Thrust

The mechanism for generating thrust is encompassed by Newton’s Third law. That is, propulsion through a fluid is achieved by applying a force to the fluid in one direction, which by Newton’s Third law results in a force on the object in the opposite direction. Equivalently, accelerating the fluid in one direction will apply a force (thrust) on the propulsion device in the opposite direction. This is illustrated in the figure below where a propulsion device ingests low velocity fluid and expels high velocity fluid (*V _{e}* >

As far as Newton’s Third Law is concerned, the exact mechanism by which the propulsion device accelerates the fluid is not relevant. The propulsion device may be a jet engine, a propeller, a flapping fin on a dolphin, etc. The design of the propulsion device does, however, affect the magnitude of the thrust it generates as well as its efficiency, as will be discussed in the Design section.

For a steady-state propulsion device like a jet engine or propeller, thrust can be expressed in terms of the flow parameters as

where

the mass flow rate of fluid entering and exiting the propulsion device, respectively, | |

the fluid velocity upstream and at the exit of the propulsion device, respectively, | |

the fluid pressure upstream and at the exit of the propulsion device, respectively, and | |

the cross-sectional area at the exit of the propulsion device. |

Mass flow rate can be expressed as where the density (r), area (*A*), and velocity (*V*) at the appropriate location are used.

For many cases (such as propellers and subsonic jets), *p _{e} = p*

Clearly *V _{e}* >

For an unsteady propulsion device, the relationship between the flow parameters and thrust is much more complex, but Newton’s Third Law is still the basic principle governing the resulting propulsive thrust.

Example Problem: Engine Comparison |