text: the cambered airfoil of 
  Next, consider 
  figure 1.21 at zero lift. 
  To produce zero lift, 
  the upper and lower surface lifts must be equal. 
  One difference noted from the symmetrical air- 
  foil is that the upper and lower surface lifts are 
  not opposite one another. While no net lift 
  the couple produced by 
  exists on the airfoil, 
  the upper and lower surface lifts creates a nose 
  down moment. As the angle of attack is in- 
  creased, the upper surface lift 
  increases while 
  lower surface lift decreases. While a 
  the 
  change in lift has taken place, no change in 
  moment takes place about the point where 
  the lift change occurs. Since the moment 
  about the aerodynamic center is the product 
  of a force (lift at the c.P.) and a lever arm 
  (distance from c.9. to a.~.), an increase in lift 
  moves the center of pressure toward the aero- 
  dynamic center. 
  
  DEVELOPMENT OF AERODYNAMIC 
  
  PITCHING MOMENTS 
  
  The distribution of pressure over a surface 
  is the ,source of the aerodynamic moments as 
  well as the aerodynamic forces. A typical 
  example of this fact is the pressure distribution 
  acting on the cambered airfoil of figure 1.21. 
  The upper surface has pressures distributed 
  which produce the upper surface lift; the lower 
  surface has pressures distributed which pro- 
  duce the lower surface lift. Of course, the  It should be noted that the symmetrical air- 
  foil at zero lift has no pitching moment about 
  the aerodynamic center because the upper and 