The effect of a wingtip-mounted propeller on the shift of the wingtip vortex core trajectory and the increase in aerodynamic efficiency of the wing is discussed in dependence of the propeller position and inclination angle. It is shown, that the quadrantal position of the propeller hub enables the wingtip vortex to be moved in the spanwise and vertical direction by a WTP. Depending on the quadrant of the propeller slipstream in which the roll-up of the wingtip vortex core takes place and the inclination of the slipstream, the vortex trajectory will be affected. Thus, the outward displacement of the wingtip vortex is not a general consequence of the increase in aerodynamic efficiency, but a relative effect of the slipstream velocities. The increase in the effective vortex span previously associated with the shift of the wingtip vortex and correspondingly the effective aspect ratio (vortex attenuation) is of lesser importance in the observed case than the effect due the covering of the wing surface by the propeller slipstream (swirl recovery).