text: deflection when subject to load, the tendency 
  may be to lower 
  to static 
  stability and reduce the damping contribution. 
  Thus, the problem of adequate stability of the 
  various airplane motions is aggravated. 
  
  the contribution 
  
  The static directional stability of an air- 
  plane decreases with Mach number in super- 
  sonic flight. The influence of the fuselage and 
  lift curve slope 
  the decrease in vertical tail 
  bring about this condition. 
  
  the airplane 
  generally deteriorates with Mach number in 
  supersonic flight. Since a large part of the 
  damping depends on the 
  tail surfaces, the 
  decrease in lift curve slope with Mach number 
  will account in part for the decrease in damp 
  ing. Of course, all principal motions of the 
  aircraft must have satisfactory damping and 
  if the damping is not available aerodynami- 
  to 
  it must be provided synthetically 
  cally 
  obtain satisfactory flying qualities. For many 
  high speed configurations the pitch and yaw 
  systems, etc., 
  stabilization 
  dampers, flight 
  are basic necessities rather than luxuries. 
  
  The dynamic stability of 
  
  The pilot may purposely induce various 
  motions to the airplane by the action of the 
  controls. 
  In additron, certain undesirable 
  motions may occur due to inadvertent action 
  on the controls. The most important con- 
  the short period longitu- 
  dition exists with 
  dinal motion of 
  the airplane where pilot- 
  control system response lag can produce an 
  unstable oscillation. 
  The coupling possible 
  system-airplane combi- 
  the pilot-control 
  in 
  nation is most certainly capable of producing 
  damaging flight 
  loads and loss of control of 
  the airplane. 