Torque-Speed Characteristics for Electrically Powered Rim Driven Fans and Thrust Comparison with a Small Conventional Fan Jet Engine

The aim of this paper is to provide a method of estimating torque versus speed characteristics of single and dual-stage electrically powered Rim Driven Fans which are intended for aircraft propulsion. The methodology is based on the well-known Euler equation which considers the change in angular momentum of the air as it passes through the fan rotors. A derivation of the useful and versatile Specific Work parameter (Y) is provided along with its important relationship with the fan Work Co-efficient ($\psi$) and an explanation of the relevance of the Fan Flow Co-efficient ($\emptyset$) in determining the flow of air through the RDF device. An equation is derived which relates the fan torque to its rotational speed and a specimen calculation of a 200 mm inlet diameter RDF has been provided. Electrical performance graphs, generated with Motor-CAD LAB software, are included to illustrate an example of a suitably optimised RDF motor circuit. Finally, a thrust performance comparison is made between a theoretical dual-stage RDF and a commercially available fan-jet engine. It is demonstrated that the dual-stage RDF technology could offer a viable solution to power high-speed medium and large commercial transport aircraft, making them particularly suited to distributed thrust system architectures and blended-wing-body aircraft designs.