text: greatly to improving 
  the helicopter. 
  
  the flying qualities of 
  
  hovering and large trim moments upon entering 
  autorotation will 
  the stabilizer size to 
  a relatively small surface. Usually the hori- 
  zontal stabilizer is used only to give the fuse- 
  lage the desired moment characteristics. 
  
  instability characteristic is 
  is 
  the helicopter 
  particularly 
  in 
  expected to be used for instrument flight 
  all-weather operations. 
  In fact, a seriously 
  divergent phugoid mode would make instru- 
  ment flight impractical. For this reason, the 
  flying qualities specification requires that 
  helicopters with 
  instrument capability 
  exhibit varying degrees of stability or insta- 
  bility depending on the period of the oscilla- 
  tion. Long period oscillations (over 20 sec- 
  onds) must not double in amplitude in less 
  than 15 seconds whereas short period oscil- 
  lations (under 10 seconds) must damp to half 
  amplitude in two cycles. 
  
  This dynamic 
  
  important  if 
  
  The angle of attack stability of a tandem- 
  rotor helicopter is adversely affected by the 
  downwash from 
  the forward rotor reducing 
  the angle of attack and thrust of the rear 
  rotor. 
  This reduction of thrust behind the 
  cg. causes the helicopter to pitch up to a 
  higher angle of attack, thereby adding to the 
  angle of attack instability. 