# Advancements and Applications of Rim-Driven Fans in Aerial Vehicles: A Comprehensive Review
## 3 Innovation along with Development  
In a German patent from 1940, [PERSON] presented the basic design of an RDT [24], with the rotor mounted on a ring around the propeller and the stator coils placed in the duct of the thruster. Several RDT patents have been issued as an outgrowth of this work [29; 30; 31; 32; 33; 34; 35].  
With the development of modern electric motor technologies, the practical implementation of an efficient, compact RDT became possible in the following decades [36; 37; 38; 39; 40; 41; 42; 43]. In recent years, a growing number of research institutions have engaged in researching RDT technology. This technology has been thoroughly explored in relation to incompressible flows, its potential applications in the marine industry have been extensively studied, and some products have been implemented on operational ships [24; 44; 45; 46; 47; 48; 49]. Efforts are being made to investigate the use of rim-driven thrusters, or RDFs, in compressible fluids, as it has not been explored in depth despite its potential. This technology is becoming increasingly relevant to the aerospace industry, leading to a growing interest in studying its applications in compressible flows.  
The NASA conceptual design investigation centered on creating a levitated ducted fan (referred to as a rim-driven fan) with a diameter measuring 32 inches [50]. The study specifically concentrated on the electromagnetic structure of an individual rotor within the rim-driven fan mechanism. The fan consists of 24 blades that extend from the rim towards an empty core rotor assembly. The propellers in question exhibit enhanced efficiency and mitigate the constraints that are commonly encountered in traditional aircraft propulsors, including but not limited to bearing wear, leaks, seals, and friction loss. The levitated ducted fan exhibits several advantages pertaining to its maintainability, reliability, and safety. A significant proportion of maintenance expenses in traditional engine systems can be attributed to malfunctions in the lubrication system and mechanical bearings. The implementation of magnetic suspension systems effectively mitigates apprehensions that are commonly associated with conventional bearings, including but not limited to active lubrication, wear, and restricted rotational velocity [50]. While the study focuses on the electromagnetic circuit design, it does not discuss structural, aerodynamic, or thermodynamic aspects such as loadings, blade aerodynamics, heat dissipation, torque and speed control, and thrust-to-weight ratio. It would have been useful to see the airflow pattern through the central portion and to have more information regarding the design's estimated thrust-to-weight ratios.  
A design for a compressible fluid capable of achieving a weight-to-thrust ratio of one-to-one was suggested by [PERSON] et al. [25]. However, their prototype featured a centerline bearing that disrupted the core flow, appearing similar to a traditional hub fan design in terms of aerodynamics.  
A proposal for a small rim-driven fan jet engine has been introduced to aid in VTOL and STOL for mid-sized and large airplanes [51]. The 50 kg engine is expected to provide 900 kg thrust using a high-efficiency motor and fat-blade fans. The compact design allows for installation on rotating wings and a 7 m triangular airplane with nine RDF jets is suggested. A new 3D HK super-capacitor is proposed for high power, along with lightweight aero gas-turbine generators. However, this is just a proposal and further development is needed.