Discussion
Spiral stability requires which combination of derivatives to be positive?
Answer: C
The classical criterion is \( L_\beta N_r - N_\beta L_r > 0 \), which is the sign of the last coefficient of the lateral quartic. Read physically, dihedral effect acting with yaw damping must outweigh fin stiffness acting with the rolling moment due to yaw rate. That is why dihedral improves spiral stability and a large fin degrades it, and why the two cannot both be pushed hard. Adding the terms instead of subtracting them abandons the balance the criterion expresses; the other two combinations are not the criterion at all.
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