Gliding flight is heavier-than-air flight without the use of thrust; the term volplaning also refers to this mode of flight in animals. It is employed by gliding animals and by aircraft such as gliders. This mode of flight involves flying a significant distance horizontally compared to its descent and therefore can be distinguished from a mostly straight downward descent like a round parachute. Although the human application of gliding flight usually refers to aircraft designed for this purpose, most powered aircraft are capable of gliding without engine power. As with sustained flight, gliding generally requires the application of an airfoil, such as the wings on aircraft or birds, or the gliding membrane of a gliding possum. However, gliding can be achieved with a flat (uncambered) wing, as with a simple paper plane, or even with card-throwing. However, some aircraft with lifting bodies and animals such as the flying snake can achieve gliding flight without any wings by creating a flattened surface underneath.
Glider aircraft
Most winged aircraft can glide to some extent, but there are several types of aircraft designed to glide:
Glider, also known as a sailplane Hang glider Paraglider Speed glider Ram-air parachute Rotor kite, if untethered, known as a rotary glider, or gyroglider. Military glider Paper aeroplane Radio-controlled glider Rocket glider Wingsuit The main human application is currently recreational, though during the Second World War military gliders were used for carrying troops and equipment into battle. The types of aircraft that are used for sport and recreation are classified as gliders (sailplanes), hang gliders and paragliders. These two latter types are often foot-launched. The design of all three types enables them to repeatedly climb using rising air and then to glide before finding the next source of lift. When done in gliders (sailplanes), the sport is known as gliding and sometimes as soaring. For foot-launched aircraft, it is known as hang gliding and paragliding. Radio-controlled gliders with fixed wings are also soared by enthusiasts. In addition to motor gliders, some powered aircraft are designed for routine glides during part of their flight; usually when landing after a period of a powered flight. These include:
Experimental aircraft such as the North American X-15, which glided back having used their fuel Spacecraft such as the Space Shuttles, SpaceShipOne and the Russian Buran Aircraft which are not designed for glide may forced to perform gliding flight in an emergency, such as all engine failure or fuel exhaustion. See list of airline flights that required gliding flight.
Gliding animals
Birds A number of animals have separately evolved gliding many times, without any single ancestor. Birds in particular use gliding flight to minimise their use of energy. Large birds are notably adept at gliding, including:
Albatross Condor Vulture Eagle Stork Frigatebird Like recreational aircraft, birds can alternate periods of gliding with periods of soaring in rising air, and so spend a considerable time airborne with a minimal expenditure of energy. The great frigatebird in particular is capable of continuous flights up to several weeks.
Mammals
To assist gliding, some mammals have evolved a structure called the patagium. This is a membranous structure found stretched between a range of body parts. It is most highly developed in bats. For similar reasons to birds, bats can glide efficiently. In bats, the skin forming the surface of the wing is an extension of the skin of the abdomen that runs to the tip of each digit, uniting the forelimb with the body. The patagium of a bat has four distinct parts:
Propatagium: the patagium present from the neck to the first digit Dactylopatagium: the portion found within the digits Plagiopatagium: the portion found between the last digit and the hindlimbs Uropatagium: the posterior portion of the body between the two hindlimbs Other mammals such as gliding possums and flying squirrels also glide using a patagium, but with much poorer efficiency than bats. They cannot gain height. The animal launches itself from a tree, spreading its limbs to expose the gliding membranes, usually to get from tree to tree in rainforests as an efficient means of both locating food and evading predators. This form of arboreal locomotion is common in tropical regions such as Borneo and Australia, where the trees are tall and widely spaced. In flying squirrels, the patagium stretches from the fore- to the hind-limbs along the length of each side of the torso. In the sugar glider, the patagia extend between the fifth finger of each hand to the first toe of each foot. This creates an aerofoil enabling them to glide 50 metres or more. This gliding flight is regulated by changing the curvature of the membrane or moving the legs and tail.
Fish, reptiles, amphibians and other gliding animals
In addition to mammals and birds, other animals notably flying fish, flying snakes, flying frogs and flying squid also glide.
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