A Comparison of Acoustic Predictions with Model Rotor Test Data from the NASA 14' X 22' Wind Tunnel

A study to correlate the predictions of the NASA-developed ROTONET rotorcraft acoustic prediction code and the Sikorsky in-house rotorcraft acoustic prediction code with model wind tunnel tests is presented. The main rotor alone portion of the NASA/Sikorsky Main Rotor/Tail Rotor Interaction Test has been selected as the experimental database from which to make acoustic predictions. The prediction methodology models thickness, steady and unsteady loading effects, with the unsteady loading derived from forward flight and simple wake models. The predictions have been compared with the acoustic data on the basis of similarity of the acoustic pressure time histories. Of the test points selected, two are considered minimal Blade-Vortex Interaction (BVI) conditions while one is known as a strong BVI condition. The computational model does not have, at this time, a model of BVI nor the capability to deal with the non linearities of transonic flow. On the whole, the codes show reasonable ability to predict the thickness and harmonically varying loading noise with the expected inability to deal with the transonic effects and to calculate wake geometry with sufficient accuracy to satisfy acoustic prediction requirements. The deviations of the predictions from experimental time histories are considered to be due to blade-wake interactions (BWI) and BVI or non-linearities caused by transonic effects, all of which could not be completely eliminated by choosing data from the test matrix.