Importance of High Accuracy Blade Motion and Airloads Prediction for Acoustic Analysis

It is generally acknowledged that the computation of rotor acoustics requires accurate descriptions of both the aerodynamic blade loads and realistic blade positions and deformations. However, acoustic predictions, once loads were defined, have typically considered the blade as rigid and the motion of the blade as being defined by rigid flapping, feathering and lead-lag motions. Indeed acoustic predictions made with the noise prediction code WOPWOP using measured high resolution blade loads and rigid blade motions have compared well for some observer locations and operating conditions. For operating conditions where blade vortex interactions (BVI) occur, the comparison have been good for out-of-plane locations where BVI noise is dominant and to a lesser degree for in-plane locations. The larger differences for the in-plane locations have been attributed to several possibilities including high speed effects, transonic BVI effects and accuracy of the blade motions. This paper addresses the latter: that of the importance of using accurate blade motions in the acoustic analysis in low to moderate speed flight. Comparisons are made between measured acoustics and predictions from a modified version of WOPWOP which uses elastic blade motion rather than rigid blade motion. The importance of considering accurate blade motions and airload models in improving noise prediction is established. Overall advances in BVI noise prediction, due to improved airloads and rotor blade motion definitions is demonstrated. However the results also show that transonic BVI effects, which is a quadrupole noise component and is not accounted for in WOPWOP, may well be needed to fully account for the in-plane BVI noise measured.