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Mar10-10, 06:57 AM
P: 4,512
Quote Quote by sophiecentaur View Post
One definition of stability is that, for stable equilibrium, a perturbation will increase the potential. For a mass hanging on a string or a mass hanging below whatever is suspending it (i.e. rotor or wing) the potential increases with tilt.
If the cm were above the centre of effort of the blades (if that's all that's involved) then any perturbation will decrease the potential - so it's unstable.

So called Low (fixed) Wing aircraft mostly have dihedral, giving a centre of effort higher than it may appear and a cm which is below where it may appear to be (like double decker buses).
They will mostly be inherently stable (you could hold them up by the wing tips and they would not pitch) but, even if not, when moving through the air, the tail and other surfaces will tend to keep them stable (roll is self corrected).
Dihedral, itself provides more lift on the side that is tilted down, which gives a righting moment so this may also apply to a helicopter rotor, which is tilted up - more lift on the appropriate side to help with stability.

More manouverable aircraft have no dihedral and are much less stable (can be put into a spin, for instance).
You reflect my thoughts almost exactly. Rather than speak in general, why would a dihedral winged aircraft be more stable to perturbations with the center of mass below the wing than above?