SVN solovelanet

47 SVN Solovelanet Global Trim in neutral position Positive trim generates greater lift 2 Negative dihedral angle WIND Rudder Foil The lift of the foil also generates a righting moment 1 Lift Weight Righting moment Righting moment WIND V F D L Resolution of force F in the components: L = Lift D = Drag V = Vertical lifting force 3 Overturning moment Moreover, the foils are designed with a non-zero dihedral angle. This means that the two "arms" of the "T" are not perpendicular with the stem, but instead form an angle with this. All the four teams of this America's Cup chose to have a negative angle, meaning that the arms point downwards (just a little for the Kiwis, definitely more for Luna Rossa). If, on one hand, a negati- ve dihedral reduces stability, on the other hand it enhances agility. In other words, the boat is more sensible to rolling perturbations, but it can also react more quickly to them. This all contri- butes to make the glide even more unstable. A possible reason for such choice is that, with a negative dihedral, the internal aerodynamic sur- face of the submerged foil is relatively horizon- tal, optimizing the lift direction, while the exter- nal surface will be definitely more tilted and will produce more righting moment thanks to a lon- ger arm with respect to boat's longitudinal axis (see Figure 1). Interesting news of this edition is also the possi- bility to act on the foil's trailing edge by modifying the so-called trim surface. Just like aircrafts have flaps and tabs, it will be possible to increase or reduce the lift produced by the hydrofoil by acti- vely tilting a small portion of the foil itself located on its rear part. By tilting this surface down (po- sitive trim), more lift is produced, and vice versa (see Figure 2). The rudder deserves a separate explanation, though. For all the four teams it has a "T" shape, it is obviously symmetric on the lon- gitudinal plane, and has a zero dihedral angle. Apart from tacking, it will have the additional role to actively compensate the boat's pitching an- gle, by means of continuous adjustments of the rudder's trim angle, most likely both with positive and negative angles. The introduction of the trim angle in the AC75 allows a full control of the boat. The AC72 and AC50 did not have any movable trailing edge on the lateral foils, and the amount of lift produced by them was adjustable by means of the cant and rake angles. In the next America's Cup the hydrofoils will be handled by hydraulic actuators, which use electricity stored into batteries carried onboard. What a relief for the grinders, which now have to paddle for generating electricity for the rig only, and not for the foils. This might save them a lot of sweat, since the AC72's cycles are not allo - wed in this edition.

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