Gliding flight

Lore & Background

Gliding flight is employed by gliding animals and by aircraft such as gliders. Most powered aircraft are capable of gliding without engine power. Gliding generally requires the application of an airfoil, such as wings or a gliding membrane, but can be achieved with a flat wing, as with a simple paper plane, or even with card-throwing. 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.

Types of glider aircraft include gliders (sailplanes), hang gliders, paragliders, speed gliders, ram-air parachutes, rotor kites (if untethered, known as rotary gliders or gyrogliders), military gliders, paper aeroplanes, radio-controlled gliders, rocket gliders, and wingsuits. The main human application is currently recreational, though during the Second World War military gliders were used for carrying troops and equipment into battle. Some powered aircraft are designed for routine glides during part of their flight, usually when landing after powered flight, including experimental aircraft such as the North American X-15 and spacecraft such as the Space Shuttles, SpaceShipOne and the Russian Buran.

A number of animals have separately evolved gliding many times. Birds use gliding flight to minimise energy use; large birds notably adept include albatross, condor, vulture, eagle, stork, and frigatebird. Mammals such as gliding possums and flying squirrels glide using a patagium, but with much poorer efficiency than bats. Other gliding animals include flying fish, flying snakes, flying frogs, and flying squid.

Reader's Guide

Gliding flight is notable as a mode of locomotion that allows both aircraft and animals to travel horizontally over significant distances while descending, without the use of thrust. The article describes how the energy for overcoming drag comes from the loss of gravitational potential energy. The lift-to-drag ratio (L/D ratio) determines the glide slope angle or glide ratio; a higher L/D ratio leads to a better glide. The effect of airspeed on the rate of descent is depicted by a polar curve, an inverted U-shape, showing airspeeds for minimum sink and best L/D ratio. Gliding flight has evolved independently in numerous animal groups, including birds, mammals, fish, reptiles, and amphibians, with adaptations such as patagia in mammals and pseudo concave wings in flying snakes. The article notes that flying snakes can glide better than flying squirrels despite lacking limbs or wings. In human application, gliding is primarily recreational, with gliders, hang gliders, and paragliders designed to repeatedly climb using rising air and then glide. Military gliders were used during the Second World War for troop and equipment transport. Aircraft not designed for glide may be forced to perform gliding flight in an emergency, such as engine failure or fuel exhaustion.

Did You Know?

• Flying fish can use updrafts at the leading edge of waves to cover distances up to 400 m (1,300 ft).
• Flying snakes glide by sucking in their abdomen and flaring out ribs to form a pseudo concave wing, making a continual serpentine motion to stabilise direction.
• The great frigatebird is capable of continuous flights up to several weeks using gliding and soaring.
• Sugar gliders have patagia extending between the fifth finger of each hand to the first toe of each foot, enabling glides of 50 metres or more.