Gaonkar, Gopal H.

The effects of dynamic stall, lift, drag and pitching moment on the aeroelastic stability of hingeless rotors are predicted. The emphasis is on correlating the predictions with the measured lag-damping levels of a three-bladed model rotor operated in an untrimmed mode. The correlation covers a wide range of test conditions for several values of rotor speed, collective pitch angle, shaft-tilt angle and advance ratio. The database includes cases that vary from near zero-thrust conditions in hover to highly stalled forward-flight conditions with dimensionless speed or advance ratios as high as 0.55 and shaft angles as high as 20$\sp\circ$. The aerodynamic representation is based on the ONERA dynamic stall models comprising virtually independent unified lift, drag and pitching-moment models. The nonlinear equations of blade motion and stall dynamics are perturbed about a periodic forced response, and the damping is evaluated by Floquet analysis. The extensive correlation study, based on a rigid-blade flap-lag analysis, demonstrates the viability of Floquet analysis in predicting lag-mode damping under dynamically stalled forward-flight conditions. It also demonstrates the limitations of the linear and quasisteady stall aerodynamic theories. In comparison to these theories, the theory with dynamic stall lift and quasisteady stall drag qualitatively improves the correlation and is viable over the entire range of the database. Addition of dynamic stall drag provides further quantitative improvement. Also presented is a comparative study of two dynamic stall drag models: circulation-like drag variables incorporating unsteady air-velocity variations and conventional drag-coefficient variables. While the formulation with drag coefficients exhibits computational sensitivity to convergence with respect to blade discretization for some isolated cases, the formulation with circulation-like drag variables removes this sensitivity and is computationally robust. The investigation concludes with an elastic blade analysis that includes blade flexibility in lag bending, flap bending and torsion as well as root-flexure elasticity. This analysis shows increasing sensitivity to structural refinements in blade and root-flexure modeling, and this sensitivity increases with increasing pitch setting. Correlation and parametric studies show that the root-flexure elasticity introduces significant bending-torsion couplings that have considerable impact on the stability of a rotor for which the root-flexure is soft, and the blade is stiffer in comparison with the root. This research is expected to serve as a reference comparison with other correlations based on different approaches of modeling dynamic stall and the elasticity of hingeless rotor blades and root-flexure.