Numerical Study of Distributed Variable Geometry Ducted Fans for eVTOLs

The effects of key design parameters of tilting distributed ducted fans are investigated through steady-state CFD simulations to assess the benefits of using variable geometry ducts in urban air mobility applications. The analysis is made on three adjacent ducted fans mounted at the trailing edge of a semi-span wing. The fans are represented by body forces calculated using the blade element theory. The duct expansion ratio, the duct thickness and the fan design expansion ratio are varied along with the fan speed, the crosswind speed in hover and the airspeed in forward flight. For each combination of the parameters, the hover Figure of Merit and crosswind stall speed as well as the forward flight lift coefficient, thrust coefficient and propulsive efficiency are evaluated. From these results, variable geometry ducted fans are benchmarked against fixed geometry ducted fans using a simplified 1 hour mission with 10% of hover time. It is found that a ducted fan equipped with a Krueger flap and a variable expansion ratio diffuser consumes nearly 11% less energy to complete the mission.