text: C%C. versus lift coefficient for several repre-. 
  sentative sections. The sign convention ap- 
  plied to moment coefficients is that the nose-up 
  moment is positive. 
  
  lower surface lifts act along the same vertical 
  line. An increase in.lift on the symmetrical 
  airfoil produces no change in this situation and 
  the center of pressure remains fixed at the aero- 
  dynamic center. 
  
  The NACA Ooog airfoil is a symmettical sec- 
  tion of 9 percent maximum thickness. Since 
  the mean line of this airfoil has no camber, 
  the coefhcient of moment about the aerody- 
  namic center is zero, i.e., the c.p. is at the ac. 
  The departure from zero cno.+ occurs only as the 
  airfoil approaches maximum lift and the stall 
  produces a moment change in the negative 
  (nose-down) direction. The NACA 4412 and 
  63,-412 sections have noticeable positive cam- 
  ber which cause relatively large moments about 
  the aerodynamic center. Notice that for each 
  sectionshowninfrgure 1.22, the c,,,.... isconstant 
  for all lift coefficients less than cl,-.  The location of the aerodynamic center of an 
  airfoil is not affected by camber, thickness, and 
  In fact, two-dimensional in- 
  angle of attack. 
  compressible airfoil 
  theory will predict the 
  aerodynamic center at the 25 percent chord point 
  for any airfoil regardless of camber, thickness, 
  and angle of attack. Actual airfoils, which 
  are subject to real fluid flow, may not have the 
  lift due to angle of .attack concentrated at the 
  exact 25 percent chord point. However, the 
  actual location of the aerodynamic center for 
  various sections is rarely forward of 23 percent 
  or aft of 27 percent chord point. 