Discussion
An aeroplane climbs to a high cruise altitude at constant Mach number. Its short-period damping ratio generally
Answer: B
Every dimensional aerodynamic derivative carries a factor of dynamic pressure, and at constant Mach number dynamic pressure falls with density as the aeroplane climbs. Frequency goes as the square root of that, while the heave and pitch damping terms scale more nearly with it directly, so the damping ratio degrades with altitude. This is a familiar result: an aeroplane comfortable at low level can feel loose in pitch in the cruise, and the pitch damper earns its place there. Higher true airspeed does not rescue the damping. Fixing Mach number fixes neither density nor dynamic pressure. Nothing special happens exactly at the tropopause.
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