Principles of Flight

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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.


The physics of flight

Lighter-than-air aircraft are able to fly without any major input of energy
Main article: Aerodynamics
There are different approaches to flight. If an object has a lower density than air, then it is buoyant and is able to rise and float in the air without using energy; a lighter than air craft is known as an aerostat. 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.

Forces for flight

Main forces on a heavier-than-air aircraft
Main article: Aerodynamics
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).

Lift to drag ratio

Speed and drag relationships for a typical flight article
Main article: 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.

Thrust to weight ratio
Main article: Thrust-to-weight ratio
If thrust-to-weight ratio is greater than one, then flight can occur without any forward motion.
If the thrust-to-weight ratio is greater than the lift-to-drag ratio then takeoff is possible.

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

Power to weight ratio
Main article: 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.

Mechanical flight
Main article: Aviation

Mechanical flight: A Robinson R22 Beta helicopter

A Bombardier Global 5000 business jet takes off
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.

The 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[4]. Durant in his book “the story of Civilisation”, quoting Al-Makkari who mentioned that Ibn Farnas indeed constructed a flying machine[5]. 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.
Supersonic flight
Main article: 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.

Hypersonic flight
Main article: 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.

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.

See also

Wikimedia Commons has media related to:
Category:Flight

Look up flight in Wiktionary, the free dictionary.
Aerodynamics
Aviation
Flying and gliding animals
Aviation history
Levitation
Transvection (flying)
Aircraft

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.
^ 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.

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

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Categories: Aerodynamics

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This page was last modified on 16 July 2008, at 08:57.
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