Wind-tunnel yawing coefficients Cn for aircraft A/B/C are −0.030/−0.025/+0.040 at β = −5°, zero for all at β = 0°, and +0.030/+0.025/−0.040 at β = +5°. Which directional-static-stability statement is true?
All three are stable.
Only C is stable; A and B are unstable.
C is unstable; A and B are stable, with A more stable than B.
C is unstable; A and B are stable, with A less stable than B.
Correct answer: (C) C is unstable; A and B are stable, with A more stable than B.
Explanation
Directional static stability requires $C_{n\beta}>0$ under the standard convention. Slopes per degree are 0.006 for A, 0.005 for B and −0.008 for C. A and B are stable, C unstable, and A has the larger restoring slope. Thus C.
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