As for the aerodynamic moment, which confuses me:
- This moment is like that of a couple of forces, isn't it? the moment is always the same no mater what point you choose, right? It's like a "pure" moment.
- Why is it always zero when the airfoil is symmetrical?
- Does it changes with Lift in cambered airfoils? Why?
I think there may be some misunderstanding here. Let me put it a simply as I can
Aerodynamic (pitching) moment is made up of two bits:
The pitching moment at zero lift = Cmo
plus
The pitching moment due to the lift acting at the aerodynamic centre which is normally close to 25% chord, but which may be distant from the point to which moments are referred = CL*(moment arm between a.c and moment reference point)
Cmo is a constant. It does not change as AoA changes and it does not change with pitching moment reference point. It depends on the shape of the camber line, both in height and location of the 'peak'. For a symmetric airfoil (obviously) the camber is zero and Cmo is also zero.
The lift bit is with you always, whatever the camber, so symmetric airfoils are no different to cambered airfoils. But if you choose to measure pitching moments about 25% chord (as is often the case) then the pitching moment due to lift will be zero because there isn't any moment arm. Camber simply shifts the datum AoA at which you get zero lift.
So in the special case of a symmetric airfoil measured about 25% chord the aerodynamic moment will always be zero.
Hope that helps!
CliveL