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Flight is the process by which an object achieves sustained movement either through the air (or movement beyond earth's atmosphere, in the case of spaceflight) by aerodynamically generating lift, propulsive thrust or aerostatically using buoyancy.
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[edit] Physics
There are different approaches to flight. If an object has a lower density than air, then it is buoyant and is able to float in the air without using energy. A heavier than air craft, known as an aerodyne, includes flighted animals and insects, fixed-wing aircraft and rotorcraft. Because the craft is heavier than air, it must use the force of lift to overcome its weight. The wind resistance caused by the craft moving through the air is called drag and is overcome by propulsive thrust except in the case of gliding.
Some vehicles also use thrust for flight, for example rockets and Harrier Jump Jets.
[edit] Relevant forces
Forces relevant to flight are[1]
- Propulsive thrust: (except in gliders)
- Lift: created by the reaction to an airflow
- Drag: created by aerodynamic friction
- Weight: (created by gravity)
- Buoyancy: for lighter than air flight
These forces must be balanced for stable flight to occur.
The stabilization of flight angles (roll, yaw and pitch) and the rates of change of these can involve horizontal stabilizers (i.e. 'a tail'), ailerons and other movable aerodynamic devices which control angular stability i.e. flight attitude (which in turn affects altitude, heading).
[edit] Lift to drag ratio
When lift is created by the motion of an object through the air, this deflects the air, and this is the source of lift. For sustained level flight lift must be greater than weight.
However, this lift inevitably causes some drag also, and it turns out that the efficiency of lift creation can be associated with a lift/drag ratio for a vehicle; the lift/drag ratios are approximately constant over a wide range of speeds.
Lift to drag ratios for practical aircraft vary from about 4:1 up to 60:1 or more. The lower ratios are generally for vehicles and birds with relatively short wings, and the higher ratios are for vehicles with very long wings, such as gliders.
[edit] Thrust to weight ratio
If thrust-to-weight ratio is greater than one, then flight can occur without any forward motion or any aerodynamic lift being required.
If the thrust-to-weight ratio is greater than the lift-to-drag ratio then takeoff using aerodynamic lift is possible.
[edit] Energy efficiency
To create thrust to push through the air to overcome the drag associated with lift takes energy, and different objects and creatures capable of flight vary in the efficiency of their muscles, motors and how well this translates into forward thrust.
Propulsive efficiency determines how much thrust propeller and jet engines gain from a unit of fuel
[edit] Power to weight ratio
All animals and devices capable of sustained flight need relatively high power to weight ratios to be able to generate enough lift and/or thrust to achieve take off.
[edit] Types
[edit] Animal
The most successful groups of living things that fly are insects, birds, and bats. The extinct Pterosaurs, an order of reptiles contemporaneous with the dinosaurs, were also very successful flying animals. Each of these groups' wings evolved independently. The wings of the flying vertebrate groups are all based on the forelimbs, but differ significantly in structure; those of insects are hypothesized to be highly-modified versions of structures that form gills in most other groups of arthropods.[2]
Bats are the only mammals capable of sustaining level flight. However, there are several gliding mammals which are able to glide from tree to tree using fleshy membranes between their limbs; some can travel hundreds of meters in this way with very little loss in height. Flying frogs use greatly enlarged webbed feet for a similar purpose, and there are flying lizards which employ their unusually wide, flattened rib-cages to the same end. Certain snakes also use a flattened rib-cage to glide, with a back and forth motion much the same as they use on the ground.
Flying fish can glide using enlarged wing-like fins, and have been observed soaring for hundreds of meters using the updraft on the leading edges of waves.[citation needed] It is thought that this ability was chosen by natural selection because it was an effective means of escape from underwater predators. The longest recorded flight of a flying fish was 45 seconds.[3]
Most birds fly (see bird flight), with some exceptions. The largest birds, the ostrich and the emu, are earthbound, as were the now-extinct dodos and the Phorusrhacids, which were the dominant predators of South America in the Cenozoic period. The non-flying penguins have wings adapted for use under water and use the same wing movements for swimming that most other birds use for flight. Most small flightless birds are native to small islands, and lead a lifestyle where flight would confer little advantage.
Among living animals that fly, the wandering albatross has the greatest wingspan, up to 3.5 meters (11.5 ft); the great bustard has the greatest weight, topping at 21 kilograms (46 pounds).[4]
Among the many species of insects, some fly and some do not (See insect flight).
[edit] Mechanical
Mechanical flight is the use of a machine to fly. These machines include airplanes, gliders, helicopters, autogyros, airships, balloons, ornithopters and spacecraft. Gliders provide unpowered flight. Another form of mechanical flight is parasailing where a parachute-like object is pulled by a boat. In an airplane, lift is created by the wings; the shape of the wings of the airplane are designed specially for the type of flight desired. There are different types of wings: tempered, semi-tempered, sweptback, rectangular, and elliptical. An aircraft wing is sometimes called an airfoil, which is a device that creates lift when air flows across it.
[edit] Supersonic
Supersonic flight is flight faster than the speed of sound. Supersonic flight is associated with the formation of shock waves that form a sonic boom that can be heard from the ground, and is frequently startling. This shockwave takes quite a lot of energy to create and this makes supersonic flight generally less efficient than subsonic flight at about 85% of the speed of sound.
[edit] Hypersonic
Hypersonic flight is very high speed flight where the heat generated by the compression of the air due to the motion through the air causes chemical changes to the air. Hypersonic flight is achieved by reentering spacecraft such as the Space Shuttle and Soyuz.
[edit] Study of flight
In 8th century Cordoba, Ibn Firnas studied the dynamism of flying and carried out a number of experiments. After one of his flights he fell on his back and he commented that he now understands the role played by the tail when birds alight on the ground, telling his close friends that birds normally land on the root of the tail which did not happen in that occasion, hence a reference to the missing tail[5]. Durant in his book “the story of Civilisation”, quoting Al-Makkari who mentioned that Ibn Farnas indeed constructed a flying machine[6]. However, he does not elaborate on how the machine works nor whether it was the one Ibn Farnas used nor on its destiny.
Leonardo da Vinci is one of the best-known early students of flight. He made many prototypes of parachutes wings and ornithopters.[citation needed]
[edit] In religion, mythology and fiction
In religion, mythology and fiction, human or anthropomorphic characters sometimes have the ability to fly. Examples include angels in the Hebrew Bible, Daedalus in Greek mythology, and Superman in comics. Two other popular examples are Dumbo, the elephant created by Disney who use his ears to fly, and Santa Claus whose sleigh is pulled by flying reindeers. Other non-human legendary creatures, such as some dragons and Pegasus, are also depicted with an ability to fly.
The ability to fly may come from wings or other visible means of propulsion, from superhuman or god-like powers, or may simply be left unexplained.
[edit] See also
- Aerodynamics
- Aviation
- Flying and gliding animals
- Aviation history
- Levitation
- Transvection (flying)
- Aircraft
[edit] References
- ^ Four forces on an aeroplane
- ^ Averof, Michalis. "Evolutionary origin of insect wings from ancestral gills." Nature, Issue 385, volume 385, February 1997 pp. 627–630.
- ^ "BBC article and video of flying fish". bbc.co.uk. Retrieved on 2008-05-20.
- ^ The Trumpeter Swan Society - Swan Identification
- ^ Al-Makkari, ed. Nafh Al-Teeb Volume 4. Dar Al-Fikre, Egypt, 1986, pp. 348–349.
- ^ Durant, Will. The Story of Civilisation vol. 13. New York: Simon and Schuster, 1967.
[edit] External links
- See how it flies: a new spin on the perceptions, procedures, and principles of flight
- 'Birds in Flight and Aeroplanes' by Evoluntionary Biologist and trained Engineer John Maynard-Smith Freeview video provided by the Vega Science Trust.
- The First Try of Human Flight
- Aerodynamic forces and control surfaces of an airplane explained
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