Each airfoil has its own aerodynamic centre. When you have a wing and a tailplane you have a single "aerodynamic system". Just like you define the AC of a wing as the point about which the pitching moment coefficient becomes independent of the AOA, you can also find the point about which the pitching moment coefficient of the whole system (the aircraft) becomes independent of pitch attitude. That's the neutral point. The CofG must be ahead of the neutral point.
I'm new at this so I don't know if the experts use the term aerodynamic centre in respect of the entire aircraft. Your diagram shows the AC for each wing separately. For a flying wing design - one wing with no tailplane, the neutral point is at the AC. When you have a second wing you have to take into account its effect on stability (that's what it's there for). So the same concept of aerodynamic centre can be applied to the aircraft (and ought to take into account the fuselage aerodynamics, undercarriage aerodynamics, when extended etc), and that's when it's called the neutral point.
I know that it's possible for the tailplane to create lift in some situations, but in these illustrations it will produce lift even at fwd CG limit. The camber of the tailplane is also positive in these illustrations...
I'm reaching the limits of my knowlege here, but if you're designing a large aircraft you're looking for every ounce of efficiency which means never flying with the tail producing a down-force. I don't have any experience with the actual figures involved for big planes though.
Isn't that the most aft point for the CG with the aircraft NOT beeing unstable? The point where the tailplane arm is to short to create enough pitching motion?
The
rear CofG limit isn't particularly linked to how much pitching motion the tail can create. If that were the case then a CofG out of limits to the rear would always cause the nose to pitch up. The stability of the aircraft is actually related to how the pitching moment
changes with a disturbance, not to how big it is. A CofG out of limits to the rear will cause the nose to pitch up if it's disturbed in a nose-up direction, but will cause the nose to pitch
down further if it's disturbed in a nose-down direction.