Discussion
Statement I: Very strong directional stability with weak lateral stability tends towards spiral instability.
Statement II: The weathercock turns the nose into a developing turn faster than dihedral can level the wings.
In the light of the above statements, choose the correct answer.
Answer: A
Correct answer: Both Statement I and Statement II are correct
Both are true and the second describes the mechanism of the first. In a developing turn a sideslip appears; a strong fin yaws the nose into it quickly, which steepens the turn and increases the bank still further.
Weak dihedral effect cannot generate the rolling moment needed to level the wings against that, so bank continues to increase and the aeroplane enters a slowly tightening descending spiral. The balance is a genuine design trade: strengthening the fin to cure Dutch roll worsens the spiral mode, which is why yaw dampers are fitted rather than the fin simply being enlarged.
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