SYSTEMATIC CORRELATION AND EVALUATION OF THE EHPIC HOVER ANALYSIS

Free wake analyses, although available since 1969, have never gained wide acceptance for hover performance calculations due both to the typically high CPU execution time required per case and the difficulty involved in reliably representing a hover wake using a time dependent approach. Recently Bliss and Quackenbush have evolved a novel neutral stability / non-time-marching free wake approach that has shown considerable promise and is implemented in the Continuum Dynamics EHPIC hover code. Sikorsky Aircraft has conducted a systematic correlation evaluation of this code using the Sikorsky full scale hover test base. This test base includes blade number variations from 3 to 7, and equivalent linear twist variations from 0 deg. to -18 deg. In addition rectangular and trapezoidal, swept, and swept tapered tips are included. The primary intent of this study was to track the ability of the analysis to predict test results for a wide range of rotor geometries while adhering to a single, consistent set of guidelines for program input. The latter requirement assured that results would represent prediction ability as opposed to forced “correlation” for each specific geometry. Correlation of all rotors in this data base has been found to be good to excellent, with the exception of the older technology, low twist, six bladed rotors. The poor showing of these rotors may be due to the effect of exposed anti-abrasion strips and an observed oscillatory wake interference phenomenon.