Investigation of 3D dynamic stall within the STELAR Project

Abstract

Dynamic Stall is one of the most challenging flow phenomena existing in the field of helicopter aerodynamics. Under fast forward flight or maneuver, the retreating blade of a helicopter can temporarily stall, leading to a rapid change in pitching moment. Dynamic stall results in a completely different aerodynamic behavior compared to static stall and cannot be effectively approximated by static measurements or computations. The elasticity of helicopter rotor blades, and the complexity of the aerodynamics of fast forward flight mean that experimental data of sufficient quality to validate numerical models is only possible using simplified, structurally stiff wind tunnel models. These experiments create a complex, unsteady aerodynamic flow, which is nevertheless relatively simple when compared with the free flight of a complete helicopter. The resulting flow can be directly compared with high resolution numerical computations. Historically, research into dynamic stall has strongly relied on prescribed pitching on two-dimensional (2D) models at constant Mach numbers. Wind tunnel experiments on finite span, nominally 2D models for these test cases, provide reference data for 2D and three-dimensional (3D) numerical simulations, and this method has led to significant advances in the numerical prediction of dynamic stall. To improve future rotorcraft, it is necessary to understand how 3D dynamic stall differs from 2D dynamic stall, so that the value of the existing 2D dynamic stall experimental data can be maximized.