Discussion
Three of the following raise the short-period natural frequency. The exception is
Moving the centre of gravity aft
Increasing dynamic pressure
Increasing the tailplane area
Reducing the pitching moment of inertia
Answer: D
Aft movement of the centre of gravity lowers the frequency, because it reduces pitch stiffness by shortening the arm to the aerodynamic centre; carried far enough it removes the stiffness altogether. The other three all raise frequency, since \( \omega_n \approx \sqrt{-C_{m_\alpha}\bar q S\bar c/I_{yy}} \) rises with dynamic pressure, with any change that makes \( C_{m_\alpha} \) more negative such as a larger tail, and with any reduction in pitching inertia. Reading that formula as a whole is the quickest route to the answer.
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