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Sunday, July 17, 2011

Propulsion

There are two types of aircraft propulsion: powered and unpowered. the powered category contains three sub-categories: propeller, jet and rotor. there are also two other methods of propulsion: ornithopter and rocket-powered.


Propulsion

Unpowered

Balloons drift with the wind, though normally the pilot can control the altitude, either by heating the air or by releasing ballast, giving some directional control (since the wind direction changes with altitude). A wing-shaped hybrid balloon can glide directionally when rising or falling; but a spherically shaped balloon does not have such directional control.
Kites are aircraft that are tethered to the ground or other object (fixed or mobile) that maintains tension in the tether or kite line; they rely on virtual or real wind blowing over and under them to generate lift and drag. Kytoons are balloon kites that are shaped and tethered to obtain kiting deflections, and can be lighter-than-air, neutrally buoyant, or heavier-than air.

Powered


Propeller

turboprop-engined DeHavilland Twin Otter adapted as a floatplane
propeller or airscrew spins on an axis aligned in the direction of travel to create thrust in a forward direction. The propeller is usually mounted in front of the power source in tractor configuration but can be mounted behind in pusher configuration. Variations of propellers include contra-rotating propellers and ducted fans.
Many kinds of power plant have been used to drive propellers.
The earliest designs used man power to give dirigible balloons some degree of control, and go back to Jean-Pierre Blanchard in 1784. Human powered heavier-than-air flight was achieved by the Gossamer Condor in 1977.
steam-powered dirigible balloon was flown in 1852, but the more practical internal combustion piston engine was the power source used for virtually all fixed-wing aircraft until World War II. The piston engine is still used in the majority of smaller aircraft produced, since it is efficient at the lower altitudes and slower speeds suited to propellers.
Turbine engines may be geared to drive a propeller in the form of a turboprop. Turbines provide more power for less weight than piston engines, and are better suited to small-to-medium size aircraft or larger, slow-flying types. Some designs mount the propeller directly on an engine turbine shaft, and are called propfans.
Unmanned aircraft and models have also used other power sources such as solar-powered electric motors and rubber bands.

Jet

Airbreathing jet engines take in air, burn fuel with it in a combustion chamber, and accelerate the exhaust rearwards at high speed to provide thrust. Turbojet and turbofan engines use a spinning turbine to drive one or more fans which provide thrust. An afterburner may be used to inject extra fuel into the hot exhaust, especially on military "fast jets".
Jet engines can provide much higher thrust than propellers, and are efficient at higher altitudes, being able to operate above 40,000 ft (12,000 m). They are also much more fuel-efficient at normal flight speeds than rockets. Consequently, nearly all high-speed and high-altitude aircraft use jet engines.
Use of a turbine is not absolutely necessary: other designs include the pulse jet and ramjet. These mechanically simple designs cannot work when stationary, so the aircraft must be launched to flying speed by some other method. Other variants have also been used, including themotorjet and hybrids such as the Pratt & Whitney J58 which can convert between turbojet and ramjet operation.


Rotor

Helicopter obtains lift from a powered rotary wing or rotor, which acts much like an upwards-pointing propeller. Forward propulsion is provided by angling the rotor disc slightly forward so that a proportion of its lift is directed forwards to provide thrust.
The rotor may, like a propeller, be powered by a variety of methods such as a piston engine or turbine. Also, tip jets, fed by gases passing along hollow rotor blades from a centrally mounted engine, have been experimented with. Attempts have even been made to mount engines directly on the rotor tips.



Other methods of propulsion

  • Rocket-powered aircraft have occasionally been experimented with, and the Messerschmitt Komet fighter even saw action in the Second World War. Since then they have been restricted to research aircraft, such as the North American X-15 which traveled up into space where air-breathing engines cannot work (rockets carry their own oxidant). Rockets have more often been used as a supplement to the main powerplant, typically for the rocket-assisted take off of heavily loaded aircraft, but also to provide high-speed dash capability in some hybrid designs such as the Saunders-Roe SR.53.
  • The ornithopter obtains thrust by flapping its wings. It has found practical use in a model hawk used to freeze prey animals into stillness so that they can be captured, and in toy birds.
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For details, please visit http://en.wikipedia.org/wiki/Aircraft

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