Aircraft classified by propulsion

Aircraft use propulsion for their forward (or onward) motion, that is to move between distant points on the ground. Some aircraft use artificial sources of propulsion, while others do not need them. For example, many lighter-than-air aircraft use the wind for propulsion. Many heavier-than-air aircraft, however, need engines for propulsion. Therefore, attending to their method of propulsion, aircraft can be classified into two main categories: powered aircraft and unpowered aircraft (Wikipedia, 20091).

Unpowered aircraft
Unpowered aircraft do not use any source of artificial energy for propulsion (although many need some sort of mechanical help to become airborne1).
Glider Gliders glide from one place to another without the need for a source of propulsion. Gliding is a way of managing the lift produced by wings while the aircraft is falling due to the effect of gravity. Although they don't need a source of propulsion, gliders need some source of power to become airborne in the first place, such as forward-launching from high altitude, pulling into the air on a tow-line or using a powered 'tug' aircraft. Gliders, hang-gliders, paragliders and rotor-kites are typical gliders. Yet, fix-wing aircraft, such as airliners, enter frequently in gliding mode when flying. (Image: a Schleicher ASK 21 —embedded from Wikipedia on 11 March 2009.) K21_glider.jpg
Drifter Drifters use the wind as a source of propulsion, letting it push the aircraft in the general direction of travel. Balloons are the typical drifters. Balloons manage lift in order to ascend or descend to different levels in the atmosphere until they find winds propitious to their direction of travel. (Image: a hot-air balloon —embedded from Wikipedia on 11 March 2009.) 2006_Ojiya_balloon_festival_011.jpg
Powered aircraft
Powered aircraft use the energy provided by engines for propulsion. Many aircraft also achieve lift in the same manner.
Propeller aircraft Propeller aircraft use propellers (i.e. engines with vertically-mounted blades) for propulsion. The blades in the propeller forms a helical surface and, when spinning, "cuts" the air in front of the blades in a screwing fashion. This, in turn, generates a forward motion on the aircraft. The power from the engines is also used for controlling the speed of the aircraft, which is achieved by adjusting the propeller's rotational speed and/or its angle of attack. Typical propeller aircraft are propeller airplanes, which also use the forward motion generated by the propeller to lift the aircraft and sustain flight. Propellers are also a common source of propulsion used by airships, autogyros and gyrodynes, which don't need them for lifting but only for navigating. (Image: an Aquila AT01 —embedded from Wikipedia on 27 March 2010.) 220px-Aquila_en_volm.jpg
Jet aircraft Jet aircraft use jet engines for propulsion2. Jet engines take air at the front of the engines, compresses it at intense pressure and release it through a narrow end at the back of the engines, thus generating a forward motion. The power from the engines is also used for controlling the speed of the aircraft. Typical jet aircraft are jet airplanes, which also use the forward motion generated by the engines to lift the aircraft and sustain flight. (Image: a Boeing 737-300 —embedded from Wikipedia on 27 March 2010.) 300px-Jet2_aeroplane_landing_at_EDI.jpg
Rocket aircraft Rocket-powered aircraft use rockets for propulsion. Rockets work with stored fluids, which are ignited and released through a narrow end at the back of the engine, thus generating a forward motion. Rockets are typically used in spacecrafts. (Image: SpaceShipOne —embedded from Wikipedia on 28 March 2010.) 220px-Spaceship_One_in_flight_1.jpg
Helicopter Helicopters use rotors (i.e. engines with rotating top-mounted blades) in order to achieve both lift and onward motion (i.e., to move forward, backwards, upwards, etc). (Image: a HAL Dhruv helicopter —embedded from Wikipedia on 27 March 2010.) 220px-Indian_air_force_dhruv_helicopter_j4042_arp.jpg
1. WIKIPEDIA (2009). Aircraft. Retrieved from Wikipedia on 11 March 2009.

Want to know more?

AviationKnowledge - Aircraft
Here you can find other classifications for aircraft.
HowStuffWorks offers more information on how airplanes and helicopters work.
NASA - Beginner's guide to propulsion
NASA introduces and expands the concept of propulsion in airplanes.
Onpedia - Balloon
This page tells you more about balloons.
Wikipedia - Aircraft
This Wikipedia page offers more detailed information on aircraft classifications, including this one.

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