text: For the conventional subsonic airplane, the 
  second mode stick-fixed 
  is characterized by 
  heavy damping with a time to damp to half 
  amplitude of approximately 0.5 seconds. IJsu- 
  ally, if the airplane has static stability stick- 
  fixed, the pitch damping contributed by the 
  horizontal tail will assume sufficient dynamic 
  stability for the short period oscillation. How- 
  ever, the second mode stick-free has the possi- 
  bility of weak damping or unstable oscilla- 
  tions. This is the case where static stability 
  does not automatically 
  imply adequate dy- 
  namic stability. The second mode stick-free is 
  essentially a coupling of motion between the 
  airplane short period pitching motion and ele- 
  vator in rotation about the hinge line. Ex- 
  treme care must be taken in the design of the 
  control surfaces to ensure dynamic stability for 
  this mode. The elevators must be statically 
  balanced about the hinge line and aerodynamic 
  balance must be within certain limits. Control 
  system friction must be minimized as it con- 
  tributes to the oscillatory tendency. 
  If insta- 
  bility were to exist in the second mode, “por- 
  poising” of the airplane would result with 
  possibility of structural damage. An oscilla- 
  tion at high dynamic pressures with 
  large 
  changes in angle of attack could produce severe 
  flight loads. 