Wing moment - providing stability or not It is said that wing alone is unstable, because it provides a nose up moment, so tailplane is needed to damp it. But normally wing provides a pitch down moment (as far as I know, apart from very high aoa situation) because centre of pressure is behind the centre of gravity. So shouldn’t wing moment be actually stabilising? I’m confused about stability, because we one time we consider stationar lift acting in aerodynamic centre and moving centre of gravity, next time moving centre of pressure with stationary centre of gravity and so on... please help me out.
It's not about the moment but its change with AoA.
Any wing can be trimmed when the cg is at the center of pressure. What counts is how things change when AoA changes.
If you want stable behavior, use an inverted airfoil. A regular, cambered airfoil will cause unstable behavior.
For a trimmed airplane the moment around the center of gravity is zero. Now it is important how this moment changes when the airplane moves away from this trim point. If AoA increases and the moment becomes negative (nose down), the airplane is stable.
If the center of gravity is too much forward, the airplane cannot be trimmed in the first place because control authority is insufficient to produce enough downforce at the tail in order to compensate for the pitch-down moment of the lift force on the wing around the more forward center of gravity.