Discussion
Excessive dihedral effect causes spiral instability. Is this correct?
Answer: A
Correct answer: No, it causes a poorly damped Dutch roll
The two lateral-directional failures lie at opposite ends of the same balance. Excessive dihedral effect relative to directional stability gives a poorly damped Dutch roll; excessive directional stability relative to the dihedral effect gives spiral instability.
So strong dihedral in fact stabilises the spiral, by levelling the wings faster than the fin can tighten a developing turn. Longitudinal instability belongs to the pitching plane and is unaffected by either. Fin size matters, but by shifting the balance rather than by changing which failure follows which excess.
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