On the propeller efficiency calculated by lifting line theories

Theoretical investigations recently reported in the literature claim that it is possible to increase the propeller efficiency above the so called Lerbs’s optimum by increasing the tip load. The critical examination given below indicates however that this result is questionable, the main reason being inconsistencies in the lifting line theory. Thus check calculations show that the thrust derived from the momentum law is lower than the thrust computed by the law of Kutta-Joukowsky, especially for tip loaded propellers. To get the same thrust by the two principles a certain reduction of the pitch of the helical tip vortices is necessary.