text: if all 
  moments in pitch, roll, and yaw are equal to 
  zero. The establishment of equilibrium at 
  various conditions of flight is the function of 
  the controls and may be accomplished by 
  pilot effort, trim 
  tabs, or bias of a surface 
  actuator. 
  
  An aircraft  is said to be trimmed 
  
  the 
  ability of the aircraft to respond to control 
  surface displacement and achieve the desired 
  condition of flight. Adequate controllability 
  takeoff and 
  must be available 
  landing and accomplish the various maneuvers 
  in flight. An important contradiction exists 
  between stability and controllability 
  since 
  adequate controllability does not necessarily 
  exist with adequate stability. 
  In fact, a high 
  degree of stability tends to reduce the controlla- 
  bility of the aircraft. The general relation- 
  ship between static stability and controlla- 
  bility 
  
  The term “controllability”  refers to 
  
  Chart F of figure 4.2 illustrates the mode of 
  a divergent oscillation. This motion is stat- 
  tends to return to the 
  ically stable since it 
  equilibrium position. However, each subse- 
  quent return to equilibrium is with increasing. 
  velocity such that amplitude continues to 
  insta- 
  increase with 
  bility exists. The divergent oscillation occurs 
  when energy is supplied to the motion rather 
  than dissipated by positive damping. The 
  most outstanding illustration of the divergent 
  the short period pitch- 
  oscillation occurs with 
  If a pilot un- 
  ing oscillation of an aircraft. 
  knowingly supplies control functions which 
  are near the natural frequency of the airplane 
  in pitch, energy is added to the system, nega- 
  induced 
  tive damping exists, and the “pilot 
  oscillation” 
  
  is illustrated by figure 4.3. 