cases is a fixed, built-in angle. When the leading edge of the  wing is higher than the trailing edge, the angle of incidence is  said to be positive. The angle of incidence is negative when  the leading edge is lower than the trailing edge of the wing. Before beginning the discussion on AOA and its effect on  airfoils, first consider the terms chord and center of pressure  (CP) as illustrated in  Figure 2-5 . The chord of an airfoil or wing section is an imaginary  straight line that passes through the section from the leading  edge to the trailing edge, as shown in  Figure 2-5 . The chord  line provides one side of an angle that ultimately forms  the AOA. The other side of the angle is formed by a line  indicating the direction of the relative airstream. Thus, AOA  is defined as the angle between the chord line of the wing and  the direction of the relative wind. This is not to be confused  with the angle of incidence, illustrated in  Figure 2-4 , which  is the angle between the chord line of the wing and the  longitudinal axis of the aircraft.  On each part of an airfoil or wing surface, a small force is  present. This force is of a different magnitude and direction  from any forces acting on other areas forward or rearward  from this point. It is possible to add all of these small forces  mathematically. That sum is called the “resultant force”  (lift). This resultant force has magnitude, direction, and  location, and can be represented as a vector, as shown in  Figure 2-5 . The point of intersection of the resultant force  line with the chord line of the airfoil is called the center of  pressure (CP). The CP moves along the airfoil chord as the  AOA changes. Throughout most of the flight range, the CP  moves forward with increasing AOA and rearward as the  AOA decreases. The effect of increasing AOA on the CP is  shown in  Figure 2-6 .  The AOA changes as the aircraft’s attitude changes. Since the  AOA has a great deal to do with determining lift, it is given  primary consideration when designing airfoils. In a properly  designed airfoil, the lift increases as the AOA is increased.  When the AOA is increased gradually toward a positive  AOA, the lift component increases rapidly up to a certain  point and then suddenly begins to drop off. During this action  the drag component increases slowly at first, then rapidly as  lift begins to drop off.  When the AOA increases to the angle of maximum lift, the  burble point is reached. This is known as the critical angle.  When the critical angle is reached, the air ceases to flow  smoothly over the top surface of the airfoil and begins to  burble or eddy. This means that air breaks away from the  upper camber line of the wing. What was formerly the area  of decreased pressure is now filled by this burbling air.  When this occurs, the amount of lift drops and drag becomes  Figure 2-6.  Effect on increasing angle of attack.  Figure 2-5.  Airflow over a wing section.  Resultant lift Center of pressure Angle of attack Relative airstream Drag Lift Chord line Resultant Center of pressure Positive pressure Relative airstream Negative pressure pattern Resultant Positive pressure Relative airstream Negative pressure pattern moves forward Resultant Positive pressure Relative airstream Center of pressure moves forward Positive pressure Wing completely stalled Angle of attack = 0° A Angle of attack = 6° B Angle of attack = 12° C Angle of attack = 18° D Angle of Attack (AOA) 2-6