Dihedral on outer portion of wing span

[rdj]

In Raymer's book "Simplified Aircraft Design for Homebuilders" 1st Edition on page 30 there is this statement:

If you want a flat center section with the dihedral only on the outer wing panels, draw it so the wing tip is at about the same height as it would be if the dihedral started at the center. Watch out--if the dihedral break occurs much more than 50% out on the span, you can get excess and maybe uncontrollable rolling near the stall.

I can't find any mention of this 'uncontrollable rolling' effect in his textbook (3rd edition). Nor can I find any mention of it elsewhere. Does anyone know of any other reference that describes 'why' in more detail? If I had, say, a straight wing for 60% span, and then canted the last 40% up to give the same tip height as say, 2% dihedral, what is the mechanism of this 'uncontrollable rolling' effect? I've seen a number of RC sailplanes with wings that look like that, and they don't have 'uncontrollable rolling' at the stall. Is the small amount of aft sweep caused by the dihedral enough to change the stall pattern that dramatically? Or is there some other aerodynamic effect I'm missing here?

Bob

 

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