Discussion
Directional stability at high angle of attack often deteriorates because
Answer: B
Correct answer: The fin is blanketed by the wing and fuselage wake
At high angle of attack the wing and fuselage shed a broad, low-energy wake, and on a conventional layout the fin sits in it. Reduced local dynamic pressure means less side force for the same sideslip, so the fin's contribution to the yawing moment derivative falls.
The loss is worst where directional control is most needed, near the stall and in spin entry, which is why remedies exist for it: ventral fins that sit below the wake, twin fins placed outboard of it, and strakes that re-energise the flow. Sideslip can certainly occur at high incidence, and fin area is a fixed geometric quantity.
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