Design and Analysis of a Liftfan for eVTOL Aircraft

Abstract Ducted liftfans can provide greater hovering efficiency to eVTOL aircraft than open propellers of an equal disk area. The goal of liftfan design is to minimise length in order to limit the weight added by propulsors and the drag incurred during forward flight. The increased complexity of liftfans over open propellers poses three challenges which are addressed in this paper. First, instead of a single row propeller, the liftfan consists of a complete stage which must be designed as a whole. The preliminary and 3D design variables of the rotor row and splittered-diffuser stator row are optimised together using 3D CFD to maximise the hovering figure of merit, a non-dimensional measure of power consumption. The resulting prototype liftfan design is validated through experimental testing. Second, unlike an open propeller, a liftfan rotor is enclosed within a complete nacelle. To be viable, it must perform effectively during edge-on forward flight, as well as in pure hover. A low-order model is developed to investigate the maximum range for different fan designs with varying diffuser area ratios and forward flight tilt angles. Design selections are validated using full annulus 3D CFD and experimental wind tunnel tests. Third, as the electric motor of a liftfan is mounted within the hub, fan-driven active cooling is necessary to prevent overheating. The stagnation pressure losses incurred by cooling airflow must be minimised without impeding heat transfer. Low-order models are developed to predict motor heat transfer and loss and to guide the design of a mixed-flow cooling fan. The addition of forward sweep and adoption of a high blade count are found to reduce cooling fan loss, confirmed through 3D CFD simulations and experimental testing.