Classification by propulsion (Unpowered)

Gliders

Heavier-than-air unpowered aircraft such as gliders (i.e. sailplanes), hang gliders and paragliders and other gliders do not usually employ propulsion once airborne. Take-off may be by launching forwards and downwards from a high location, or by pulling into the air on a tow-line, by a ground-based winch or vehicle, or by a powered "tug" aircraft. For a glider to maintain its forward air speed and lift, it must descend in relation to the air (but not necessarily in relation to the ground). Some gliders can 'soar'- gain height from updrafts such as thermal currents. The first practical, controllable example was designed and built by the British scientist and pioneer George Cayley who many recognise as the first aeronautical engineer.^[2]

Balloons

Balloons drift with the wind, though normally the pilot can control the altitude either by heating the air or by releasing ballast, giving some directional control (since the wind direction changes with altitude). A wing-shaped hybrid balloon can glide directionally when rising or falling; but a spherically-shaped balloon does not have such directional control.

Kites

Kites are aircraft^[3] that are tethered to the ground or other object (fixed or mobile) that maintains tension in the tether or kite line; and rely on virtual or real wind blowing over and under them to generate lift and drag. Kytoons are balloon kites that are shaped and tethered to obtain kiting deflections, and can be lighter-than-air, neutrally buoyant, or heavier-than air.