SVN solovelanet

SVN Solovelanet Global The latter will likely have a negative trim, in order to produce downforce, hence providing some ex- tra righting moment. The head part of the main- sail will also be counter-intuitively trimmed such to produce downforce (lift in windward direction), hence righting moment. Basically, the boat will accelerate in a delicate equilibrium and with little heel. As soon as it gains speed, the aerodynamic forces increase their magnitude, since they are function of the velocity squared (see Deepening Box "The Lift"). Slowly but consistently both the foils must be opened. This will reduce the overall stability, since the boat center of mass moves up, but it is necessary. The boat accelerates in this un - stable equilibrium of forces: on one side the wind pushing on the sails, on the other the two foils and the upper mainsail portion providing righting mo- ment. Reached a critical speed, the boat starts to glide and to lift. This is a critical moment, when the crew must coordinate perfectly between opening the foils and trimming. The upper portion of the mainsail will be reversed in its normal position; the mainsail becomes powerful enough to allow take off and foiling. The windward foil, now completely open, is totally emerged and contributes to the righting moment only thanks to its weight. Tacking and gybing Tacking while foiling is everything but easy. Avoi- ding a nosedive is essential. The hardest part is to coordinate the foils angle variation with the yaw imposed by the rudder. In fact, there is a moment when the windward foil must get into the water and the leeward get out of it. If the former goes down too soon, the boat loses righting moment and capsizes downwind. If done too late, the foil might not be ready to provide enough lift when the boat tacks, resulting to a capsize on this side as soon as it turns its bow through the wind. When approaching a tack, the upwind foil should be downed just below the surface and its trim adjusted to a negative angle in order to produce downforce. On so doing, it "sticks" to the water and still provides a sufficient amount of righting moment. Meanwhile, the rudder starts yawing. The action of immersing the upwind foil into the water is very similar to hand braking in a kids snow sledge on one side only. You basically get two effects: on one hand the foil act as a pivot and helps the boat to turn. On the other hand, it acts like a brake and slows the boat down. L et's assume to select an aerodynamic surface (e.g. a wing) and to slice it perpendicularly. The 2D section that we are watching is technically cal- led "profile" and it is the very essential part of the wing, what defines its properties, basically its DNA. Then, let's imagine connecting the leading edge and the trailing edge (the furthermost and the rearmost points of the profile, respectively). Such line is refer - red to as profile's chord. It's worth reminding that a profile does not produ- ce two separate forces called lift and drag, like we often hear, but only one. Generally, this force points towards the high-pressure side of the profile and slightly backwards. Hence, the distinction into lift and drag is only a force decomposition in the two directions of interest: the lift is the component per - pendicular to the incoming flow, the drag is the pa- rallel one. These two forces can mathematically be expressed as: where ρ is the fluid density, A is a proper reference area, v is the fluid velocity at leading edge, and C_L and C_D are the lift and drag coefficients respecti - vely. First, let's keep in mind that water is circa 1000 times denser than air. Hence, the aerodynamic forces ge- nerated from a foil working in water are 1000 times greater than those generated from the same profile traveling in air, all the rest being constant. That is why sails are much larger than foils or keels, and ne - vertheless there is equilibrium. Moreover, because of the shape of the aerodynamic profiles, the lift coefficient is generally much greater than the drag coefficient, with a lift-to-drag ratio CL/ CD ranging from 10 to 50 and even higher for very advanced uses (e.g. on gliders). That is why, often, sailing close-hauled is said to be more "efficient" than running downwind. Not only because the ap - parent wind in running downwind is lower, as we all know, but also because the propulsion mechanism itself is definitely less efficient. The lift

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