Experimental Study of the Rotor–Arm Interaction Noise in Single and Contra-Rotating Rotors of eVTOL Aircraft

The installation effects of arms connected to single and contra-rotating rotors of electric vertical take-off and landing (eVTOL) aircraft are of great importance for noise radiation in the far field. In this study, experiments were conducted in a fully anechoic chamber to investigate the rotor–arm interaction noise in single and contra-rotating rotors with three configurations: (1) a single rotor with an arm placed downstream of the rotor wake, (2) a single rotor with an arm placed upstream of the rotor wake, and (3) contra-rotating rotors with an arm placed in the middle of the rotors. The rotor–arm distances were adjusted over a wide range, and two types of arm lengths were considered. The results show that for single-rotor configurations, the close proximity of the arm increases the tonal noise at all emission angles, and more-pronounced blade passing frequency (BPF) harmonics are increased by the Venturi effect when the arm is placed upstream of the rotor. The tonal noise at the upstream and downstream angles for the case in which the arm is extended across the entire rotor disk is higher than that for the case in which the end of the arm is located at the hub center, and noise suppression was observed at the sideline angles due to phase cancellation. For contra-rotating rotors, the rotor–arm interaction primarily amplifies the BPF harmonic tones of the downstream rotor, and does not influence the interaction tones between the rotors. Details of the noise directivity, noise features, and noise generation mechanisms are discussed. The findings of this study are expected to provide guidance for the installation of arms on the rotors of eVTOL aircraft.