text: AC. as a limit but as stall occurs, the drop in 
  suction near the leading’ edge cause the c.p. to 
  move aft. 
  
  If the two symmetrical airfoils are subject 
  to an upgust, an increase in lift will take place 
  at the 4.c. If the c.g. is ahead of the ax., the 
  change in lift creates a nose down moment 
  about the c.g. which tends to return the air- 
  foil to the. equilibrium angle of attack. This 
  stable, “weathercocking” 
  tendency to return 
  is a very necessary feature in 
  to equilibrium 
  any airplane. 
  If the c.g. is aft of the a.~., the 
  change in lift due to the upgust takes place at 
  the AC. and creates a nose up moment about 
  the c.g. This nose up moment tends to displace 
  the airplane farther from the equilibrium and 
  is unstable-the airplane is similar to a ball 
  balanced on a peak. Hence, to have a stable 
  airplane, the c.g. must be located ahead of the 
  airplane rl.c. 
  
  Of course, if the airfoil has negative camber, 
  or a strongly reflexed trailing edge, the moment 
  about the aerodynamic center will be positive. 
  In this case, the location of the aerodynamic 
  remain at 
  center will be unchanged and will 
  the quarter-chord position. 
  
  The aerodynamic center is the point on the 
  chord where the coefficients of moment are 
  constant-the point where all changes in lift 
  take place. The aerodynamic center is an cx- 
  tremely important aerodynamic reference point 
  and the most direct application is to the longi- 
  tudinal stability of an airplane. To simplify 
  the airplane is a 
  the problem assume that 
  tailless or flying wing type. 
  In order for this 
  type airplane to have longitudinal stability, 
  the center of gravity must be ahead of the 