Impact of Rotor-to-Rotor Interactions on the Tonal Noise Characteristics of Different Octocopter Configurations

Rotor-to-rotor interactions have been found to significantly contribute to acoustic characteristics. The present research presents a novel idea to conduct comparative studies on the aeroacoustics characteristics of conventional, coaxial, and hybrid octocopter configurations in hover and steady forward flight to identify the configuration that has minimal noise levels. Moreover, the influence of vertical spacing in the coaxial octocopter is explored, and an aeroacoustics comparison of coaxial and hybrid configurations with larger-diameter rotors in the same confined vehicle space for missions that require a high thrust is presented at the end. The virtual blade method (VBM) is employed herein for aerodynamic analysis due to its high computational efficiency, and a numerical analysis code based on FWH equations is developed for the acoustics analysis. The hover results show that coaxial configurations produce a peak SPL value of 93 dB, which is 5 dB louder than the conventional configuration and 3 dB louder than the hybrid configuration. The coaxial configuration with 0.125 R vertical spacing produced a peak SPL of 92 dB, which is 5 dB louder than the 0.5 R and 10 dB louder than the 1.5 R. In steady forward flight with normal-sized rotors, the hybrid configuration outperformed the others with a peak SPL value of 85 dB, which is 7 dB lower than that of the conventional configuration; meanwhile, the other configurations had similar noise values. The noise attributed to larger rotors in a confined vehicle space during hover flight in the coaxial configuration is 5 dB less than that of the hybrid configuration at almost all elevation angles in the farfield; meanwhile, a 5 dB difference was observed in the front half of the vehicle in forward flight and minute differences were found in the rear half.