Aircraft Acceleration Through Thrust

In this series we have examined how aircraft overcome gravity by producing lift and the aerodynamic force of drag. Now let's consider how the aircraft's jet engines produce the thrust required to overcome that drag.

In level flight, if thrust is greater than drag, the aircraft will accelerate. This acceleration is in accordance with Newton's second law of motion -- the acceleration of an object is proportional to the net accelerating force and inversely proportional to its mass. Thrust is also what allows an aircraft to climb. A climbing jet has to overcome the component of gravity acting along its flight path, as does a car driving up a hill. The more thrust a jet can produce, the steeper the angle it can climb. Jet fighters that can produce more thrust than they weigh can climb like a rocket, straight up.

In an automobile engine, called a reciprocating engine, air is taken into the cylinders where it is mixed with fuel, compressed, ignited, burned and expelled. These same events occur in a gas turbine or turbojet engine, although they occur continuously and at different engine locations.

Air which is pulled into the jet engine passes through the inlet diffuser and then into the compressor. The intake air is squeezed as it moves through the compressor, resulting in high pressures and temperatures. The hot, high pressure gas is then mixed with fuel and burned. The volume of the burning gas increases, causing an acceleration of the gas flow through the engine. It is this acceleration that creates the force of thrust. Not all of the energy released from the expanding exhaust is converted into thrust. Remember the compressor? It takes energy to squeeze the air to the high pressure necessary for combustion. The compressor is connected to a turbine that is powered by the high energy exhaust gases before they exit the engine.

The thrust of the jet engine is also used to slow the aircraft on the landing roll. Components of the engine, called thrust reversers, are activated by the pilot after touchdown. This is why jet engine noise becomes louder after landing and why passengers feel the strong deceleration.

There are a number of advantages to the turbojet engine. It allows jet transport aircraft to fly at high altitudes where the air is smooth and clear. Turbojet engines pressurize and air condition the aircraft's cabin, keeping the passengers comfortable. Most important, the turbojet engine allows passengers to travel safely at speeds close to the speed of sound. Without jet engines, transport aircraft couldn't fly above the bumpy weather, and would take three times longer to reach their destinations. If not for the advent of the turbojet engine, you might be reading this magazine while riding on a train!

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