drop in pressure. This lowered pressure is a component of  total lift. The pressure difference between the upper and  lower surface of a wing alone does not account for the total  lift force produced.  The downward backward flow from the top surface of an  airfoil creates a downwash. This downwash meets the flow  from the bottom of the airfoil at the trailing edge. Applying  Newton’s third law, the reaction of this downward backward  flow results in an upward forward force on the airfoil.  A certain amount of lift is generated by pressure conditions  underneath the airfoil. Because of the manner in which air  flows underneath the airfoil, a positive pressure results,  particularly at higher angles of attack. However, there is  another aspect to this airflow that must be considered. At a  point close to the leading edge, the airflow is virtually stopped  (stagnation point) and then gradually increases speed. At  some point near the trailing edge, it again reaches a velocity  equal to that on the upper surface. In conformance with  Bernoulli’s principle, where the airflow was slowed beneath  the airfoil, a positive upward pressure was created (i.e., as  the fluid speed decreases, the pressure must increase). Since  the pressure differential between the upper and lower surface  of the airfoil increases, total lift increases. Both Bernoulli’s  Principle and Newton’s Laws are in operation whenever lift  is being generated by an airfoil.  From experiments conducted on wind tunnel models and on  full size airplanes, it has been determined that as air flows  along the surface of a wing at different angles of attack  (AOA), there are regions along the surface where the pressure  is negative, or less than atmospheric, and regions where the  pressure is positive, or greater than atmospheric. This negative  pressure on the upper surface creates a relatively larger force  on the wing than is caused by the positive pressure resulting  from the air striking the lower wing surface.  Figure 4-7  shows  the pressure distribution along an airfoil at three different  angles of attack. The average of the pressure variation for  any given AOA is referred to as the center of pressure (CP).  Aerodynamic force acts through this CP. At high angles of  attack, the CP moves forward, while at low angles of attack  the CP moves aft. In the design of wing structures, this CP  travel is very important, since it affects the position of the  air loads imposed on the wing structure in both low and high  AOA conditions. An airplane’s aerodynamic balance and  controllability are governed by changes in the CP.  Although specific examples can be cited in which each of  the principles predict and contribute to the formation of lift,  lift is a complex subject. The production of lift is much more  complex than a simple differential pressure between upper  and lower airfoil surfaces. In fact, many lifting airfoils do  not have an upper surface longer than the bottom, as in the  case of symmetrical airfoils. These are seen in high-speed  aircraft having symmetrical wings, or on symmetrical rotor  blades for many helicopters whose upper and lower surfaces  Airfoil Behavior  4-8  Pressure Distribution  High Pressure Below  f High angle of attack  Normal angle of attack  Low angle of attack  at ta c k  o A ng l e  -8 °  CP  a t a c k  A n g l e o f  + 1 0 °  CP  att ac k  o f  A n g l e  + 4 °  CP  Figure 2-8. Pressure distribution on an airfoil & CP changes with an angle of attack. Figure 4-7.  Pressure distribution on an airfoil and CP changes  with AOA. 