A powered aircraft uses an engine to move forward and the air flowing over its wings generates lift to sustain flight.  A sailplane trades height (potential energy) for forward motion (kinetic energy) to achieve the same result.  At altitude, the weight of the sailplane is being pulled towards the earth by gravity.  Therefore, the sailplane must always descend at a minimum rate.  The weight may be considered as a force opposite of the Lift, L, called WL plus a force opposite to the Drag, D, called WT which may be combined to give the Resultant Force=Weight=Mg  The force propelling the glider forward is the force WHence the Equivalent.  Thrust=WT+Mg.sinø

 

Glide ratio - The most modern Sailplanes can reach glide ratios of 60 to 1 in still air such that if a sailplane is 1000m above ground it can glide...60Km before it will touch the ground!