Unpowered aircraft can remain airborne for a significant period of time without onboard propulsion. They can be classified as fixed-wing
gliders, lighter-than-air
balloons and tethered
kites. This requires a trajectory that is not merely a vertical descent such as a
parachute. In the case of kites, lift is obtained by tethering to a fixed or moving object, perhaps another kite, to obain a flow of wind over the lifting surfaces. In the case of balloons, lift is obtained through inherent
buoyancy and the balloon may or may not be tethered. Free balloon flight has little directional control. Gliding aircraft include
sailplanes,
hang gliders, and
paragliders that have full directional control in free flight.