The aerodynamic derivative C_{n\beta} represents directional (weathercock) stability. For an aircraft to be statically directionally stable, C_{n\beta} must be:

(A) Strictly positive (C_{neta} > 0)

(B) Strictly negative (C_{neta} < 0)

(C) Strictly zero

(D) Equal to elevator power C_{m\delta_e}

Answer Key:

A

### Solution: A positive sideslip angle eta (wind from the right) must generate a positive yawing moment N (nose right) to align the aircraft into the relative wind, requiring C_{neta} = dC_n/deta > 0. Correct: A.

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