Gliders or sail planes are aircraft designed for unpowered flight.Most gliders are intended for use in the sport of gliding and so have high aerodymamic efficiency.Lift to drag ratios may exceed 70 to 1.After launch,the energy for sustained gliding flight is obtained through the skillful exploitation rising air in the atmosphere.Glider flights of thousands of kilometers at average speeds over 200km/hr have been achieved.The glider is most commonly launched by a tow plane or by a winch.Some gliders,called motor gliders are equipped with engines and some are capable of self-launching.Military gliders have been used in war to deliver assault troops,and specilized gliders have been used in atmospheric and aerodynamics research