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Like the motor car engine, the gas turbine is an internal
combustion engine. In both, air is compressed, fuel
added, the mixture ignited, and the rapid expansion
of the resultant hot gas produces the power. However,
combustion in a motor car engine is intermittent and
the expanding gas produces shaft power through a piston
and crank, whereas in a jet engine combustion is continuous
and its power results from expanding gas being forced
out of the rear of the engine.
One of Newton's principles is that to every action
there is an equal and opposite reaction. The expanding
gas flow is an action which creates a reaction of equivalent
force. This force - thrust - is transmitted through
the engine to the aircraft, propelling it through the
air. An inflated party balloon can demonstrate this:
with the neck closed, the air inside presses equally
in all directions; open the neck, and the air is released
as an action, creating a reaction on the opposite surface
of the balloon which drives it forward. Thrust is measured
in pounds force (lbf ), kilograms force (kgf ), or Newtons
(N).
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