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FAQ

Forces Acting on an Airplane in Flight - Level 3

Thrust

Thrust is the force that provides the forward motion of the airplane through the air. There are several ways to produce this force—jets, propellers or rockets— but they all depend on the principle of pushing air backward with the object of causing a reaction, or thrust, in the forward direction. The effect is the same whether the thrust is produced by a propeller moving a large mass of air backward at a relatively slow speed or by a jet moving a small mass of air backward at a relatively high speed.

For jet aircraft, the means of thrust is the gas turbine engine. The figure below shows the inlet and exhaust flows of the turbojet. The negative thrust due to bringing the freestream air almost to rest just ahead of the engine is called momentum drag or ram drag. The resulting thrust is given by following equation,

Schematic of a turbojet engine.

where: = is weight flow rate of the air passing through the engine.
= jet stream velocity
= static pressure across propelling nozzle
= atmospheric pressure
= propelling nozzle area
= aircraft speed

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Updated: March 12, 2004