NAVWEPS D&801-80  BASIC AERODYNAMICS  CHANGE IN LIFT  DUE TO UPGUST  CHANGE IN LIFT  DUE TO UPGUST  C:G. 1  O.C.  t  (UNSTABLE)  C:G.  t  LIFT  1  WEIGHT  Figure 1.23. Application to Stability  aerodynamic center. This very necessary  fea-  ture can be visualized from the illustrations of  figure 1.23.  AC. as a limit but as stall occurs, the drop in  suction near the leading’ edge cause the c.p. to  move aft.  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  center will be unchanged and will remain at  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 problem assume that the airplane is a  tailless or flying wing type.  In order for this  type airplane to have longitudinal stability,  the center of gravity must be ahead of the  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  to equilibrium is a very necessary  feature in  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.  51 