Changes

==Etymology==

==Etymology==

[Middle English propellen, from Latin propellere,  from pro- before + pellere to drive — more at felt

[Middle English propellen, from Latin propellere,  from pro- before + pellere to drive — more at felt

*Date: [http://www.wikipedia.org/wiki/15th_Century 15th century]

*Date: [https://www.wikipedia.org/wiki/15th_Century 15th century]

==Definition==

==Definition==

*to drive forward or onward by or as if by means of a [[force]] that imparts [[motion]]

*to drive forward or onward by or as if by means of a [[force]] that imparts [[motion]]

==Description==

==Description==

Air '''propulsion''' is the [[act]] of moving an object through the air. The most common [[types]] are [http://en.wikipedia.org/wiki/Propeller propeller], jet engine, turboprop, ramjet, [http://en.wikipedia.org/wiki/Rocket_propulsion rocket propulsion], and, [[experiment]]ally, scramjet, pulse jet, and [http://en.wikipedia.org/wiki/Pulse_detonation_engine pulse detonation engine]. [[Animals]] such as birds and insects obtain propulsion by flapping their wings.

Air '''propulsion''' is the [[act]] of moving an object through the air. The most common [[types]] are [https://en.wikipedia.org/wiki/Propeller propeller], jet engine, turboprop, ramjet, [https://en.wikipedia.org/wiki/Rocket_propulsion rocket propulsion], and, [[experiment]]ally, scramjet, pulse jet, and [https://en.wikipedia.org/wiki/Pulse_detonation_engine pulse detonation engine]. [[Animals]] such as birds and insects obtain propulsion by flapping their wings.

The [[word]] is derived from two [[Latin]] words: pro [[meaning]] before or forwards and pellere meaning to drive. Propulsion means to push forward or drive an object forward. A propulsion [[system]] is a [[machine]] that produces thrust to push an object forward. On airplanes, thrust is usually generated through some [[application]] of Newton's third [[law]] of [[action]] and [[reaction]]. A gas, or working [[fluid]], is accelerated by the engine, and the [[reaction]] to this acceleration produces a [[force]] on the engine. A general derivation of the thrust [[equation]] shows that the amount of thrust generated depends on the [[mass]] [[flow]] through the engine and the exit [[velocity]] of the gas. [[Different]] propulsion [[systems]] generate thrust in slightly different ways.  

The [[word]] is derived from two [[Latin]] words: pro [[meaning]] before or forwards and pellere meaning to drive. Propulsion means to push forward or drive an object forward. A propulsion [[system]] is a [[machine]] that produces thrust to push an object forward. On airplanes, thrust is usually generated through some [[application]] of Newton's third [[law]] of [[action]] and [[reaction]]. A gas, or working [[fluid]], is accelerated by the engine, and the [[reaction]] to this acceleration produces a [[force]] on the engine. A general derivation of the thrust [[equation]] shows that the amount of thrust generated depends on the [[mass]] [[flow]] through the engine and the exit [[velocity]] of the gas. [[Different]] propulsion [[systems]] generate thrust in slightly different ways.  

Why are there different types of engines? If we think about [http://en.wikipedia.org/wiki/Newton%27s_first_law_of_motion Newton's first law of motion], we [[realize]] that an airplane propulsion system must serve two [[purposes]]. First, the thrust from the propulsion system must [[balance]] the drag of the airplane when the airplane is cruising. And second, the thrust from the propulsion system must exceed the drag of the airplane for the airplane to accelerate. In [[fact]], the greater the [[difference]] between the thrust and the drag, called the excess thrust, the faster the airplane will accelerate.

Why are there different types of engines? If we think about [https://en.wikipedia.org/wiki/Newton%27s_first_law_of_motion Newton's first law of motion], we [[realize]] that an airplane propulsion system must serve two [[purposes]]. First, the thrust from the propulsion system must [[balance]] the drag of the airplane when the airplane is cruising. And second, the thrust from the propulsion system must exceed the drag of the airplane for the airplane to accelerate. In [[fact]], the greater the [[difference]] between the thrust and the drag, called the excess thrust, the faster the airplane will accelerate.

Some [http://en.wikipedia.org/wiki/Aircraft aircraft], like airliners and cargo planes, spend most of their life in a cruise condition. For these airplanes, excess thrust is not as important as high engine efficiency and low fuel usage. Since thrust depends on both the amount of [[gas]] moved and the [[velocity]], we can generate high thrust by accelerating a large [[mass]] of [[gas]] by a small amount, or by accelerating a small mass of gas by a large amount. Because of the aerodynamic [[efficiency]] of propellers and fans, it is more fuel efficient to accelerate a large mass by a small amount. That is why we find high bypass fans and turboprops on cargo planes and airliners.

Some [https://en.wikipedia.org/wiki/Aircraft aircraft], like airliners and cargo planes, spend most of their life in a cruise condition. For these airplanes, excess thrust is not as important as high engine efficiency and low fuel usage. Since thrust depends on both the amount of [[gas]] moved and the [[velocity]], we can generate high thrust by accelerating a large [[mass]] of [[gas]] by a small amount, or by accelerating a small mass of gas by a large amount. Because of the aerodynamic [[efficiency]] of propellers and fans, it is more fuel efficient to accelerate a large mass by a small amount. That is why we find high bypass fans and turboprops on cargo planes and airliners.

Some aircraft, like fighter planes or [[experiment]]al high speed aircraft, require very high excess thrust to accelerate quickly and to overcome the high drag [[associated]] with high speeds. For these airplanes, engine efficiency is not as important as very high thrust. Modern military aircraft typically employ afterburners on a low bypass turbofan core. [[Future]] hypersonic aircraft will employ some type of ramjet or rocket propulsion.

Some aircraft, like fighter planes or [[experiment]]al high speed aircraft, require very high excess thrust to accelerate quickly and to overcome the high drag [[associated]] with high speeds. For these airplanes, engine efficiency is not as important as very high thrust. Modern military aircraft typically employ afterburners on a low bypass turbofan core. [[Future]] hypersonic aircraft will employ some type of ramjet or rocket propulsion.