Investigation of Dynamic Stall Effects on Isolated Rotor Flap‐Lag Stability with Experimental Correlation

The effects of dynamic stall lift and drag on the flap‐lag stability of an isolated hingeless rotor blade are investigated. The emphasis is on the correlation with measured regressing lead‐lag mode damping levels of a soft‐innlane, three‐bladed model rotor, operated untrimmed. The correlation covers a wide range of test conditions for several values of rotor speed, collective pitch angle, shaft tilt angle, and advance ratio. It includes cases that vary from near zero thrust condition in hover to highly stalled forward‐flight conditions with advance ratios as high as 0.55 and shaft tilt angles as high as 20°. Both the experimental and analytical blade models represent a simple model of a hingeless rotor with rigid blades and spring restrained flap‐lag hinges. The aerodynamic representation is based on the ONERA dynamic stall models comprising virtually independent unified lift and drag models. The nonlinear equations of blade motion and stall dynamics are perturbed about a periodic forced response, and the damping is evaluated by the Floquet eigenanalysis. In comparison to the linear and quasisteady stall aerodynamic theories, the theory with dynamic stall lift and quasisteady stall drag qualitatively improves the correlation, and adding dynamic stall drag provides further quantitative improvement.