Toward a Better Understanding of Ducted Rotor Antitorque and Directional Control in Forward Flight

Despite decades of very successful yaw-control and anti-torque applications, the aerodynamics of ducted rotors in low-power, near-edgewise flow conditions are not well understood. Motivated by phenomena discovered during the development of the RAH-66 Comanche’s directional axis control laws, a research program was initiated to use CFD to improve the understanding of the dynamic relationship between ducted rotor thrust and applied collective pitch, especially when the rotor is operating in near-edgewise flight conditions. This paper is a presentation of the initial results of this study. Numerical solutions of the inviscid Euler equations were obtained for the flow over the Comanche fuselage with a uniform actuator disk and blade element models for the FANTAILTM; the main rotor is excluded in this study. The solutions were obtained by running a modified PUMA 2 (Parallel Unstructured Maritime Aerodynamics) computational fluid dynamics code with an unstructured grid with 2.8 million tetrahedral cells. Excellent correlation between the calculations and a variety of static test data are presented and discussed. Subsequent efforts will examine the important aspects of the dynamics of the thrust response, and allow further comparisons with flight test data.