Rotor Performance in the Wake of a Large Structure

This paper is based on an investigation into the effects of rotorcraft operating in the aerodynamic wake of a large upstream object, Reference 1, that was conducted at the Fluid Mechanics Laboratory (FML), NASA Ames Research Center, Moffett Field, CA. The effort focused on basic fluid mechanics issues of a flapping rotor system, and employed both experimental and analytic techniques, using visual, measured, and calculated results for comparison. Experimental data showed that upwind building presence tended to decrease induced velocities at the leading edge of the rotor disc, which is consistent with the presence of a recirculation region between the rotor and building. The analytic effort involved development and limited validation of a mathematical rotor blade element model, which was used to predict various aspects of rotor blade performance. Final comparison with experimental spanwise induced velocity measurements showed that the blade element vortex (BEV) model, with the implementation of Kocurek and Tangler's prescribed wake model, a Biot-Savart ground effect correction, and a simple wake skew correction, resulted in 5 to 10 percent error for the BEV model. This model was applied to the case of rotorcraft operating in the aerodynamic wake of a large building using superposition. The measured wake was vastly (>50 percent speed) different from that predicted by a superposition assumption. This is because a rotor operating close to a structure develops a completely new flow field, which cannot be determined by simple vector addition/subtraction of flow fields. The acquired data also facilitated a review of the validity of the superposition principle, which is used extensively in rotorcraft flight simulator applications, and was proven to not be accurate.