Gliders are aircraft that are supported in flight through the dynamic reaction of air against their lifting surfaces without an engine. Some gliders have small engines to extend flight or for launching. The Wright Brothers spent years experimenting with gliders to develop efficient airfoils and flight control before attempting powered flight. Modern gliders include sailplanes for recreation, hang gliders that are foot-launched, and almost ready-to-fly models that require minor assembly. Gliders find lift through thermals or slope lift and pilots circle to remain in rising air. Winches are commonly used to launch larger sailplanes a few hundred feet and electric motors now assist some gliders. Efficiency is measured by glide ratio and half the drag