The book "The Jet Engine" by Rolls Royce gives the following version of the equation for propulsive efficiency for a jet engine.
Propulsive Efficiency = (Work done on the aircraft) / (Work done on the aircraft + Work done on the exhaust)
We can modify this to apply for a propeller aircraft as follows.
Propulsive Efficiency = (Work done on the aircraft ) / (Work done on the aircraft + Work done on the propwash)
We can also modify it to apply to your car on the dynamometer as follows:
Propulsive Efficiency = (work done on the car) / (Work done on the car + Work done on the dynamometer)
The work done on the jet engine exhaust, the propwash and the dynomometer are all wasted work.
In each case if the aircraft or the car are not moving then the work done on them is zero and the propulsive efficiency is zero.
If we have zero propulsive efficiency this means that we are wasting all of the power that is being generated by our propulsion system. This means that we have no thrust horsepower.
Last edited by keith williams; 3rd Apr 2012 at 20:04.