Flying Shape and Aerodynamics of a Full-Scale Flexible Olympic Windsurf Sail

Abstract The introduction of hydrofoils in windsurfing has fundamentally changed the aerodynamic loads on modern rigs. While past studies including recent ones in windsurfing or sailing aerodynamics have relied on reduced-scale rigid sail models and numerical simulations, there is a lack of experimental data on the full-scale flying shape of deformable sails. In particular, iQFOil class - new Olympic windsurf class - still lacks of direct measurements to potentially validate Fluid Structure Interaction model, in order to improve the accuracy of Velocity Program Prediction. The 3D flying shape of a real-scale 8 m2 iQFOiL class windsurf sail is measured in steady state sailing configurations. The outdoor conditions are simulated in a large-scale wind tunnel and the flying shape is reconstructed with a stereo camera imaging technique. Together with the sail shape, we measure simultaneously the aerodynamic forces and moments applied to the sail. With the measured forces and moments, the lift, drag and roll coefficients are determined for wind velocities ranging from 4 to 8 m/s. A systematic decrease of these coefficients is observed as compared to previous studies on reduced-scale rigid sail model, because of the rigging deformation due to wind loading. We thus establish experimentally benchmark data for the iQFOiL sail and more generally for compliant rigging. Keywords windsurfing; sail aerodynamics; wind tunnel test; full scale experiment; fluid-structure interaction; photogrammetry