ROTOR INFLOW VARIABILITY WITH ADVANCE RATIO

Comparison of the inflow calculations among several analytical rotor wake methods and rotor inflow measurements using a laser velocimeter is presented. The inflow measurements were made near a rectangular four-bladed rotor system operating in forward flight at a thrust coefficient of 0.0064. The inflow measurements were made at azimuthal increments of 30 degrees from ψ=0, at 3.0 inches (approximately one chord) above the plane formed by the tips of the blades, and radial location from r/R=0.2 to r/R=1.1. The experimental measurements showed that as the advance ratio increased, the induced upflow region moved progressively from the forward 20% of the rotor disc at μ=0.15 to covering most of the complete forward half of the rotor disc at μ=0.30. The induced flow characteristics at all advance ratios were found to be unsymmetrical about the longitudinal center-line. The maximum downwash was found to be in the rear portion of the disc and skewed toward the advancing blade side. The comparisons among the analytical methods showed that the region of induced upflow over the rotor disc was effectively modeled only at the advance ratio of 0.15. Very large values of induced inflow were computed with the UTRC free-wake method near the tips of the rotor blades at 60° and 300° azimuth, and were a result of very close passage of concentrated vorticity. At the higher advance ratios, the results of the Beddoes method, as implemented by Komerath and Schreiber, deteriorated to the point that the appropriate input values did not result in an inflow distribution that was at all representative of the measured values. The other four methods consistently indicated the largest values of induced inflow ratio to be on the retreating-blade side of the rotor disc, which was opposite from that measured.