# A Computational Study on the Aeroacoustics of a Multi-Rotor Unmanned Aerial System
## 1 Introduction
A similar study was conducted by [PERSON] [33] for three angles of attack, including \(\theta=0^{\circ},3^{\circ}\), and \(6^{\circ}\), where results suggested that the noise level at the first BPF remains unchanged with increase in \(\theta\), while it increases at higher harmonics. This discrepancy between the results of the two studies may be due to having different configurations or using different coordinate systems. [PERSON] [33] has also found that for a configuration of a single propeller, a wing and a fuselage, the sound emitted from the fuselage surface is smaller than that of the wing, since it experiences a smaller amount of load and interaction with the propeller's wake. Apart from the noise generated due to interaction of the propeller with wing-nacelle-fuselage, some recent studies have focused on the sound generated by multirotor configurations, which is a common configuration for recent UAS [3]. It is observed that the SPL generated by two counter-rotating propellers is higher than that of a single propeller, and the increase is dominant at BPF, suggesting mainly increase in tonal noise [34; 35]. Additionally, tilting the propellers shifts the location of maximum SPL downstream. Increasing the number of propellers to four, leads to a significant growth in noises at high-frequency or broadband noise [22], in addition to manifestation of a low-frequency disturbance, which is not present in the isolated propeller noise spectra [36]. When the spacing between the tip of the adjacent propellers in a quadrotor is small, the level of fluctuations they impose on each other's loads becomes significant [35]. Studies have also shown the rotation direction of multiple rotors affects the noise generation when lifting surfaces such as wings are immersed in the propeller wakes. For instance, the CFD analysis of two counter-rotating propellers interacting with a wing, conducted by [PERSON] et al. [37] showed that the top-in layout results in lower noise generation while the top-out layout provides a better aerodynamic efficiency due to the swirl effect. Although, the UAS noise signature has been investigated in several studies as mentioned earlier, these studies are often not for a fully appended UAS and limited to a narrow range of operational conditions. The aim of the present work is to provide a comprehensive acoustics analysis around a biplane tailsitter quadrotor that initially takes off vertically, in hover mode, and then changes its pitch angle and flies horizontally to use wings for the forward mode flight to enhance its performance. To understand the acoustic signature of such a quadrotor, a wide range of pitch angles covering hover and transition modes, as well as different vehicle velocity and propeller RPM are considered. The physics involved in the acoustics performance of the vehicle is discussed by comparing SPL at various locations and their correlations with local flow characteristics. In addition, the role of different interaction mechanisms on the CRC-3 noise level as well as the structure-born noise due to unsteady loadings are discussed.