A linearized analytical method for stability analysis of the helicopter rotor wake is given.The wake vortex filaments are discretized into a number of straight-line vortex segments and perturbations of the vortex filaments are described by the end-position change of vortex segments.Self-induced and mutually-induced effects of the tip vortices as well as the interaction between rotor blade and tip vortex are considered.Taking UH-1H and AH1G model rotors as numerical examples,stability of the rotor wakes in both hover and forward flight is analyzed.It is shown that there is a positive eigenvalue in rotor wake motion and the wake is intrinsically unstable.Furthermore,a regular change of the maximum divergence rate with wave number is observed.The maximum wake divergence rate is smaller and the instability is weaker in forward flight compared with that in hovering flight.