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Stability Analysis of Helicopter Rotor Wakes in Axial Flight

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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

Affiliations: 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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  • The Journal of the American Helicopter Society is the world's only scientific journal dedicated to vertical flight technology. It is a peer-reviewed technical journal published quarterly by AHS International and presents innovative papers covering the state-of-the-art in all disciplines of rotorcraft design, research and development. (Please note that AHS members receive significant discounts on articles and subscriptions.)

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    Authors can find submission guidelines and related information on the AHS website.

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