Abstract: The state of the art in the analysis of rotor wake stability has its limitation that it will obtain impractically large divergence rate where the vortex filaments get too close. In order to eliminate this unreasonable interference in the analysis of rotor wake stability coming from highly distorted wakes in forward flight, a linearized method of wake stability analysis associated with wake distortion revision under a sinusoidal perturbation is presented. In this method, the wake vortex filaments are discretized into a series of vortex elements with a sinusoidal perturbation distributed on the nodes along the vortex filaments, where the self/mutually-induced velocity of wakes and the influence of blades is considered. A fake vortex core is novelly applied to revise this unreasonable interference in the analysis of rotor wake stability coming from highly distorted wakes. Taking the H-34 model rotor as a numerical example, the influence of wake drifting, size of vortex core and advance speed on rotor wake stability are calculated by this method and analyzed individually in forward flight. The results show that the fake vortex core can effectively revise the impractically large divergence rate where the vortex filaments are getting too close. Furthermore, in forward flight, the change of the size of vortex core or advance speed, etc., cannot change the inherent instability of rotor wakes, but it can slightly influence the instabilities under some conditions.

Key words: stability, rotor wake, sinusoidal perturbation, eigenvalue analysis, fake vortex core