MODERN GLIDERS ARE AMONG the most graceful and aerodynamically efficient of all aircraft. Unpowered but with a large wingspan (up to about 82 ft, or 25 m), gliders use currents of hot, rising air (thermals) to stay aloft, and a rudder, elevators, and ailerons for control. Modern gliders have achieved flights of more than 900 miles (1,450 km) and altitudes above 49,000 ft (15,000 m). Hang-gliders consist of a simple frame across which rigid or flexible material is stretched to form the wings. The pilot is suspended below the wings in a harness or body bag and, gripping a triangular
A-frame, steers by shifting weight from side to side. Like gliders, hang-gliders rely on thermals for lift. Ultralights are basically powered hang-gliders. A small engine and an open fiberglass car (trike),
which can hold a crew of two, are suspended beneath a stronger version of a hang-glider frame; the frame may have rigid or flexible wings. Ultralight pilots, like hang-glider pilots, steer by shifting their weight against an A-frame. Ultralights can reach speeds of up to 100 mph (160 kph).
