While a stable glider without
any control input may have straight line stability in the horizontal plane, hydrostatic restoring forces and moments are
prone to destabilize the glider in the vertical plane. The stability of a glider is controlled by a moving internal mass or.
Alternatively, the dynamic stability of a glider may be
controlled by external fixed wings and a vertical rudder. The
Routh stability criteria for dynamic stability in heave and pitch
are given in Eqs. (4) and (5)