Glider Aircraft
Sailplanes, hang gliders and paragliders can gain some initial energy from a launch process, but can then gain additional potential energy from rising air such as thermals and ridge lift. The launch may be by pulling the aircraft into the air with a tow-line, with a ground-based winch or vehicle, or with a powered "tug" aircraft. For foot-launched aircraft, there is also the option of merely stepping off a high location.
For glider aircraft to generate lift, they must maintain sufficient forward air speed. This is achieved by a gradual descent, though if the air is rising faster than the aircraft is descending, it will also ascend. Today, the majority of the use of all types of glider aircraft is recreational.
In the past unpowered aircraft have been used for military applications.
Read more about this topic: Unpowered Aircraft