A VTOL-UAV inlet flow distortion reduction concept using a new flow control approach: Double-ducted-fan (DDF)

This paper briefly describes a novel ducted fan inlet flow-conditioning concept that will significantly improve the performance and controllability of ducted fans operating at high angle of attack. High angle of attack operation of ducted fans is very common in VTOL (vertical takeoff and landing) UAV systems. The new concept that will significantly reduce the inlet lip separation related performance penalties in the edgewise/forward flight zone is named DOUBLE-DUCTED FAN (DDF). The current concept uses a secondary stationary duct system to control inlet lip separation related momentum deficit at the inlet of the fan rotor occurring at elevated edgewise flight velocities. The DDF is self-adjusting in a wide edgewise flight velocity range and its corrective aerodynamic effect becomes more pronounced with increasing flight velocity due to its inherent design properties. In this manuscript, after a comprehensive discussion of VTOL inlet flow distortion issues, a conventional baseline duct without any lip separation control feature is compared to two different double-ducted fans named DDF CASE-A and DDF CASE-B via 3D, viscous and turbulent flow computational analysis. Both hover and edgewise flight conditions are considered. Significant relative improvements from DDF CASE-A and DDF CASE-B are in the areas of vertical force (thrust) enhancement, nose-up pitching moment control and recovery of fan through-flow mass flow rate in a wide horizontal flight range.