Thrust and Force
Thrust is the force which moves any aircraft through the air. Thrust is generated by the propulsion system of the aircraft. Different propulsion systems develop thrust in different ways, but all thrust is generated through some application of Newton’s third law of motion. For every action there is an equal and opposite reaction. In any propulsion system, a working fluid is accelerated by the system and the reaction to this acceleration produces a force on the system. 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.
Gas Turbine/Jet Engine
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 sometimes call this engine a jet engine. Early gas turbine 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 turbine engine does not work in outer space because there is no surrounding air. For a gas turbine engine, the accelerated gas, or working fluid, is the jet exhaust. Most of the mass of the jet exhaust comes from the surrounding atmosphere. Most modern, high speed passenger and military aircraft are powered by gas turbine engines. Because gas turbine engines are so important for modern life, we will be providing a lot of information about turbine engines and their operation.
Gas Turbine Engine
Turbine 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 of 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.
Turbine Engine Propulsion
Jet Engine Interactive Simulator is an interactive Java applet which allows you to study 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.