Discussion
Consider the following pairs of lateral derivative and its usual sign on a conventional aeroplane.
Rolling moment with sideslip - Negative
Rolling moment with roll rate - Negative
Yawing moment with yaw rate - Negative
Yawing moment with sideslip - Negative
How many of the pairs given above are correctly matched?
Answer: B
The first three are negative and the fourth is positive, so three pairs match. Directional stability requires a positive yawing moment with sideslip: sideslip from the right must produce a nose-right moment to turn the aeroplane into the relative wind. The other three are all restoring or damping terms and are negative by convention. Getting this one sign right matters, because it is the difference between weathercock stability and directional divergence.
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