text: HELICOPTER STABILITY AND CONTROL 
  
  (2) Coupled motion stable and acceptable. 
  
  In discussing many of the problems of sta- 
  bility and control that occur in high speed 
  airplanes, one might be prone to believe that 
  the slow flying helicopter does not have any 
  such problems. Unfortunately, 
  is not 
  the case. Flying qualities that would be con- 
  sidered totally unsatisfactory by fixed-wing 
  standards ate normal for helicopters. Heli- 
  copter pilots are living evidence that an un- 
  stable aircraft ca. k‘.: controlled. Also, they 
  are evidence ~a. control without stability 
  requires constant attention and results in con- 
  siderable pilot fatigue. 
  
  The behavior of the airplane is stable and 
  adequately damped to allow acceptable 
  target tracking. The amplitudes of motion 
  are too slight to result in structural failure 
  or loss of control. 
  
  (3) Coupled motion divergent and unacceptable. 
  
  The rate of divergence is too rapid for the 
  pilot to recognize the condition and recover 
  prior’ to structural failure or complete loss 
  of control. 
  
  (4) Coupled. motion divergent but acceptable. 
  
  For such a condition the rate of divergence 
  is quite slow and considerable roll displace- 
  ment is necessary to produce a critical ampli- 
  tude. The condition can be recognized 
  easily in time to take corrective action. 
  There are available various means to cope 
  the problem of roll coupling. The fol- 
  the 
  items can be applied to control 