Nonsymmetrical Symmetrical C a m ber  of up pe rs ur fa ce Camber of l owe r su r fa ce Trailing edge Leading edge Mean camber line Chord line Figure 2-13.  The upper and lower curvatures are the same on a  symmetrical airfoil and vary on a nonsymmetrical airfoil. Figure 2-12.  Aerodynamic terms of an airfoil. 2-8 relative wind experienced by the airfoil may not be  exactly opposite its direction of travel. The chord line connects the ends of the mean camber  line. Camber refers to curvature of the airfoil and  may be considered as curvature of the mean camber  line. The shape of the mean camber is important for  determining aerodynamic characteristics of an airfoil  section. Maximum camber (displacement of the mean  camber line from the chord line) and its location help  to define the shape of the mean camber line. The  location of maximum camber and its displacement  from the chord line are expressed as fractions or  percentages of the basic chord length. By varying the  point of maximum camber, the manufacturer can tailor  an airfoil for a specific purpose. The profile thickness  and thickness distribution are important properties of  an airfoil section. Airfoil Types Symmetrical Airfoil Nonsymmetrical Airfoil (Cambered) The symmetrical airfoil is distinguished by having identical  upper and lower surfaces.  [Figure 2-13]  The mean camber  line and chord line are the same on a symmetrical airfoil,  and it produces no lift at zero AOA. Most light helicopters  incorporate symmetrical airfoils in the main rotor blades. The nonsymmetrical airfoil has different upper and lower  surfaces, with a greater curvature of the airfoil above the  chord line than below.  [Figure 2-13]  The mean camber line  and chord line are different. The nonsymmetrical airfoil design  can produce useful lift at zero AOA. A nonsymmetrical design  •  Chord line—a straight line intersecting leading and  trailing edges of the airfoil.  [Figure 2-12] •  Chord—the length of the chord line from leading edge  to trailing edge; it is the characteristic longitudinal  dimension of the airfoil section. •  Mean camber line—a line drawn halfway between the  upper and lower surfaces of the airfoil.  [Figure 2-12] •  Trailing edge—the rearmost edge of an airfoil.  •  Induced flow—the downward flow of air through the  rotor disk.  •  Resultant relative wind—relative wind modified by  induced flow. •  AOA—the angle measured between the resultant  relative wind and chord line.  •  Angle of incidence (AOI)—the angle between the  chord line of a blade and rotor hub. It is usually  referred to as blade pitch angle. For fixed airfoils,  such as vertical fins or elevators, angle of incidence  is the angle between the chord line of the airfoil and  a selected reference plane of the helicopter. •  Center of pressure—the point along the chord line of  an airfoil through which all aerodynamic forces are  considered to act. Since pressures vary on the surface  of an airfoil, an average location of pressure variation is  needed. As the AOA changes, these pressures change,  and the center of pressure moves along the chord line. •  Leading edge—the front edge of an airfoil.  [Figure 2-12] •  Flightpath velocity—the speed and direction of  the airfoil passing through the air. For airfoils on  an airplane, the flightpath velocity is equal to true  airspeed (TAS). For helicopter rotor blades, flightpath  velocity is equal to rotational velocity, plus or minus  a component of directional airspeed. The rotational  velocity of the rotor blade is lowest closer to the hub  and increases outward towards the tip of the blade  during rotation. •  Relative wind—defined as the airflow relative to  an airfoil and is created by movement of an airfoil  through the air. This is rotational relative wind for  rotary-wing aircraft and is covered in detail later. As  an induced airflow may modify flightpath velocity, 