Discussion
Assertion (A): An aeroplane can have excellent static directional stability and still an unacceptable Dutch roll.
Reason (R): Directional stability supplies stiffness, while the damping of the oscillation comes from separate rate derivatives that fall away with altitude.
Select the correct answer from the options below.
Answer: A
Both are true and the reason explains the assertion. A large fin makes the yawing moment with sideslip strongly positive, raising the frequency, but damping depends on the yaw rate and side force derivatives, which grow only in proportion to the tail arm and shrink with density. The result is a fast, poorly damped wag at altitude. It is the lateral echo of the longitudinal case, where a large static margin gives a quick short period that may still need a pitch damper.
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