An eigenvalue analysis has been developed to study the stability characteristics of helicopter rotor wakes in hover and axial climb. The results are supported by experimental observations of the rotor wake behavior. The hovering helicopter rotor wake is shown to be intrinsically unstable,
with the tip vortices exhibiting several possible unstable deformation modes. The wake divergence (instability) rates associated with each wake deformation mode depend on the relative phase of the perturbations produced on tip vortex filaments generated by different blades. The wake divergence
rates increase sharply after the initial radial contraction of the wake below the rotor. Divergence rates are governed by the vortex‐indnced 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 one unstable deformation mode of the rotor wake. It is also shown that in numerical solutions of the wake using free‐vortex methods this deformation mode can he
artificially excited because of numerical errors. A proper choice of a numerical time integration algorithm is necessary to prevent non‐physical growth of these numerical errors.
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Document Type: Research Article
Department of Aerospace Engineering, Glenn L. Martin Institute of Technology, University of Maryland, College Park, Maryland 20742
Publication date: 2000-07-01
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