Discussion
Consider the following pairs of longitudinal derivative and its usual sign.
Pitching moment with incidence - Negative
Pitching moment with pitch rate - Negative
Normal force with incidence - Negative
Pitching moment with elevator deflection - Negative
How many of the pairs given above are correctly matched?
Answer: B
All four are negative in the usual stability-axis convention. The first is static stability, the second pitch damping, and the fourth says that a trailing-edge down elevator gives a nose-down moment. The third is the one that catches people: \( Z_\alpha \) is negative because the normal force axis points downward in the standard body-axis set, so an increase in incidence produces a more negative \( Z \) even though it produces more lift. Sign conventions are worth pinning down before any derivative is used in anger.
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