# Advancements and Applications of Rim-Driven Fans in Aerial Vehicles: A Comprehensive Review
## 4 Type of Electric Motor
Through a series of tests involving motors with matching active radial dimensions, it was determined that the slot-less motor exhibited lower efficiency compared to the slotted motor. Compared to the slotted motor, the slot-less motor had a longer active length, longer end windings, and bulkier magnets. Despite this, [PERSON] and [PERSON] [58] insisted that the slot-less motor design was superior, especially when the motor was mass-produced. As the current density in the stator increases, heat is generated, which can affect the performance of the motor and potentially lead to failure. To mitigate these effects, various design strategies have been investigated, including the use of materials with high thermal conductivity, such as copper or aluminum, improved cooling techniques, and reducing the stator's electric resistance. As mentioned earlier, [PERSON] and [PERSON] [59] focus on optimizing the design of a slot-less permanent-magnet brushless motor with helical edge-wound laminations for rim-driven thrusters. They use a thermal finite element analysis to evaluate the temperature distribution inside the motor and investigate design parameters that can improve heat dissipation. Additionally, they propose a new method for winding the stator using helical edge winding laminations to reduce the thermal stress on the stator. Powered by a permanent-magnet (PM) motor, [PERSON] devised and manufactured a prototype RDT with 120 hp at 500 rpm [60]. The propeller, housings, structural blading, motor canning, and fairings were all made of composite materials to reduce weight and cost and to prevent eddy current losses in the motor. The composite motor's efficacy was 6% greater than that of a metallic motor (predicted 98.188% for composite, 92.285% for metallic). Moreover, the cost of this composite RDT was 35% less than that of a metal instrument. In the study by [PERSON] et al. [61], a Halbach array was incorporated into the design of an RDT PM alternating-current (AC) motor to increase the magnetic density in the air gap and reduce the magnetic leakage that occurs in conventional PM motors. The motor's rotor was quite narrow, and the air gap was quite substantial. The air gap flux density increased as the PM thickness increased, indicating that the Halbach array would be advantageous if the PM thickness reached a certain threshold. [PERSON] et al. highlight the importance of using two-segment Halbach arrays with unequal segment arcs, which enables a thin rotor and large air gap for an efficient propulsion system and provides an analytical solution to the magnetic field to aid in the discussion of rotor structures [62]. They conclude that the Halbach array improves the air gap flux density and flux density distribution while the rotor core improves the air gap magnetic field. The paper also provides an optimization method for the Halbach array and proposes different design processes for the stator of the integrated motor, with a slotted stator design being found most effective. The optimized designs are validated through static FEA. Experimental data of the integrated motor are presented, and water tunnel experiments of the IMP prototype show good agreement with the calculated results, but the measured rotational speed and output thrust were less than expected, due to friction loss on the rotor surface. It was found that the friction loss is mainly on the outer surface of the rotor through CFD calculations. [PERSON] and [PERSON] came up with the idea of a rim-driven high-temperature superconducting (HTS) motor in order to further cut down on power loss, boost the efficiency of the PM motor [38], and make the RDT more compact. The HTS motor is more efficient electromagnetically, as well as being more compact and thinner than the PM motor. A decreased motor thickness results in a significant reduction in unwanted drag force. This structure can enable the construction of an improved electric propulsion system that combines the advantages of rim-driven and superconducting motors, which can propel a vehicle in a superior manner.