# A Computational Study on the Aeroacoustics of a Multi-Rotor Unmanned Aerial System
## 1 Introduction  
The remarkable progress in manufacturing electric batteries in recent decades has facilitated the development of vertical take-off and landing (VTOL) multi-rotor unmanned aerial systems (UAS), which may be utilized for a wide range of tasks such as reconnaissance, package delivery, aerial resupply, and surveillance, among others [1; 2; 3]. While such systems have a great range of movement and maneuverability [1], their major issues including efficiency and noise emission have recently grabbed the scientific community's attention. Among these challenges, resolving the noise emission is relatively more demanding, due to recently raised concerns regarding negative effect on military applications, civil usage, and the environment. Some examples are cardiovascular and metabolic disease [4], sleep disturbance [5], depression and anxiety [6], and chronic stress [7], which are considered side effects of long-term exposure to the UAS noise [8]. Most studies on the noise signatures in the literature are on an isolated propeller, focusing on several aspects including geometry effect [9; 10; 11], number of blades and spacing effects [12; 13; 14], and coaxial rotors [15; 16; 17]. Studies have also investigated the effect of operational conditions on noise level [18; 19; 20]. For instance, it is known that an increase in inflow velocity causes growth in sound pressure level (SPL), which is ascribed to the asymmetry between advancing and retreating blades at high velocity [18]. The same effect is observed for increasing RPM, which results in rise of SPL in tonal and broadband noises [19]. Broadband noise is often due to the interaction of rotor blades with turbulence, resulting in random blade loading variations [21]. It is also reported that the vortex sheddingnoise and flow separation on the blades manifest as a second hump in the broadband noise of an isolated propeller [22]. Furthermore, increase in the pitch angle decreases SPL at first and second blade pass frequencies (BPF) [18; 20]. While the major source of noise in the UAS is propellers, the propeller interaction with UAS structures has a significant contribution to the UAS noise signature, particularly for some of the novel multi-rotor UAS designs with hybrid configurations (e.g., quadrotor biplane tail-sitter VTOL [1]), in which propellers' wake and tip vortices might even interact with fixed wings used to enhance the UAS efficiency in forward flight mode. Such an interaction imposes unsteady loads on the UAS structure (wing and fuselage) and propeller blades, which are the source of the structure-born noises and alter the noise level [23; 24; 25; 26; 27; 28; 29; 30; 31; 32]. In an experimental study, [PERSON] [29] found that for a tractor configuration of a propeller-wing-nacelle system, the noise due to the propeller-wing interaction may be reduced by decreasing the propeller loading, because the blade tip vortices are the main source of structure-born noise. [PERSON] [29] also showed that the noise level increases with decreasing propeller advance ratio (\(J=U_{0}/nD_{p}\), with \(U_{0}\), \(n\) and \(D_{p}\) denoting the inflow velocity (m/s), rotating speed (rev/s) and propeller diameter (m)) by increasing the propeller's rotating speed. Similar finding was also reported in [32]. However, such a relationship between noise and propeller advance ratio is not always the case, because the unsteady loading is not monotonically dependent on \(J\)[29]. The noise level was also shown to be correlated with the spacing between the propeller and the wing/fuselage such that increasing the spacing would reduce the load fluctuation and consequently the corresponding interference noise [29]. [PERSON] et al. [32] investigated the effect of pitch angle on the noise generated by the propeller-wing system and have shown that this effect is negligible, although it should be noted that the tested range was limited to \(-5^{\circ}<\theta<10^{\circ}\).