Ducted Rotor Performance Calculations and Comparisons with Experimental Data

*† ‡ Ducted fan performance was calculated using the AMI panel method VSAERO coupled with the ROTOR propeller module. Calculations were performed for a set of experimental data measured on a test model in the NASA Ames facility. The ducted fan model was a generic representation of a class of proposed small UAV systems. Good correlation of duct and rotor thrust and aerodynamic pitching moments was found for all conditions under which the duct lip flow was attached. Empirical extensions to stalled duct lip results can be made for high rotor thrust. I. Introduction new class of unmanned aerial vehicles under development are small ducted fans. These vehicles have no separate lifting surfaces but instead rely on rotor and fan-induced thrust for lift and maneuvering. These UAVs operate primarily at low speeds, high thrust, and high angles of attack. An area of particular concern for these UAVs is translational performance in hover. In this flight condition (nearly 90 degrees angle of attack with respect to the duct thrust axis) a large aerodynamic pitching moment is generated which tends to pitch the vehicle away from the direction of translation. This pitching moment requires additional control authority to overcome and in many cases can severely limit the translational speed of the hovering vehicle. A computer code well suited to the task of calculating these flows is the ROTOR module of the AMI panel method VSAERO. ROTOR computes the coupled effects of the propeller induced flow and the external flow around the duct. The code combines a blade element theory propeller calculation with the VSAERO potential flow solution for arbitrary bodies in subsonic flow. The code takes into account induced effects of the propeller such as asymmetric inlet lip suction and the effects of the exhaust flow in thrust vectoring. Computations for a generic ducted fan UAV compare favorably with test results. The overall lift, drag, and pitching moments developed by the vehicle can be calculated with the VSAERO/ROTOR method for all flight conditions in which attached flow is maintained around and through the duct inlet. This includes the high angle of attack and high thrust conditions (representing crosswinds in hover) which are of particular concern to this class of vehicle. While the very high duct angles of attack would initially seem to preclude the successful use of a potential flow method, the high duct thrust and resulting high mass flows through the duct maintain attached flow through the duct. The resulting flow field can be successfully calculated with this method. . Empirical correlations of duct stall can be used to provide limits of attached flow behavior and guidelines for estimates of aerodynamic forces and moments beyond duct stall.