Unpowered aircraft

A glider, (also known as a sailplane) is a airplane with no power source. It is towed into the air and then glides back to the ground.

Gliders must be unusually efficient to stay in the air long, because they have no engine. The designs try to improve the lift to drag ratio. Besides a very narrow and smooth fuselage, gliders have long, slender wings to reduce induced drag. Basically, this trick increases the amount of lifting wing, minimizing the effect of the wingtips. Wingtips cause vortices that have much drag.

New materials and computer-aided design have increased performance. Gliders of the World-War II era had a glide ratio of 5 to 1. Modern high-performance sailplanes can reach 50 to 1.

Modern sailplanes are often so light that they cannot fly smoothly. Some include a bladder of water for extra weight. The bladder is dropped just before landing to reduce stresses on touchdown.

Many sailplanes now include a small engine to help them take off. The "dead weight" of the engine is counterbalanced by modern materials and design. The engine allows the craft to climb to altitude more quickly than a tow plane. Also, the aircraft is safer, because the pilot can avoid storms, and can go to an airstrip to land.

Sailplanes can stay in the air indefinitely, using naturally upward-flowing air masses. Sailplanes have a special insrument called a variometer to locate small changes in the sink rate. Variometers often have an audible alarm for positive changes in altitude.

The most common techniques for finding sources of lift are thermalling and ridge running.

In thermalling, the pilot searches for bubbles of air that are moving up because they were heated by sunlit earth. Thermals are often found over freshly ploughed fields and asphalt roads.

Thermalling is most effective in mid-latitudes during the summer. Other areas have air temperatures that are more constant over different altitudes. During the winter the ground is too cold.

When a pilot encounters a thermal, the pilot banks sharply to keep the plane turning in a small circle within the thermal. This way the sailplane can ride the thermal upward until it cools off and stops rising.

When ridge running, a pilot looks for air that is mechanically lifted as it flows up the sides of hills or other landscape (including buildings in some cases). Ridge running works in any clime or weather, but can only be used in certain locations. Most sailplane distance records were set by ridge-running long mountain ranges, and using thermalling to cross gaps in the ridge lines.

Rotor-riding is a variation of ridge-running. In some areas, notably the eastern Sierra Nevadas of the U.S., a rolling cylinder of air forms against a mountain range. In some cases, this rotor induces counter-rotating rotors above the main rotor. Most sailplane altitude records were set by riding rotors.

Gliders were the first heavier-than-air aircraft to be built and flown, dating to the late 1880's. Many attempts were made to fit power sources to gliders, but none were successful until the Wright Flier which was an entirely new machine built to this purpose.

Gliders were used during WWII to support paratroop operations. The troops landing by glider were referred to as airlanding as opposed to paratroops. A typical cargo plane could carry only 8 to 10 paratroops, but that same plane could tow a glider with 20 men in it. Furthermore the glider could be released at some distance from the actual target. This made it more difficult to guess the landing place. Larger gliders were used to land heavy equipment like anti-tank guns and jeeps. These improved the power of the otherwise lightly-armed paratroops.

Today gliders are used solely for recreation and sport (see gliding).