Discussion
A fin stall increases the yawing moment derivative. True or false?
Answer: C
Correct answer: False, the derivative collapses when the fin stalls
Beyond the stall the flow over the fin separates and its side force falls away, so the fin's contribution to \( C_{n\beta} \) collapses. The derivative can approach zero or go negative, since the destabilising fuselage and propeller terms remain unchanged.
That is the essence of the problem at large sideslip: the aeroplane loses the very restoring moment it needs at the moment it needs it most, and rudder lock may follow from the same separation. A dorsal fin does not make a stalled fin effective - it delays the stall, so the derivative never collapses in the first place.
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