# Advancements and Applications of Rim-Driven Fans in Aerial Vehicles: A Comprehensive Review
## 5 Design  
It is absolutely necessary to match the aerodynamic parameters of the propeller to the characteristics of the motor in order to obtain efficient RDFs. According to [PERSON] et al. [57], the pitch ratio of the propeller is the single most critical metric. The hydrodynamic efficiency can also be affected by the structural design of the propeller, the form and size of the duct, the thickness of the air gap, and a number of other parameters. In a study by [PERSON] et al. [63], numerical investigations to analyze the impact of blade count on the hydrodynamic performance with a fixed blade area ratio and pitch ratio are conducted. Their analysis focuses on comparing and contrasting the thrust coefficient, torque coefficient, and efficiency of RDTs with varying blade numbers. Furthermore, this study successfully identifies and meticulously analyzes the pressure distribution on the blade surface, as well as the velocity distribution surrounding the blade and within its wake. It is observed that the thrust coefficient and torque coefficient of the RDT exhibit an upward trend with the advance coefficient, while the efficiency of the RDT displays a downward trend with the increase in the number of blades, as per the findings. A feasibility study explored the potential of using an induction-motor rim drive to operate an aircraft boundary-layer-ingestion (BLI) fan [64]. The primary focus of the research was to identify efficient power density and determine the induction motor rim drive's effectiveness in meeting structural requirements when operating at high tip speeds. Through the proposed 250 kW, 14,000 rpm design example, an efficiency of over 97% was achieved with an active mass of 20 kg. The paper also highlighted a variation of the baseline design, which consisted of 0.35 mm thick cobalt-iron laminations and aluminum rotor bars. This design achieved an efficiency of 96.4% with an active mass of 19 kg, where the aluminum rotor cage kept the rotating mass to a minimum and helped reduce centrifugal stress in the rotor teeth. The high-tensile-stress cobalt-iron laminations used could withstand centrifugal stress, and the addition of support banding could accommodate the aluminum rings easily onto the ring spigots. The research also revealed that replacing aluminum in the rotor cage with copper only resulted in marginal efficiency improvement, leading to a severe increase in centrifugal stress within the rotor. This stress could not be resolved with available high-saturation lamination materials unless additional support banding was added. In summary, the feasibility study demonstrated the potential for using an induction-motor rim drive to power aircraft BLI fans while identifying crucial design considerations needed to optimize power density and efficiency. The results established that efficiencies in excess of 97% could only be achieved using 0.1 mm thick laminations in the stator for the baseline design with the aluminum rotor cage. In the analysis of the slot-less winding characteristics of motors, crucial factors driving the design include geometrical relationships and magnetic requirements [65]. These factorsencompass the width of each phase coil side, the skew angle, and the winding arrangement. Additionally, a preliminary design study of a winding model derived from a Faulhaber configuration has been subjected to scrutiny. It was determined that in order to ensure optimal efficiency, the skew angle must be maintained at a low level. However, it is also imperative that the angle be of sufficient magnitude to meet both the geometric and magnetic criteria. The investigation by [PERSON] et al. [66] delves into the effects of gap geometry on a conventional hubless radial-flow turbine through the manipulation of its axial path length and inlet and outlet oblique angles. The hydrodynamics of the RDT were simulated using OpenFOAM's k-\(\omega\) shear stress transport turbulence model.