Abstract: A model is developed for the calculation of the initial rotor tip vortex core radius aiming at eliminating the empirical parameters of the tip vortex in rotor free-vortex wake analysis. This method makes use of the conservation of wake vorticity, the moments of vorticity with the rotation effect and modified Vatistas's vorticity distribution function, and establishes a nonlinear relationship between the initial core radius and the spanwise bound circulation distribution. The new initial tip vortex model and the vortex core diffusion and stretching model together form a complete tip vortex model. The complete model is used in the free-vortex wake analysis for two model rotors in hover. The computed initial core radius and span station of the tip vortex are shown to compare favorably with experimental measurements. What is more, the effect of turbulent momentum diffusion on free-vortex wake analysis is discussed, and it is found that this effect has important influence on the convergence of rotor free wake analysis.

Key words: helicopter, rotor, tip vortex, vortex core, vorticity, free wake