NAVWEPS DD-8OT-80  STABILITY AND CONTROL  &ease the angle of attack, the aircraft would  be trimmed at the higher angle of attack by  a push force to keep the aircraft from con-  tinuing in the displacement direction.  Such  control force reversal would evidence the aii-  plane instability; the pilot would be supply-  ing the stability by his attempt to maintain  the equilibrium.  An unstable aircraft can be  flown if the instability is slight with a low  rate of divergence. Quick reactions coupled  with effective controls can allow the pilot to  cope with some degree of static instability.  Since such flight would require constant at-  tention by the pilot, slight instability can be  tolerated only in airships, helicopters, and  certain minor motions of the airplane. How-  ever, the airplane in high speed flight will  react rapidly to any disturbances and any in-  stability would create unsafe  conditions. Thus,  it is necessary  to provide some positive static  stability  to the major aircraft degrees of  freedom.  and maintain it in the displaced position, a  force must be supplied in rhe direction of  displacement co balance the inherent tendency  to return to equilibrium.  This same stable  tendency in an aircraft resists displacement  from trim by pilot effort on the controls or  atmospheric disturbances.  The effect of increased stability on con-  trollabilicy  is illustrated by rhe ball in a  steeper  trough.  A greater force is required to  “control”  the ball to the same lateral dis-  placement when the stability is increased.  In this manner, a large degree  of stability tends  to make the aircraft less controllable.  It is  necessary to achieve the proper balance be-  tween stability and tontrollability  during rhe  design of an aircraft because  the ~ppcr limits  of stability arc set by the lower 1imitJ of controlla-  bility.  The effect of reduced stability on .controlla-  bility is illustrated by the ball on a flat surface.  When neutral static stability exists, the ball  may be displaced from equilibrium and there  is no stable tendency to return. A new point  of equilibrium is obtained and no force is  required to maintain the displacement. As  the static stability approaches  zero, controlla-  bility increases  to infinity and the only resist-  ance to displacement is a resistance to the  motion of displacement-damping.  For this  reason, the lower Limits of stability may be Set  by the upper limits of controllability. If the  stability of the aircraft is too low, control  deflections may create exaggerated displace-  ments of the aircraft.  In order to visualize the forces and moments  on the aircraft; it is necessary  to establish a  set of mutually perpendicular reference axes  originating at the center of gravity.  Figure  4.4 illustrates a conventional right hand axis  system. The longitudinal or X axis is located  in a plane of symmetry and is given a positive  direction pointing into the wind.  A moment  about this axis is a rolling moment, L, and the  positive direction for a positive rolling moment  utilizes the right hand rule.  The vertical or 2  axis also is in a plane of symmetry and is estab-  lished positive downward.  A moment about  the vertical axis is a yawing moment, N, and a  positive yawing moment would yaw the air-  craft co the right (right  hand rule).  The  lateral or Y axis is perpendicular to the plane  of symmetry and is given a positive direction  out the right side of the aircraft.  A moment  about the lateral axis is a pitching moment, M,  and a positive pitching moment is in the nose-  up dlrection.  The effect of static instability. on controlla-  bility is illustrated by the ball on a hill.  If  the ball is displaced from equilibrium at the  top of the hill, the initial tendency is for the  ball td continue in the displaced direction.  In order to “control”~the  ball to some lateral  displacement, a force must be applied oppo&  to the direction of displacement. This effect  would be appreciated during flight of an un-  stable aircraft by an unstable “feel”  of the air-  craft. If the controls were deflected co in-  AIRPLANE REFERENCE AXES  249 