text: Graph A of figure 4.5 illustrates the variation 
  of pitching moment coefficient, C,, with 
  lift 
  coefficient, C,, for an airplane with positive 
  Evidence of 
  static 
  static stability is shown by the tendency to re- 
  ,t,urn to equilibrium-or 
  upon dis- 
  .,placement. The airplane described by graph A 
  is in trim or equilibrium when C,=O and, if the 
  ‘airplane is disturbed to some different C,, the 
  pitching moment change tends to return the 
  If the airplane 
  aircraft to the.point of trim. 
  ‘were disturbed to some higher C, (point Y), a 
  negative or nose-down pitching moment is de- 
  veloped which tends to decrease angle of attack 
  back to the trim point. 
  If the airplane were 
  disturbed to some lower C,, (point X), a posi- 
  tive, or nose-up pitching moment is developed 
  tends to increase the angle of attack 
  which 
  back to the trim point. Thus, positive static 
  longitudinal stability is indicated by a negative 
  slope of C, versus C,, i.e., positive stability is 
  evidenced by a decrease in CM with an increase 
  in C,. 
  
  longitudinal  stability. 