Discussion
Which of the following is not a fixed surface added for directional stability?
The rudder
A dorsal fin
A ventral fin
A strake ahead of the fin
Answer: B
Correct answer: The rudder
The rudder is a movable control surface, hinged to the fin and deflected by the pilot to produce yawing moments on demand. It provides control, not stability.
The other three are fixed surfaces added to improve directional stability: the dorsal fin, which delays the fin stall at large sideslip; the ventral fin, which works below the wake at high angle of attack; and a strake ahead of the fin, which sheds a vortex to keep the fin's flow attached. Keeping the two functions distinct matters because they are sized by different criteria - stability by the derivative, control by the largest moment to be balanced.
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