An airplane that tends to gradually increase a pitching moment set into motion exhibits?

An airplane that tends to gradually increase a pitching moment set into motion is indicative of poor longitudinal stability. In this context, poor longitudinal stability signifies that once a disturbance causes the aircraft's nose to pitch up or down, it is unlikely to return to its original flight attitude without pilot intervention. The continuous increase in the pitching moment means that the aircraft will either climb or descend increasingly, rather than stabilizing after the initial disturbance.

In contrast, high longitudinal stability would imply that any disturbance would result in forces or moments that work to return the airplane to its original position. Neutral stability suggests that after a disturbance, the aircraft would neither return to its original position nor diverge further away, effectively having a balanced state. Adverse yaw, on the other hand, refers to a condition where an aircraft yaws in the opposite direction of a turn owing to differential lift and drag between the wings, which is not directly related to longitudinal stability.

Understanding these dynamics is crucial for pilots and engineers when designing and flying aircraft, as the stability characteristics directly affect handling and safety.