text: Many devices can be added to a control 
  force 
  system to modify or tailor 
  stability to desired levels. If a spring is added 
  to the control system as shown in figure 4.16, 
  it will 
  tend to center the stick and provide a 
  force increment depending on stick displace- 
  ment. When the control system has a fixed 
  gearing between stick position and surface 
  deflection, the centering spring will provide a 
  contribution to stick force stability according 
  to stick 
  to stick position. The contribution 
  force stability will be largest at low 
  flight 
  large control deflec- 
  speeds where relatively 
  tions are required. The contribution will be 
  smallest at high airspeed because of the smaller 
  control deflections required. Thus, .the stick 
  increase the airspeed 
  centering bungee will 
  and maneuvering stick force stability but the 
  contribution decreases at high airspeeds. A 
  variation of this device would be a spring 
  stiffness which would be controlled to vary 
  In this case, the 
  with dynamic pressure, 4. 
  force 
  contribution of 
  stability would, not diminish with. speed. 
  
  the stick 
  
  A “downspring” 