Discussion
A fighter at very high angle of attack in a departure shows almost no tendency to weathercock, although its fin is large and its geometry unchanged.
What explains the loss?
Answer: D
Correct answer: The fin is blanketed by the wake
The geometry is unchanged, so the loss must be aerodynamic. At very high angle of attack the wing and fuselage shed a broad wake of low-energy air, and a conventional centreline fin sits inside it.
Reduced local dynamic pressure means far less side force for a given sideslip, so a large fin can behave as though it were a small one. That is the point of the question: fin area is a fixed quantity and does not "effectively reduce", but the dynamic pressure acting on it does. Sideslip certainly occurs at high incidence - departure and spin entry are precisely such cases - which is why ventral fins and strakes exist.
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