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.

In urban settings, buildings create complex turbulent conditions, affecting helicopter flight performance during missions and increasing safety risks during takeoff and landing. A numerical study on rotor–building coupled flow field is carried out to address rotor aerodynamic performance under building interferences in natural atmospheric conditions. A high-fidelity atmospheric boundary layer (ABL) model described by an exponential law is established herein. The solution of the coupled flow field is based on the Reynolds-averaged Navier–Stokes (RANS) equations, with the rotor’s rotation achieved through the overset grid method. Based on the dominant wind features, the building flow field is distributed into four regions, where the updraft along the headwind side impacts the rotor, bringing about a 76% increase in pitching moment. On the lateral side of the building, distorted rotor wake squeezed upward into the rotor disk, leading to severe blade–vortex interaction (BVI). During low-altitude hovering over rooftops, the mixing of building shed vortices with forward flow wakes causes the formation of a circulation region on the rotor’s windward side, resulting in a thrust loss of approximately 7.8%. Meanwhile, the flow environment on the leeward side of the buildings is more stable. Therefore, it is recommended that helicopters adopt a headwind approach during rooftop operations. However, an 11.4% loss in the average hover figure of merit is observed due to consistent thrust losses caused by the recirculation region.