure 7-8C]  Aerodynamic  twisting force tends to turn the blades to a high blade angle.  [Figure 7-8D]  Centrifugal twisting force, being greater than  the aerodynamic twisting force, tends to force the blades  toward a low blade angle. At least two of these forces acting on the propellers blades  are used to move the blades on a controllable pitch propeller.  Centrifugal twisting force is sometimes used to move the  blades to the low pitch position, while aerodynamic twisting  force is used to move the blades into high pitch. These forces  can be the primary or secondary forces that move the blades  to the new pitch position. A propeller must be capable of withstanding severe stresses,  which are greater near the hub, caused by centrifugal force and  thrust. The stresses increase in proportion to the rpm. The blade  face is also subjected to tension from the centrifugal force and  additional tension from the bending. For these reasons, nicks or  scratches on the blade may cause very serious consequences.  These could lead to cracks and failure of the blade and are  addressed in the repair section later in this chapter. A propeller must also be rigid enough to prevent fluttering,  a type of vibration in which the ends of the blade twist back  and forth at high frequency around an axis perpendicular  to the engine crankshaft. Fluttering is accompanied by a  distinctive noise, often mistaken for exhaust noise. The  7-4$^{Tip section}$