Modeling Aerodynamics, Including Dynamic Stall, for Comprehensive Analysis of Helicopter Rotors.

To fulfill the objective of a predictive tool for rotorcraft, comprehensive analysis (CA) needs to be capable of providing both accurate and time-efficient predictions of rotor air loads and structural loads. The more recent methodology based on comprehensive analysis coupled with high-fidelity computational fluid dynamics (CFD) has shown improved predictions of air loads, but it has not the strength of computational efficiency and the versatility of stand-alone CA. The present article is concerned with modeling aerodynamics about helicopter rotors for CA. The aerodynamics about rotors are very complex, encompassing subsonic to transonic flow with unsteady, stalled behavior and 3D effects. CA treats aerodynamics as separated into local and global flows. Semi-empirical models of dynamic stall were created in the 1970s-1990s for modeling unsteady local aerodynamics, including stalled flow. Most of them fail to provide good predictions of experimental results and also suffer problems of numerical convergence. The main effort in this study is about modeling local aerodynamics based on the revised "ONERA-Hopf bifurcation model". It is implemented in the comprehensive analysis code of ONERA according to a scheme that ensures numerical convergence. The experimental results obtained in the Wind Tunnel S1 of Modane (France) in 1991 on the Rotor 7A are considered for validation of the analysis under three flight test conditions: high-speed test, high-thrust tests with light stall and deep stall, respectively. There is a reasonable agreement between the predictions of CA with experimental results. The distinct features of the stall model are the modeling of the boundary-layer effects and the vortex-shedding phenomenon. [ABSTRACT FROM AUTHOR]