Thrust is the force which moves an aircraft through the air. Thrust is generated by the propulsion system of the aircraft. Different types of engines develop thrust in different ways, although thrust is usually generated through some application of Newton's third law - action <-> 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.
During World War II, a new type of airplane engine was developed independently in Germany and in England. This engine was called a gas turbine engine. We normally call the engine a jet engine. Early jet engines worked much like a rocket engine creating a hot exhaust gas which was passed through a nozzle to produce thrust. But unlike the rocket engine which must carry its oxygen for combustion, the turbine engine gets its oxygen from the surrounding air. (A jet engine will not work in outer space because there is no surrounding air.) For a gas turbine engine, the accelerated gas is the jet exhaust and most of the mass of the jet exhaust comes from the surrounding atmosphere. Most modern passenger and military aircraft are powered by gas turbine engines. Because jet engines are so important for modern life, we will be providing a lot of information about jet engines and their operation.
Jet engines come in a wide variety of shapes and sizes because of the many different aircraft missions. All gas turbine engines have some parts in common, however. On the slide we see pictures of four different aircraft equipped with gas turbine engines. Each aircraft has a unique mission and therefore a unique propulsion requirement. At the upper left is a DC-8 airliner. Its mission is to carry large loads (passengers or cargo) for a long distance at high speed. It spends most of its life in high speed cruise. At the lower left is an F-14 fighter plane. Its mission is to shoot down other aircraft in air-to-air combat. It spends most of its life in cruise, but needs high acceleration when in combat. At the lower right is a C-130 cargo aircraft. Like the DC-8, it carries cargo a long distance, but it does not have the high speed requirement of the DC-8. At the upper right is a T-38 trainer. It is used to teach pilots how to fly jet aircraft and does not have the acceleration requirements of the F-14. The DC-8 is powered by four high-bypass turbofan engines, the F-14 by two afterburning low-bypass turbofans, the C-130 by four turboprop engines, and the T-38 by two turbojet engines.
EngineSim is an interactive Java applet which allows you to test different types of jet engines. You can learn the fundamentals of turbine engine propulsion with the EngineSim simulator. RangeGames is an interactive Java applet which allows you to study how different types of aircraft use different types of engines to meet their mission.
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