# Advancements and Applications of Rim-Driven Fans in Aerial Vehicles: A Comprehensive Review
## 6 Noise and Vibration  
The development of quiet propulsion systems has become increasingly important in the field of unmanned aerial vehicle (UAV) operations, particularly in reconnaissance and surveying missions where low noise levels are imperative. The use of rim-driven thrusters has been shown to improve both the aerodynamic and acoustic capabilities of aerial vehicles compared to traditional propellers. However, external noise can still come from various sources, including aerodynamic and electromagnetic propeller noise and friction and impact noise caused by unbalanced force and thrust. This study offers insight into the potential for rim-driven fans to operate more quietly compared to conventional impellers, highlighting the importance of reducing both ring and blade commotion in future motor designs. The research delves into how the cogging torque of a rim-electric driven propeller (REDP) is influenced by factors such as the slot number, pole number, tooth geometry, and pole arc coefficient. This investigation offers valuable insights to establish a theoretical basis for studying the motor's impact on vibration and noise [69]. The findings suggest that for a motor with frictional slots and concentrated windings, the cogging torque's amplitude decreases as the lowest common multiple increases. Furthermore, there appears to be a discernible pattern in the relationship between the pole arc coefficient and cogging torque, with fluctuations observed. It is generally known that the typical propeller produces an audible acoustic signature due to blade tip vortices and other flow features [70; 71; 72]. The tangential velocities are reduced and the tip vortices may be minimized if the blade tips are located closer to the center of the fan. The rim-driven thruster is an attractive choice for thrust production since it improves both the aerodynamic and acoustic capabilities of aerial vehicles. This is especially relevant in the field of tiny unmanned aerial vehicle (UAV) applications, which are finding widespread usage in reconnaissance and surveying operations that need a low noise footprint [73; 74; 75]. The frequency spectra of acoustic signal for fanless motors and hubless fans have been plotted for comparison [23]. The absence of air movement-associated broad-band frequencies was evident in the absence of a fan. The fan vibration frequencies drowned out the blade pass frequencies (BPF). The absence of a peak near the BPF may indicate that a rim-driven fan has the potential to operate more quietly compared to a conventional impeller. Efforts are currently being made to reduce ring vibration, and it is necessary to assess the common emanating from the blades themselves. Because conventional propulsion systems rely on drive shafts and reduction gear, shaft-less RDT propulsion devices offer a more comfortable riding experience. This is because these devices do not require the use of these components. However, external noise can come from a variety of sources, including aerodynamic propeller noise, electromagnetic propeller noise, vibration noise created by friction in the bearings, and impact noise caused by the propeller impacting the structure when subjected to unbalanced force and thrust force. The volume of these noises is fairly intense throughout a broad spectrum of frequency ranges. In the next years, it will also be extremely important to make efforts to reduce the sensitivity of RDTs to vibration and noise. In the context of RDF technology structural health monitoring (SHM), [PERSON] et al. propose a novel approach to detect damage in the non-metal sheath of the rim-driven thruster (RDT) motor using helical guided waves [76]. This method employs a pair of MFC sensors on a non-metal hollow cylinder to excite and receive helical guided waves, with damage identification based on energy entropy comparisons in the non-destructive state.