NAVWEPS 00-801-80  STABILITY AND CONTROL  greatly to improving the flying qualities of  the helicopter.  hovering and large trim moments upon entering  autorotation will limit 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.  This dynamic instability characteristic is  particularly important if the helicopter is  expected to be used for instrument flight in  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  an 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.  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.  As in the airplane, several oscillatory modes  of motion are characteristic of the dynamic  stability of a helicopter. The phugoid is the  most troublesome for the helicopter.  The  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  results in pilot fatigue.  The elimination of  the dynamic instability  would  contribute  The only immediate solution for the dynamic  instability is an attitude stabilization system  which  is essentially an autopilot.  Other  solutions to the dynamic instability problem  involve mechanical, aerodynamic, or elec-  tronic control feedback of pitch attitude,  pitch velocity, normal acceleration, or angle  of attack.  The improvement of the heli-  copter ’ s stability is mandatory to fully utilize  its unique capability.  As more of the heli-  copter problems are analyzed and studied, the  flying qualities of helicopters wiI1 improve  and be comparable to the fixed wing aircraft.  324 