NAVWEPS DD-EOT-80  STABMTY AND CONTROL  The first four items can be effected,only  during  design or by design changes. Some roll per-  formance restriction is inevitable since all of  the desirable characteristics are difficult to  obtain without serious compromise elsewhere  in the airplane design. The typical high  speed airplane will have some sort of roll pet-  formance limitation provided by flight restric-  tions or automatic control devices to prevent  reaching some critical condition from which  recovery is impossible. Any roll restriction  provided an airplane must be regarded as a  principal flight operating limitation since the  more severe motions can cause complete loss  of control and structural failure.  design features  which result in a large positive  dihedral effect are high sweepback, high wing  position, or large, high vertical tail,  When  the inertia axis is inclined below the aero-  dynamic axis at low or negative angles of  attack, the roll induced inertia couple results  in proverse  yaw.  Depending on the flight condition where the  roll coupling problem exists, four basic types  of airplane behavior are possible:  (1) Coupled motion stable but unacceptabk.  In this case the motion is stable but proves  unacceptable  because  of poor damping of the  motion.  Poor damping would  make it  dificult to track a target or the initial am-  plitudes of the motion may be great enough  to cause structural failure of loss of control.  (2) Coupled motion stable and acceptable.  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.  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, this 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.  (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.  “Inertia  coupling”  problems are relatively  new to fixed-wing aircraft but a similar effect  in the helicopter rotor has resulted in some  of its most important characteristics. This  aerodynamic-dynamic coupling effect is so im-  portant that it must be considered in discussing  both stability and control.  The helicopter  derives both longitudinal and lateral control  by tilting the main rotor and thus producing  a pltchmg or rolling moment as indicated in  figure 4.35.  The magnitude of the rotor thrust  the angle of tilt, and the height of the rotor  hub above the c.g. determine the control  moment produced. It should be noted that  low control effectiveness  would result when  the rotor thrust is low.  Some helicopters  There are available various means to cope  with the problem of roll coupling.  The fol-  lowing items can be applied to control the  problem of roll coupling:  (ZZ) Increase directional stability.  (b) Reduce dihedral effect.  (c) M’  h  1‘  mnmze t e mc mation of the inertia  axis at normal flight conditions.  (d) Reduce  undesirable  aerodynamic  coupling.  (e) Limit  roll rate, roll duration, and  angle of attack or load factor for performing  rolling maneuvers.  319  HELICOPTER STABILITY AND CONTROL 