text: As in the airplane, several oscillatory modes 
  of motion are characteristic of the dynamic 
  stability of a helicopter. The phugoid is the 
  the helicopter. The 
  most troublesome for 
  phugoid mode is unstable in the majority of 
  helicopters which operate without 
  the assist- 
  ance of artificial stabilization devices. The 
  dynamic instability of the helicopter is given 
  evidence by the flying qualities specification for 
  helicopters. 
  These specifications essentially 
  limit the rate of divergence of the dynamic oscil- 
  lations for the ordinary helicopter. Although 
  this dynamic instability can be controlled, it 
  requires constant attention by the pilot 
  and 
  fatigue. The elimination of 
  results in pilot 
  contribute 
  the dynamic  instability would 