These changing angles of attack are where the force vector diagrams come into effect. You are comparing drag force to lifting force. As the shapre presented to the apparent wind changes the magnitude of the drage force and the magnitude of th lifting force caused by the redirection of the wind chage drastically. I am not sure of the amount of lift this contributes to the total number as we had a hard time isolating just the underside of the airfoil in the wind tunnel.
Two, the coanda effect on the top of the wing. This is where the airfoil needs to have that curved geometry. We are all familiar with the water on the back of the spoon effect. The top of the wing is the back of the spoon and the air is the water. This being said, the air is then accelerated downward causing a force (F=ma). Again, I am not sure of the portion of the total lifting force this contributes, but I believe it is the majority of it.
Hobienick,
Some of what is said here is contradictory. For instance, in the first paragraph above, you state that "As the shapre presented to the apparent wind changes the magnitude of the drage force and the magnitude of th lifting force caused by the redirection of the wind chage drastically." This is demonstrably true and occurs mainly at the area immediately behind the stagnation point on any airfoil. That is the point where the maximum air mass acceleration happens. At the stagnation point, the air mass actually comes to a *stop*, then is accelerated in a new direction over the top of the wing. It continues to accelerate over the balance of the airfoil, but nowhere near as violently as at that small strip just behind the stagnation point.
You state later WRT the balance of the wing chord well behind the stagnation point: "This being said, the air is then accelerated downward causing a force (F=ma). Again, I am not sure of the portion of the total lifting force this contributes, but I believe it is the majority of it."
But relatively little acceleration, and hence reaction (lift) is actually generated here. Collectively, the balance of the wing chord does contribute significantly to lift, to wit the center of effort of the foil is said to act somewhere near the point of maximum thickness. But this is usually well forward, implying that most lift is really being created well forward of that.
I don't know if anyone in this discussion is familiar with this paper, but it is very instructive and supported with experimantal data:
http://www.tspeer.com/Wingmasts/teardropPaper.htmQuting from that paper: "When the pressure changes over some distance, any small volume of fluid will have an unbalanced pressure on each side, and this net force accelerates the fluid, either changing its speed or its direction of motion in accordance with the conservation of momentum. This links the pressure gradients, the bending of the flow direction, and the local flow velocities so as to satisfy the three conservation laws: mass, momentum and energy."
So we would expect that wherever the maximum acceleration occurs, maximum pressure gradients exist. Look at the graphs of distributed velocity and pressure contained in the paper and you will find this confirmed.
Something I found really interesting is exactly where on a typical sail the line of maximum lift is. It is actually only inches from the tip of the mast! For some boats with longer mast sections, this point may actually be located on the mast itself!
This does NOT imply in any way that the balance of the airfoil under discussion is any less important WRT shape and function simply because most of the 'action' is happening farther forward. It does tend to clarify, however what the function of the balance of the airfoil actually is; it smoothly reunites (deccelerates) the air mass on the lift side with the other side. This is essential to retaining adhesion of the airmass to the lift side of the airfoil, and thus essential for creating lift.
Great discussion, guys!
Jimbo
