# Advancements and Applications of Rim-Driven Fans in Aerial Vehicles: A Comprehensive Review
## 2 Features and Benefits of the RDF Jet  
The ducted fan concept, which uses a rotor to generate thrust enclosed in a cover, has been used since the 1960s and is widely used in manned aircraft. Electric ducted fan (EDF) is the same concept, but the rotor is driven by an electric motor. EDF offers advantages including high thrust-to-weight and thrust-to-diameter ratios, physical and environmental protection, and thrust vectoring. Design parameters, such as the end clearance and outlet opening to fan area ratio, affect performance parameters such as thrust, RPM, energy consumption, efficiency, and noise [19]. Hence, compact electric ducted fan (EDF) modules enable the integration of propulsion configurations that distribute thrust, yet their usage remains confined to scenarios involving low-speed flight, like those observed in general aviation and small unmanned aerial systems. Owing to their design centered around a central hub, diminutive single-stage rotors necessitate exceedingly elevated rotational velocities to attain higher exhaust airspeeds, a limitation that poses significant challenges [20; 21; 22]. However, rim-driven fan (RDF) technology has emerged as a possible solution. In contrast to conventional motor designs, this technique allows for smaller fan inlet diameters while still producing significant thrust and exhaust air velocity. The rim-driven fan (RDF), rim-driven thruster (RDT), or rim-driven propulsor/propeller (RDP) is an innovative electric propulsion unit. Unlike conventional propellers, which have a center hub through which the driving torque is transmitted, rim-driven thrusters only have a rotating outer ring [23]. In contrast, the blades of a rim-driven thruster are not attached to a central hub but rather to an outer ring. The electric motor's rotor, which is the ring, rotates inside the stator, which is also a ring, to provide the required torque. The stator of the electric motor is installed in the duct, and the rotor forms a ring around the propeller's rim in this innovative design (Figure 1). In recent years, the maritime sector has witnessed the emergence of a promising ship propulsion option--the shaftless rim-driven thruster (RDT) [24]. The concept of propulsion being driven by its rim is not a recent innovation. As far back as 1957, the idea of a mechanically rim-driven ship's propeller had been proposed [25]. Since then, rim-driven propeller (RDP) devices for both surface and underwater vessels have become commercially available [24], thanks to the widespread adoption of electrical rim drives in marine applications. Nevertheless, references to rim-driven technologies have been comparatively scarce within the automotive and aerospace industries, particularly with regards to aviation. The advancement of rim-driven fan (RDF) technology for aerospace applications has encountered a more gradual pace of development. This lag may be attributed to various factors, including the challenges associated with larger base forms and rotating component masses, technical complexities, and limitations in the available technology at the time. These factors might have contributed to the comparative absence of rim-driven concepts in the aerospace field. Nonetheless, recent technological advancements have reshaped this landscape. Progressions in computerized modeling and analysis methods, developments in materials science, the emergence of 3D prototyping, advancements in unmanned aircraft system (UAS) technology, and breakthroughs in power electronics and electric propulsion systems have jointly driven the progress and assessment of motorized RDFs for aerospace applications, including within the domain of unmanned aircraft. These developments have substantially reduced barriers that once hindered progress. While potential challenges and intricacies are outlined in the subsequent sections, the momentum gained through technological strides suggests that the incorporation of electric motors in aviation might offer a viable solution to the imperative of electrifying airplanes.