ORIGINAL: Nathan King
As you correctly mentioned, dihedral enhances stability about the longitudinal (roll) axis. When the airplane is rolled the lower wing has a higher angle of attack (more lift) than the high wing which will cause the wings to level back out somewhat by themselves. A negative effect of dihedral is roll coupling.
Nathan, how is the angle of attack increased? I was taught that the lift vector dropped as the roll angle of the increased. The lift vector in relation to the wing is constant, but the lift in relation to the horizon drops as the roll angle, or wing pannel angle to the horizon increases. With the dhedrial, one wings angle incrases as the other decreases as the plane rolls, so the low wing will increase it lift vector in relation to the horizon as it levels out in relation to the horizon.
The anhedrial works on the same priciple, but now the high wing is the one increasing in lift vector so the unstability is increased. You are basicly flying inverted in that case.
Correct me if I'm wrong. It wouldn't be the first time, or the last.
Don