An eigenvalue analysis has been developed to study the stability of helicopter rotor wakes in hover and axial flight. The results are supported with observed trends from hovering rotor wake experiments. The rotor wake is shown to be intrinsically unstable, with the tip vortices exhibiting several possible unstable deformation modes. The wake divergence (instability) rate associated with each deformation mode depends on the relative phase of the perturbations produced on tip vortex filaments generated by different blades. Wake divergence rates increase sharply after the initial radial contraction of the wake below the rotor, and is confirmed by experimental observations. The divergence rates are governed by the vortex-induced velocities; the divergence rate for any deformation mode decreases with increasing rotor thrust, and also with increasing climb rate. The so-called tip vortex `pairing' phenomenon, sometimes empirically observed in hovering flight conditions, is shown to be a particular unstable long-wave deformation mode of the rotor wake. It is further shown that in numerical solutions of the wake using free-vortex wake methods this deformation mode can be artificially excited by numerical errors. A proper choice of numerical time integration algorithm is necessary to prevent non-physical growth of these numerical errors.
This work was published at the 56th Annual Forum of the American Helicopter Society, Virginia Beach, VA, May 2000:
Go back to the Rotorcraft Aerodynamics main page