text: is evidenced by the develop- 
  ment of restoring moments, the c.g. must be 
  forward of the a.c. for the wing to contribute 
  to positive static longitudinal stability. 
  As 
  shown in figure 4.6, a change in lift aft of the 
  c,g. produces a stable restoring moment de- 
  pendent npon the lever arm between the a.c. 
  and c.g. 
  In this case, the wing contribution 
  would be stable and the curve of CM versus CL 
  for the wing alone would have a negative slope. 
  If the c.g. were located at the a.c., C, would 
  
  led to this conclusion. 
  
  When the aircraft is operating in subsonic 
  flight, the a.c. of the wing remains fixed at the 
  25 percent chord station. When the aircraft 
  is flown in supersonic flight, 
  the ax. of the 
  wing will approach the 50 percent chord sta- 
  tion. Such a large variation 
  in the location 
  of the a.c. can produce large changes in the 
  wing contribution and greatly alter the air- 
  The second chart 
  plane longitudinal stability. 
  
  252 