Thrust and propulsive efficiency from an instructive viewpoint

In a typical engineering or physics curriculum, the momentum equation is used for the determination of jet engine thrust. Even a simple thrust analysis requires a heavy emphasis on mathematics that can cause students and engineers to lose a physical perspective on thrust. This article provides for this physical understanding using only static pressures that act on engine surfaces. Such an alternative, but equivalent, method can offer insights into some special examples of jet engine thrust that contradict commonly held beliefs. One such example is provided by the engine bellmouth that is used for testing jet engines on the ground. The static pressure distribution clearly shows that the engine bellmouth actually experiences "forward" thrust. Another example is provided by the conic exhaust nozzle that is used at the end of some jet engines. The static pressure distribution shows that the conic nozzle does "not" experience any forward thrust (although the nozzle increases the overall thrust of an engine through higher pressures upstream of the nozzle). Following these examples, a basis for conceptualizing propulsive efficiency is discussed. This illustrates that it is more efficient to have a smaller acceleration of a large amount of air than a larger acceleration of a smaller amount of air. (Contains 4 figures and 3 footnotes.)