If you want to be able to trim a hang glider to have zero pitch trim force (pilot doesn't need to push or pull on control bar) with the airfoil shown above, at 0.5 degrees angle-of-attack, assuming drag on the pilot's body is negligible, the "hang point" would have to be vertically in line with the circle on the diagram that is just aft of the trailing edge of the wing, labelled "0.5 degrees". Connecting the hang strap right at the circle would accomplish this, but that is not only possible configuration-- it could also connect above, or below, the circle.
If drag on the pilot's body is not negligible-- so that when the pilot lets go of the control bar, the hang strap angles backwards rather than running straight up and down-- then the extended line of the hang strap will have to pass through the circle on the diagram that is just aft of the trailing edge of the wing, labelled "0.5 degrees". Connecting the hang strap right at the circle would accomplish this, but that is not only possible configuration. It could be attached below the circle and slightly aft of it, or above the circle and slightly forward of it. The key point is that the extended line of the hang strap must pass through that circle.
(Note that we're only talking about trim, not stability. Assume for simplicity that we have a straight, non-swept, constant-chord wing. Such a wing would not be stable-- the angle-of-attack would not tend to stay constant-- at least with a conventional airfoil cambered in the usual direction-- but we can still discuss the trim characteristics at any given angle-of-attack, where the forces and torques are at least momentarily in balance. Hang glider stability dynamics have been discussed elsewhere on ASE, and are fertile ground for additional future questions.)
Now imagine that we flip the wing upside down, so that we have an airfoil with "negative camber", and wish to fly at a 3-degree angle-of-attack. Now to trim the glider for hands-off flight, the extended line of the hang strap will have to pass through the circle labelled "-3" on the diagram. Again, this can be accomplished by attaching the hang strap right at the "-3" circle, but this is not the only possible configuration.
It seems somewhat counter-intuitive that the center-of-pressure could lay outside the physical dimensions of the airfoil, but it is possible. The reason that this happens is that any non-symmetrical airfoil generates a pitching moment. If the airfoil is cambered in the normal direction, the pitching moment acts in the nose-down direction. Even at the zero-lift angle-of-attack, the airfoil generates a nose-down pitch torque. The modern way to analyze the forces and torques generated by an airfoil is to treat the Lift and Drag forces as acting at a Center of Pressure which is defined to be fixed in location, at the quarter-chord point of the wing. Any nose-up or nose-down aerodynamic pitch torque that would exist if the wing were fixed on a pivot at this Center of Pressure, is expressed as a Pitching Moment Coefficient.
