Experimental Investigation of Sail Aerodynamic Behavior in Dynamic Conditions

The aerodynamic behavior of a 48’ yacht rig 3D scale model was characterized by means of wind tunnel tests. The experimental program, performed in the Politecnico di Milano Twisted Flow Wind Tunnel, allowed the aerodynamic forces to be characterized using forced motion tests and the aeroelastic behavior of the sail plan to be studied. Preliminary results are presented with reference to the typical encounter frequency range and typical pitch amplitudes corresponding to best yacht Velocity Made Good (VMG) condition for different true wind speeds and related sea states. A new representation of the aeroelastic effects is proposed in the form of aerodynamic hysteresis loops, obtained by plotting the sail plan aerodynamic coefficients against instantaneous apparent wind angle, varied using a harmonic law combining wind tunnel constant wind speed with sail plan center of effort velocity induced by yacht pitch motion. A “sail plan reduced velocity” concept is defined, and experimental results show that this reduced velocity play a fundamental role in sail plan aerodynamics. Finally, an estimate of the aerodynamic added mass effects is provided based on the comparison between the measured pitch moment during the “wind off” tests and the experimental test rig pitch moment of inertia.