text: can be assumed to take place with negligible 
  changes in velocity. The second mode consists 
  of a pitching oscillation during which the air- 
  plane is being restored to equilibrium by the 
  static stability and the amplitude of oscillation 
  decreased by pitch damping. The typical mo- 
  tion 
  is of relatively high frequency with a 
  period of oscillation on the order of 6.5 to 5 
  seconds. 
  
  control system is greatly magnified. 
  In addi- 
  tion, response lag of the controls may add to 
  the problem of attempting to forceably damp 
  the oscillation. 
  In this case, should an oscilla- 
  tion appear, the best rule is to release the con- 
  trols as the airplane stick-free will demonstrate 
  the necessary damping, Even an attempt to 
  fix the controls when the airplane is oscillating 
  may result in a small unstable input into the 
  control system which can reinforce the oscilla- 
  tion to produce failing 
  flight loads. Because 
  of the very short period of the oscillation, the 
  amplitude of an unstable oscillation can reach 
  dangerous proportions in an extremely short 
  period of time. 
  
  The third mode occurs in the elevator free case 
  
  and is usually a very short period oscillation. 
  The motion is essentially one of the elevator 
  flapping about the hinge line and, in most 
  cases, the oscillation has very heavy damping. 
  A typical flapping mode may have a period of 
  0.3 to 1.5 seconds and a time to damp to half- 
  amplitude of approximately 0.1 second. 