Airframe Interference Effects On Rotorcraft BVI

A coupled numerical procedure involving modified versions of the NASA Langley “Camrad.Mod1” lifting-line code and the NASA Ames “PMARC” potential lower-order panel code has been developed. The procedure is suitable for predicting the time-averaged or the unsteady aerodynamic interference between a helicopter rotor and the airframe. In the PMARC code, vortex wake induced effects are simulated using the “velocity” transpiration approach. This computationally efficient method is equivalent to calculating the local perturbations in the body source singularity strength, per unit area, due to the flowfield-embedded rotor vortex wake. Therefore, only modifications to the right hand side of the discretized system of algebraic equations, rather than the modification of the influence coefficient matrix, are required. Accuracy of the coupled procedure was assessed using the wake trajectory data acquired in the 14- by 22- foot subsonic wind tunnel at NASA Langley for the two-meter rotor test system (2MRTS) mounted atop a generic fuselage (ROBIN). Time-averaged wake distortion results are presented for cases with, and without, allowance for interference to help assess the influence of the airframe on both the tip vortex wake geometry and the rotor trim/performance. Results for typical landing approach conditions are presented to illustrate the effects of the airframe on the rotor wake and hence BVI. Good correlation between the measured and the computed wake trajectories was obtained for two advance ratios.