The Code of Federal Regulations (14 CFR part 1, section 1.1) states, “glider means a heavier-than-air aircraft, that is supported in flight by the dynamic reaction of the air against its lifting surfaces and whose free flight does not depend principally on an engine.”
Gliders are principally used for the air sports of gliding, hang gliding and paragliding. However some spacecraft have been designed to descend as gliders and in the past military gliders have been used in warfare. Some simple and familiar types of glider are toys such as paper planes and balsa wood gliders.
In flight, a glider has three forces acting on it as compared to the four forces that act on a powered aircraft. Both types of aircraft are subjected to the forces of lift, drag, and weight. The powered aircraft has an engine that generates thrust, while the glider has no thrust.
Glider, nonpowered heavier-than-air craft capable of sustained flight. Though many men contributed to the development of the glider, the most famous pioneer was Otto Lilienthal (1848–96) of Germany, who, with his brother Gustav, began experiments in 1867 on the buoyancy and resistance of air.
Gliders and sailplanes are names often used synonymously to refer to aircraft designed to fly without an engine. However, a sailplane is typically regarded as a high-performance glider that can soar and remain aloft almost indefinitely by relying solely on atmospheric updrafts.
In its simplest form, a glider is an unpowered aircraft, an airplane without a motor. While many of the same design, aerodynamic and piloting factors that apply to powered airplanes also apply to gliders, that lack of a motor changes a lot about how gliders work.