Neutral dynamic stability Positive dynamic stability Negative dynamic stability Center of gravity Center of lift Negative AOA (Exaggerated in  this illustration) Figure 3-21.  Three types of dynamic stability. Figure 3-22.  Use of the horizontal stabilizer angle to offset the natural tendency of a glider to nose down. Another factor that is critical to the longitudinal stability of a  glider is its loading in relation to the CG. The CG of the glider  is the point at which the total force of gravity is considered  to act. When the glider is improperly loaded so it exceeds  the aft CG limit, it loses longitudinal stability. As airspeed  decreases, the nose of a glider rises. To recover, control inputs  must be applied to force the nose down to return to a level  flight attitude. It is possible that the glider could be loaded  so far aft of the approved limits that control inputs are not  sufficient to stop the nose from pitching up. If this were the  case, the glider could enter a spin from which recovery would  be impossible. Loading a glider with the CG too far forward  is also hazardous. In extreme cases, the glider may not have  enough pitch control to hold the nose up during an approach  to a landing. For these reasons, it is important to ensure that  the glider is within weight and balance limits prior to each  flight. Proper loading of a glider and the importance of CG  is discussed further in Chapter 5, Performance Limitations. Another factor that can affect the ability to control the glider  is flutter. Flutter occurs when rapid vibrations are induced  through the control surfaces while the glider is traveling at  high speeds. Looseness in the control surfaces can result in  flutter while flying near maximum speed. Another factor  that can reduce the airspeed at which flutter can occur is a  disturbance to the balance of the control surfaces. If vibrations  are felt in the control surfaces, reduce the airspeed. Flutter 3-11