Aerodynamic Characteristics of Laminar Airfoils with Oscillating Flaps: Experimental and Numerical Study

In the flutter analysis, unsteady aerodynamics are typically modeled with linearized potential-based methods, that can be improved with higher-fidelity methods. However, most studies regarding unsteady aerodynamics including control surfaces do not consider the influence of transition on hinge moments. On modern natural laminar flow (NLF) airfoils, a turbulator is strategically placed just before the main pressure rise to prevent laminar separation bubbles and the associated drag increase. Its position is optimized for steady flow across the full flight envelope and various flap settings. An oscillation of the flap can cause a movement of the boundary-layer transition upstream of the turbulator or even a laminar separation bubble that is able to overcome the turbulator. This may have an impact on the unsteady flap forces and moments, which is not accounted for in the classic flutter prediction. Wind-tunnel measurements and computational fluid dynamics simulations with the Gamma Transition Model are performed on a two-dimensional NLF airfoil section with forced periodic flap deflections in incompressible flow at operational Reynolds numbers from Formula: see text matching full-scale flight conditions. The unsteady loads resulting from fast oscillations that may occur during control surface flutter on a sailplane wing are investigated to reduce the uncertainties in the flutter prediction.