Glide-path stability of an aircraft under speed constraint

Abstract

The motion of an aircraft operating under perfect speed constraint (e.g. by an appropriate autothrottle) is examined theoretically and it is shown that, if engine thrust acts through the aircraft centre of gravity, the aircraft will have weak flight path stability in descending flight but be unstable in climb. These effects are readily overshadowed by thrust effects in the sense that with low-slung engines the aircraft motion is destabilized and vice versa. Manual flight with autothrottle engaged is shown therefore to be potentially dangerous, as the strong flight-path stability possessed by the natural aircraft is suppressed and inadvertent glide-path errors are not self-correcting.