A propeller creates a thrust force out of the supplied power. The magnitude of this force is not constant for a given propeller, but depends on the velocity of the incoming air and the rotational velocity of the propeller itself. Thus tests of propellers usually cover a wide regime of operating conditions.
Propellers having the same shape, but are scaled by a size factor behave similar.
In order to make a comparison of propellers of different size easier, aerodynamicists try to get rid of the units. Then it is possible to use the results of a small scale wind tunnel model to predict the performance of a full scale propeller. While an airfoil can be characterized by relations between angle of attack , lift coefficient and drag coefficient , a propeller can be described in terms of advance ratio , thrust coefficient , and power coefficient . These coefficients are helpful for the comparison of propellers of differing diameters, tested under different operating conditions. Depending on the country where you live, there may be different coefficients in use.