results of the VPP. It is well known that we first sailed on foils in August 2012 in the first AC72 yacht, however the design team had been “foiling” for 12 months prior to this by utilising the predictions of the VPP. This led to many changes in the design of our wing.’ The computational methods involved 3D RANS CFD methods using ANSYS CFX for the data generation for wing forces. Extensive 2D analysis using MSES was used across a range of design para - meters for wing and flap configurations as a basis for many of the wing design trade- offs. Optimised wing and flap sections were developed using an approach that combined 3D CFX simulations with 2D design using multi-point optimisation. Early in the process it was identified that the relatively open nature of the rule created a large design space to be explored. The principal areas were identified as: 1 The number of elements – single, two- element or three-element 2 The position of the wing rotation point 3 Overall planform 4 Tack location of upwind and downwind headsails 5 Main element-to-flap ratio 6 Main element twist 7 Flap twist profile 8 Wing sectional shapes.
28 SEAHORSE
MSES results gave an early indication that a two-element wing would have good performance over a wide range of lift coef- ficients. A single element would not produce enough lift for light airs down- wind sailing and three-element wings were viewed as too complex for a fairly minimal increase in maximum lift coefficient. Placement of the ball position had to be settled early as it had a large effect on the structural design of the boat. The ETNZ position was further aft than most of the small AC72 fleet. This had desirable aero- dynamic interaction effects with the head- sail, but also balanced torsional loads in the wing, reducing mainsheet loads, easing wing trim, and allowed for a smaller and lighter mainsheet winch.
The rule specified maximum tack- forward location points, but it became apparent particularly upwind that the J- measurement could be reduced with slight performance sacrifice only in light airs. At the time the Code Zero J-measure- ment was not so clear, but it was reduced from the maximum primarily to save weight in the structure of the prod and under-rigging. As it turned out, however, the increased speed produced by foiling and improved platform aerodynamics meant that ‘whenever we were foiling it
was better to have no Code Zero at all and just use the jib’.
Similarly, considerable early attention on flap ratios, planform shapes and the flap hinge line was found to offer insignifi- cant performance potential compared with gains being found in the platform aero - dynamics. The focus shifted to producing a wing that was easy to trim quickly to its optimal shape.
Leading Element (W1) twist in the wing did show benefits, though it was mechani- cally difficult to achieve and carried a weight penalty. In the main element, twist was confined to the bottom half and was designed to max out at 16°. [The VPP showed that within the specified wind range of 5-30kt optimal twist of the main element and rear flap combined should be as much as 50° when reaching – but the team opted for a figure closer to 45]. For the section shapes three different profiles were optimised at 25%, 50% and 75% of the wing height. At the 25% point the VPP pointed to quite a thick section to promote laminar flow. However, the designers reduced that in the interests of weight saving. At the 75% position the optimiser indicated a rounder-nosed sec- tion with the position of maximum thick- ness further forward. The 50% position
CHRISTOPHE FAVREAU/DPPI
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