Discussion
An aeroplane with very strong directional stability and weak lateral stability tends towards
Answer: A
Correct answer: Spiral instability
The spiral mode is settled by the balance between the two. Strong directional stability makes the aeroplane weathercock quickly into any sideslip that develops in a turn, tightening it; weak lateral stability means dihedral cannot generate the rolling moment needed to bring the wings level.
The result is a slowly tightening spiral, with bank and rate of descent both increasing. The opposite imbalance - strong dihedral effect with a weak fin - gives a poorly damped Dutch roll instead. Longitudinal instability and phugoid divergence belong to the pitching plane and are unaffected by either.
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