Discussion
Two fins of \( 5 \ \mathrm{m^2} \) each replace one of \( 9 \ \mathrm{m^2} \). The change in total fin area is
Answer: D
Correct answer: An increase of 11 per cent
Compare the totals rather than the individual surfaces:
\[ 2 \times 5 = 10\ \mathrm{m^2} \quad \text{against} \quad 9\ \mathrm{m^2} \]
\[ \frac{10 - 9}{9} \times 100 = 11.1\% \]
So twin fins here add area rather than merely dividing it. The comparison is not the whole story, though: two fins of half the height each have a lower aspect ratio and so a shallower lift-curve slope, which is why designers do not simply halve the area when going to a twin arrangement. The gain in placing them in cleaner air often outweighs that penalty.
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