Analytic Prediction of Rotor Broadband Noise with Serrated Trailing Edges

Trailing-edge serrations are known to be an effective way to reduce broadband noise on wing sections, but their noise reduction capabilities on rotorcraft have not been fully understood. This paper develops a new approach to analytically predicting and investigating broadband noise of rotors with serrated trailing edges. To achieve rotorcraft broadband noise predictions, we extend Lyu and Ayton's semi-infinite serrated wing model to include the finite blade span, the modified scattering coefficient, and the spanwise observer distance. The validations show good agreements with experimental data for serrated trailing-edge noise of a wing section, a hovering rotor, and a forward-flight rotor. Next, the effects of serration parameters on rotor broadband noise are studied. For optimal and realistic rotor broadband noise reduction, the desirable design of a serrated-blade rotor has the serration height to wavelength ratio of 2, the radial range of serrations from 50% blade span to the tip. It is also found that the sine-wave, chopped peak, or saw-tooth serration shapes reduce noise most among various shapes that are considered. Finally, noise reduction with serrations is applied to urban air mobility (UAM) aircraft. A 6-passenger eVTOL quadrotor is found to have more than 9 dB noise reduction potential with serrated trailing edges, where higher noise reductions are observed with higher tip speeds and fewer blades. Serrated-blade quadrotors are found to have the dipole broadband noise directivity, and noise reductions are observed at all observer angles.