Discussion
Increasing the tail moment arm of an aeroplane, all else equal, has what effect on the short-period mode?
Answer: C
A longer arm increases the tail's contribution to pitch stiffness roughly in proportion to the arm, and its contribution to pitch damping roughly with the square of the arm, since the tail's local incidence change from a pitch rate is itself proportional to the arm. So the change raises frequency and damping together, which is why a long-tailed aeroplane feels crisp and well damped. The second and third options each get one of the two trends backwards. Leaving both unchanged would require the tail to contribute nothing, which is only true of a tailless configuration relying on the wing alone.
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