Autonomous Surface Vehicle Free-Rotating Wingsail Section Design and Configuration Analysis

The Atlantis Project is a wing-based autonomous sailing vessel, based on a Prindle-19 catamaran, that has demonstrated line following under automatic control to an accuracy of better than 0.3meters. This work details both the design of the wing section and the configuration analysis for a free-rotating, aerodynamically actuated wingsail that is used as themain propulsion system. Based on numerical airflow analysis using PANDAandXFOIL of the low Reynolds number aerodynamic performance, a symmetric section with a small trailing-edge flap is used in a conventional arrangement (tail behind the main section). This allows efficient sailing to both port and starboard, as well as retaining high-lift coefficients for broad reaches. The inability of the flow to withstand adverse pressure gradients at the typically low Reynolds number causes separation with flap deflection, thus necessitating a narrowchord flap that is deflected through a large angle for high-lift applications. The designed section has aCLmax of 1.8 at a Reynolds number of 229,000, and a flap deflection of 45 deg. The wing section is 21% thick, with a 13% plain flap to achieve this maximum lift coefficient. Four different configurations are evaluated for stability and swept radius (the farthest distance that the entire wingsail reaches as it rotates about the mast). The configurations evaluated are 1) conventional, 2) canard, 3)flyingwing, and 4) free-floating canard. The canard and theflyingwing cannot bemade passively stable with a high coefficient of lift, whereas the free-floating canard and the conventional configuration achieve the stability criteria. The free-floating canard has a larger swept radius and has issues with stall stability that are not covered by this simplified analysis, thus the conventional configuration (tail behind the wing) is determined best for this application.