An airplane that has a tendency to gradually increase a pitching moment that had been set into motion demonstrates:

An airplane that gradually increases a pitching moment after it has been set into motion demonstrates poor longitudinal stability. In the context of aircraft design, longitudinal stability refers to the aircraft's ability to maintain a steady pitch attitude without continuous control input from the pilot.

When an aircraft has poor longitudinal stability, any disturbance—such as a change in airspeed or configuration—tends to produce a pitching moment that exacerbates the initial condition. In this case, the tendency to gradually increase the pitching moment means that the airplane does not return to its original pitch attitude effectively. Instead, it may either keep pitching up or down, leading to difficulty in controlling the aircraft.

This stability characteristic can affect the overall flight behavior of the aircraft, making it less responsive or predictable, which poses challenges for pilots in maintaining controlled flight. Understanding these principles is crucial for recognizing how various designs and configurations influence aircraft handling and flight safety.