Experimental and Numerical Analysis of Aerodynamic Flap Hinge Moment under Unsteady Flow Conditions on a NLF Airfoil

Experience has demonstrated the importance of accurately modeling hinged control surfaces in flutter analysis. It is state of the art in the flutter analysis to model unsteady aerodynamics with linearized potential-based methods, that can be improved with higher fidelity CFD simulations or wind tunnel data. However, most studies regarding unsteady aerodynamics including control surfaces do not consider the influence of transition on hinge moments. On modern NLF airfoils, the boundary layer on the pressure side remains laminar beyond the flap hinge. To avoid the occurrence of laminar separation bubbles (LSB) and the subsequent increase in drag, a turbulator is placed just upstream of the main pressure rise. Its position is optimized for steady flow cases considering the entire flight envelope and the different flap settings. However, an oscillation of the flap can cause a movement of the boundary layer transition upstream of the turbulator or even a LSB 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 CFD simulations are performed on a 2D Natural Laminar Flow (NLF) airfoil section with forced periodic flap deflections in incompressible flow at operational Reynolds numbers from 15e5 < Re < 35e5 matching full-scale flight conditions. The goal of the study is to model unsteady forces resulting from fast oscillations that may occur during control surface flutter on a sailplane wing to reduce the uncertainties in the flutter prediction. The results obtained experimentally and numerically with the Gamma Transition model regarding pressure distributions, integral aerodynamic coefficients, and hinge moments are presented and discussed.