NAVWEPS oo-s01-80  STABILITY AND CONTROL  deflection when subject to load, the tendency  may be to lower the contribution to static  stability and reduce the damping contribution.  Thus, the problem of adequate stability of the  various airplane motions is aggravated.  Mach number. As a corollary of this increase  in stability is a decrease  in controllability and  an increase  in trim drag.  The static directional stability of an air-  plane decreases  with Mach number in super-  sonic flight.  The influence of the fuselage and  the decrease  in vertical tail lift curve slope  bring about this condition.  The dynamic stability  of  the airplane  generally deteriorates with Mach number in  supersonic flight.  Since a large part of the  damping depends on the tail surfaces, the  decrease  in lift curve slope with Mach number  will account in part for the decrease  in damp  ing.  Of course, all principal motions of the  aircraft must have satisfactory damping and  if the damping is not available aerodynami-  cally it must be provided synthetically to  obtain satisfactory flying qualities. For many  high speed configurations the pitch and yaw  dampers, flight  stabilization systems, etc.,  are basic necessities  rather than luxuries.  The pilot  may purposely induce various  motions to the airplane by the action of the  controls.  In additron, certain undesirable  motions may occur due to inadvertent action  on the controls.  The most important con-  dition exists with the short period longitu-  dinal motion of the airplane where pilot-  control system response lag can produce an  unstable oscillation.  The coupling possible  in the pilot-control  system-airplane combi-  nation is most certainly capable of producing  damaging flight loads and loss of control of  the airplane.  Generally, flight at high Mach number will  cake place at high altitude hence the effect of  high altitude must be separated for study.  All of the basic aerodynamic damping is due  to moments created by pitching, rolling, or  yawing motion of the aircraft.  These moments  are derived from the changes in angles of  attack on the tail  surfaces with  angular  rotation (see fig. 4.15).  The very high true  airspeeds common to high  altitude flight  reduce the angle of attack changes and reduce  the aerodynamic damping.  In fact, the aero-  dynamic damping is proportional to &  similar to the proportion of true airspeed to  equivalent airspeed. Thus, at the altitude of  4O,C00 ft., the aerodynamic damping would  be reduced to one-half the sea level value and  at the altitude of 100,000 ft. the aerodynamic  damping would be reduced to one-tenth the  sea level value.  When the normal human response lag and  control system lag are coupled with the air-  plane motion, inadvertent control reactions  by the pilot may furnish a negative damping  to the oscillatory motion and dynamic in-  stability exists.  Since the short period motion  is of relatively high frequency, the amplitude  of the pitching oscillation can reach dangerous  proportions in an unbelievably short time.  When the pilot  induced oscillation is en-  countered, the most effective solution is an  immediate release of the controls. Any at-  tempt to forcibly damp the oscillation simply  continues the excitation and amplifies the  oscillation.  Freeing the controls removes  the unstable (but inadvertent) excitation and  allows the airplane to recover by virtue of  its inherent dynamic stability.  The pilot induced oscillation is most likely  under certain conditions,  Most obvious is the  case of the pilot unfamiliar with the “feel”  of the airplane and likely to overcontrol or  have excessive  response  lag.  High speed  flight  at low. altitude (high 4) is most likely to  provide low stick-force gradients and periods  High dynamic pressures (high $I can be  common to flight at high Mach number and  adverse  aeroelastic effects  may be encountered.  If the aircraft surfaces,  encounter significant  314  PILOT INDUCED OSCILLATIONS 