text: Of all the modes of longitudinal dynamic 
  the second mode or porpoising oscil- 
  stability, 
  is of greatest importance. The por- 
  lation 
  the possibility of 
  poising oscillation has 
  loads and can be adversely 
  damaging flight 
  affected by pilot response lag. 
  It should be 
  remembered that when stick-free the airplane 
  will demonstrate the necessary damping. 
  
  The problems of dynamic stability are acute 
  under certain conditions of flight. Low static 
  stability generally increases the period (de- 
  creases frequency) of the short period oscil- 
  lations and increases the time to damp to half- 
  amplitude. High altitude-and 
  consequently 
  the aerodynamic damp- 
  low density-reduces 
  ing. Also, high Mach numbers of supersonic 
  flight produce a decay of aerodynamic damping. 
  The second mode has relatively short periods 
  the normal pilot 
  
  that correspond closely with 
  response lag time, e.g., 1 or 2 seconds or less. 
  There is the possibility 
  to 
  forceably damp an oscillation may actually re- 
  inforce the oscillation and produce instability. 
  true in the case of powered 
  This is particularly 
  controls where a small input energy into the 