capteric36 wrote: > Exact same way an airplane wing works. Pressure > on one side is higher than the other. > In the context of the earlier posting here, it was said that all the lift, or maybe just the vast majority of it, on an airplane wing comes from the pressure (in this case meaning a force trying to increase air pressure) on the underside of the wing.
If this was the case for a sailboat going upwind, it would mean that the fwd motion into the wind was as a result of the wind 'pushing' on the inboard side of the sail. Obviously this is NOT the case! No amount of mere 'pushing' on the inboard/upwind portion of the sail will result in the boat going upwind. If you draw vectors you will see that there must be a total force created as a result of both sides of the sail and both sides of the keel that acts to 'pull' the boat upwind. I think it is important to note if the wind acting against the sail is such that air pressure on that side of the sail would tend to be increased or decreased relative to ambient air pressure. If the only force you had was a 'pushing' force, IE a force that wanted to, or tended to, 'increase' air pressure against the sail/wing on both its sides, then going upwind would be impossible, to go upwind you must also have a force (on the outboard/lee side of the sail/wing ) that wants to 'decrease' air pressure on that outboard/lee side of the sail/wing. A force that wants to 'decrease' air pressure is commonly known as a vacuum or suction force. So on the windward side of the sail/wing there is a force that wants to 'increase' air pressure against the sail and on the leeward or outboard side of the sail there is a force that wants to 'decrease' air pressure against the sail. This can be readily seen as the sail is 'sucked' to the lee. The sum of these forces is known as 'lift'. In the case of upwind sailing the vector of the forces is pointed upwind. If you draw the vector of the 'pushing' force or that is to say a vector that points in the direction of less to more, or increasing relative air pressure acting (call this a positive vector) on the windward side of a sail going upwind you will see it points into or toward the sail and downwind (but not directly downwind as the boat is at an angle to the wind of course. In fact there would be more force trying to shove the boat sideways than there would be trying to shove it downwind if the boat is pointing higher than 45 degrees for example.) Call this a positive vector. The keel would have a vector of a force less than that of the sail (since the boat is being pushed downwind) and one pointing into the keel (opposite the wind direction) and upwind, since it is resisting the force from the direction of the wind. The sum of these two vectors would result in a vector pointing downwind, since the sails inward downwind contribution to the vector sum is the greater magnitude over the keels upwind vector. So the boat would be shoved downwind if these were the only forces acting on the sail and keel, but they are not the only forces. If you draw the vector of the force acting on the leeward side of the sail you will see it points upwind and away from the sail, because the wind splits at the sails leading edge and tries to rise off the sails backside making the decreasing air pressure vector on that side of the sail, that is a vector pointing in the direction of forces trying to make less to more relative air pressure, so it will point away from the sail and toward the leading edge or forward (call this a negative vector). The keels 'windward' vector or vector on the side of the keel facing the wind (it would be a lee vector it we considered the relative water direction for the keel, not wind direction) also points away from the keel and upwind for similar reasons. The sum of these two vectors points upwind since they both point upwind, but the one pointing upwind from the sail is the greater magnitude of the two since it is the keel that is being moved by the wind, in effect, and so the total of these two vectors would be upwind and to the lee. The sum of the downwind and upwind vectors for the sail and keel will be a vector that points upwind, and the boat will then 'lift' itself or be 'pulled' or sucked' upwind. I may have goofed things up a bit here, but I think it is correct more or less. So it seems to me that it is logical to state that suction DOES play a role on wings to make lift at least some of the time, since if the boats negative vectors did not act as they do the boat would only be shoved downwind. Upwind the vast majority of the force must come from lift generated by such a 'pulling' force, but downwind it could be that almost all the force is 'pushing'. -Ken > _______________________________________________ Liveaboard mailing list [email protected] To adjust your membership settings over the web http://www.liveaboardnow.org/mailman/listinfo/liveaboard To subscribe send an email to [EMAIL PROTECTED] To unsubscribe send an email to [EMAIL PROTECTED] The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/[email protected] The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
