Unstable Aircraft

Not all aircraft are designed to be stable without some help from the pilot or flight control system, although the complete (closed-loop) system should be stable. By using active controls, modern aircraft can relax some of the constraints associated with longitudinal stability (but not trim) and achieve performance gains.

This is, of course not without it risks as the following example illustrates. In the short video clip here, problems with the control system result in an aircraft that is not dynamically stable. This was one of the early test flights of the YF-22 in which control system gains were not properly set for this flight mode. Fortunately the pilot was not hurt.