A lifting surface performance analysis with circulation coupled wake for advanced configuration hovering rotors

Abstract

The application of numerical lifting surface theory to the calculation of rotor hover performance is presented. Special techniques are developed to combine the lifting surface method with a conventional strip analysis to introduce airfoil section characteristics into the problem. Also, a Gothert type similarity transformation is developed for the hovering rotor. This transformation provides a refined treatment of the important influence of compressibility, and permits the incompressible lifting surface theory to be correctly applied for the analysis. These analytical procedures are supported by the development of an improved prescribed wake model derived from schlieren flow visualization studies of model rotor wakes. These flow visualization studies reveal wake sensitivity to thrust coefficient, number of blades, and twist not identified in previous investigations. Numerical experimentation using the lifting surface analysis with the experimentally prescribed wake demonstrates that wake settling rates can be coupled to calculated tip vortex circulation strength such that these parameters are no longer explicitly influenced by rotor blade geometry characteristics. Thus, circulation coupling extends the analysis for application to advanced configuration rotors. These studies also indicate, however, that wake contraction rate is governed by the overall inflow distribution, and thus behaves more in the sense of traditional momentum concepts. This result is demonstrated and the circulation coupling verified through comparison of predicted and measured wake parameters. Verification of the analysis is shown through comparisons of calculations to measured rotor blade air-loading and integrated rotor performance.