What does a negative slope of Cm with respect to angle of attack (dCm/dα) indicate about static stability?

What does a negative slope of Cm with respect to angle of attack (dCm/dα) indicate about static stability?

• A. It indicates static stability
• B. It indicates instability
• C. It indicates neutral stability
• D. It indicates maximum lift

Answer: (A)

In pitch stability, what matters is how the pitching moment responds when the angle of attack changes. The coefficient Cm represents that pitching moment. If increasing the angle of attack makes Cm more negative (a negative dCm/dα), the airplane develops a nose-down moment when perturbed upward. That nose-down push acts to reduce the angle of attack back toward the original value, restoring the aircraft to its trim condition. This restoring tendency is what we call static stability in pitch. If the slope were positive, a small increase in angle of attack would produce a nose-up moment, amplifying the perturbation and leading to instability. If the slope is zero, the aircraft is neutrally stable in pitch. The option about maximum lift isn’t about how the moment changes with angle of attack, so it doesn’t describe static stability.

In pitch stability, what matters is how the pitching moment responds when the angle of attack changes. The coefficient Cm represents that pitching moment. If increasing the angle of attack makes Cm more negative (a negative dCm/dα), the airplane develops a nose-down moment when perturbed upward. That nose-down push acts to reduce the angle of attack back toward the original value, restoring the aircraft to its trim condition. This restoring tendency is what we call static stability in pitch.

If the slope were positive, a small increase in angle of attack would produce a nose-up moment, amplifying the perturbation and leading to instability. If the slope is zero, the aircraft is neutrally stable in pitch. The option about maximum lift isn’t about how the moment changes with angle of attack, so it doesn’t describe static stability.