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Engines
An engine produces a force which acts toward the rear of the aircraft which
"thrusts" the aircraft forward. For this reason, the force produced by the engine is
called thrust. Thrust is the most important force acting on an aircraft, because regard-
less of the type of aircraft, ALL need some type of thrust to propel them aloft. Even
unpowered aircraft such as gliders need a tow plane to provide an external force to
pull the aircraft into the air, where it can obtain airflow over the wings to provide the
necessary lift to remain airborne. Hang gliders use foot power to initiate movement
prior to "leaping" off a cliff. The most common means of developing thrust on pow-
ered airplanes comes from propellers or jets. Whether an aircraft has a propeller, a
turbojet, or a turbofan, all of these produce thrust by accelerating a mass of air to the
rear of the aircraft. The movement of this air to the rear creates an unbalanced force
pushing the aircraft forward.
The Wright brothers made many important things come together for their his-
toric first heavier-than-air flight. One of the most vital was an engine that efficiently
produced thrust while not weighing too much. They used propellers - the only effec-
tive means available of transferring an internal combustion engine's output into push
or pull for the airplane. Propellers are essentially revolving wings situated so that the
lift they produce is used to pull or push the airplane.
Most modern high-speed aircraft use a very different type of engine - the jet
engine. Jet engines not only look different from propellers, they operate in a very dif-
ferent manner as well. More like rocket engines, jets produce thrust by burning pro-
pellant (jet fuel mixed with air) and forcing the rapidly expanding gases rearward. In
order to operate from zero airspeed on up, jets use enclosed fans on a rotating shaft to
compress the incoming air (and suck it in if the airplane is not going very fast) and
send it into the combustion chamber where the fuel is added and ignited. The burning
gases keep the shaft turning by rotating a fan before exiting the engine.
Turbojet Engine
Some other jet engines differ from this basic pattern by the way they compress
the incoming air. Instead of forcing it down a restricting tube, the tweet's centrifugal
flow compressor literally flings the air outward into the compressor section exit,
compressing it against the outside wall.
Centrifugal Flow Jet Engine (T-37)
In a turbojet engine, the inlet area is small when compared to that of a propel-
ler. As the air exits the compressor section of the engine, it enters the combustion
chamber where fuel is added. This densely packed air/fuel mixture is ignited and the
resultant "explosion" accelerates the gases out the rear of the engine at a very high
rate of speed. This chemical acceleration of the air (combustion) adds to the thrust
produced by the engine. Most jet fighters have a system called afterburners, which
adds raw fuel into the hot jet exhaust generating even more thrust through higher ac-
celerations of the air. The jet generates large amounts of thrust by chemically accel-
erating the air as the result of combustion. The fact that the jet compresses the air as
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