Investigation of the Rotor Stall Boundary Using CFD/CSD Analysis for Passive Rotor Systems

Coupled computational fluid dynamics and structural dynamics methods offer the potential to significantly improve the ability to predict helicopter rotor performance and unsteady loads. Accurate performance and unsteady loads predictions are necessary for the optimal design of the rotor dynamic components and blades. High-thrust and extreme maneuvering conditions, which are important for design, are especially challenging for the analysis tools to predict accurately due to dynamic stall and its strong coupling with the structural dynamics. Evaluation of complex features such as highly 3D tip shapes, composite structures, and active devices further increases the need for accurate analysis tools. Fundamentally, the CFD/CSD methods capture more relevant physics than the comprehensive analyses using lifting-line, but require significantly more computational time. This paper investigates the use of OVERFLOW/DYMORE to capture the rotor stall boundary and rotor efficiency (L/De). The results are compared with comprehensive analysis (RCAS) and experimental data for a representative rotor. A few variations of these analyses were employed to investigate numerical and physical effects such as grid refinement, and laminar-turbulent transition. The performance results are presented in terms of L/De, which historically has been very difficult to predict based on first principles. Preliminary results are discussed for moving flaps at moderate-thrust conditions. Several potential quantities to identify the rotor stall boundary are examined and discussed; of the quantities considered, the steep fall-off in L/De appears to be the best measure of the rotor stall boundary. Transition modeling and grid refinement improved the correlation of L/De with experimental data; it is believed that further refinements are needed to improve high Mach-number drag.