Understanding in-plane noise and loading for active noise control

This paper is focused on gaining a deeper understanding of in-plane rotor noise due to thickness and loading; accurate prediction of in-plane noise; and ultimately the reduction of in-plane noise while minimizing any performance penalties. First, two approaches to compute thickness noise are reviewed, which are Farassat's retarded time Formulation 1A and Isom's thickness noise computation, respectively. Secondly, significant differences between in-plane loading noise computed by chordwise compact loading and distributed loading is shown and the reason for these differences is explained. On the advancing side of the rotor, the retarded time for the in-plane observer differs significantly at different chordwise positions on the blade. But these retarded time differences cannot be taken into account by the compact loading model; therefore, loading noise computed by compact loading is not accurate for observers in the rotor plane. Third, a new dual compact loading model is developed for noise prediction. Inspired by Isom's thickness noise, the dual compact loading model computation of thickness noise is in good agreement with normal thickness noise predictions for multiple cases (different blade planforms, airfoils and advance ratios) and it is approximately 25 times faster than the normal thickness noise computation. This dual compact loading model can also be used to compute actual loading noise more accurately in the rotor plane. Finally, the contribution of lift and drag to in-plane loading noise generation is studied on a generic rotor through the use of active trailing edge flaps. Flap deployment schedules for an inboard device are developed to alter the aerodynamic conditions near the tip of the blade for both thickness noise reduction and performance enhancement.