The stream tube of a low bypass ratio turbo jet engine looks completely different, because the acceleration of the flow is mainly performed through the thermal expansion of the heated air. Here the incoming stream tube will usually have a smaller diameter than the exhaust stream, depending on operating conditions. The acceleration of a propeller depends on the velocity v , thus it is generally not true that increasing the velocity v increases the thrust. But it can be said, that increasing the additional velocity , increases the thrust. For a propeller of a fixed diameter, working in a certain medium at a certain speed, thrust depends on the velocity increase only.
Power is defined as force times distance per time .
Now, thrust ist the one thing, the power to create this thrust the other. Of course we want to create as much thrust as possible from the smallest amount of power, which can be expressed by the term efficiency . Efficiency of a propeller is defined as the ratio of available power to the engine power which is . So this definition is not useful for the special case of static thrust. Unfortunately this equation cannot be easily solved for , but below, we find the relation plotted to find the efficiency for given values of power P , diameter D and density .