Dihedral is the angle between the two halfes of the wing where a flat wing is called "No Dihedral". The angle when the two wing tips are higher than the center of the wing is refered to a positive dihedral and promotes stability. If you have the wing up side down as to efectively have negative dihedral the plane will be very unstable. So if your servo in the wing is facing up out of the top of your plane then you have it with the dihedral up. If you inverted the wing to hide the servo it will be very dificult to fly.