Gliding flight
Gliding flight is heavier-than-air flight without the use of thrust; the term volplaning also refers to this mode of flight in animals.[1] 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.
For the sport of soaring in gliders (sailplanes), see Gliding.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,[2] 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.
Most winged aircraft can glide to some extent, but there are several types of aircraft designed to glide:
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:
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 in a helicopter is called autorotation.
Three principal forces act on aircraft and animals when gliding:[20]
As the aircraft or animal descends, the air moving over the wings generates lift. The lift force acts slightly forward of vertical because it is created at right angles to the airflow which comes from slightly below as the glider descends, see angle of attack. This horizontal component of lift is enough to overcome drag and allows the glider to accelerate forward. Even though the weight causes the aircraft to descend, if the air is rising faster than the sink rate, there will be a gain of altitude.
Drag[edit]
Induced drag is caused by the generation of lift by the wing. Lift generated by a wing is perpendicular to the relative wind, but since wings typically fly at some small angle of attack, this means that a component of the force is directed to the rear. The rearward component of this force (parallel with the relative wind) is seen as drag. At low speeds an aircraft has to generate lift with a higher angle of attack, thereby leading to greater induced drag. This term dominates the low-speed side of the drag graph, the left side of the U.
Profile drag is caused by air hitting the wing, and other parts of the aircraft. This form of drag, also known as wind resistance, varies with the square of speed (see drag equation). For this reason profile drag is more pronounced at higher speeds, forming the right side of the drag graph's U shape. Profile drag is lowered primarily by reducing cross section and streamlining.
As lift increases steadily until the critical angle, it is normally the point where the combined drag is at its lowest, that the wing or aircraft is performing at its best L/D.
Designers will typically select a wing design which produces an L/D peak at the chosen cruising speed for a powered fixed-wing aircraft, thereby maximizing economy. Like all things in aeronautical engineering, the lift-to-drag ratio is not the only consideration for wing design. Performance at high angle of attack and a gentle stall are also important.
Minimising drag is of particular interest in the design and operation of high performance glider (sailplane)s, the largest of which can have glide ratios approaching 60 to 1, though many others have a lower performance; 25:1 being considered adequate for training use.
Soaring animals and aircraft may alternate glides with periods of soaring in rising air. Five principal types of lift are used:[30] thermals, ridge lift, lee waves, convergences and dynamic soaring. Dynamic soaring is used predominately by birds, and some model aircraft, though it has also been achieved on rare occasions by piloted aircraft.[31]
Examples of soaring flight by birds are the use of:
For humans, soaring is the basis for three air sports: gliding, hang gliding and paragliding.