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. 
It should be noted that the symmetrical air- 
foil at zero lift has no pitching moment about 
the aerodynamic center because the upper and 
47