text: &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- 
  the pilot would be supply- 
  plane instability; 
  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 
  the major aircraft degrees of 
  stability 
  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 
  in a 
  steeper trough. A greater force is required to 
  the ball to the same lateral dis- 
  “control” 
  placement when 
  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. 
  
  to 