Discussion
Statement I: A dorsal fin cures rudder lock as well as the loss of stability at large sideslip.
Statement II: Both problems arise from the main fin stalling.
In the light of the above statements, choose the correct answer.
Answer: D
Correct answer: Both Statement I and Statement II are correct
Both are true and the second is the common cause. Beyond the fin's stall angle the side force collapses, and two things follow: the restoring yawing moment is lost, so directional stability falls away in a large sideslip; and the separated flow reverses the rudder's hinge moment, so the surface floats hard over and cannot be recentred - rudder lock.
A dorsal fin addresses both at once because it addresses the stall itself, its swept leading edge shedding a vortex that keeps the fin's flow attached to a far larger sideslip angle.
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