# Why does aircraft stability increase when it transitions from subsonic to supersonic flight?

I am puzzled why would the aircraft be more stable at high speed flight at high altitudes. Given that the Centre or Pressure moves gradually towards the 50% MAC region, once past Mcrit.

Has this got to do with moments and the neutral point?

Static longitudinal stability (also called static margin) is determined by the distance between the center of gravity and the neutral point¹. The neutral point is at a quarter of the chord at subsonic speed and moves back to half chord once the aircraft flies fully supersonically. This means that the static margin will greatly increase when moving from sub- to supersonic flight.

In order to keep the center of pressure at the center of gravity, in supersonic flight the elevator needs to create lots of downforce. The bigger the difference in lift per area between wing and elevator, the stronger pitch stability will become.

This effect makes the transition between both regimes tricky: When accelerating, aircraft experience the shift in the center of pressure as Mach tuck, and when the trim facilities are not sufficient, the aircraft might enter an uncontrollable dive.

In the reverse direction, an aircraft trimmed for supersonic flight will experience a pitch-up moment from the forward shift in the center of pressure once it slows down, which can stall it or overload the structure.

¹ I could also talk of the center of pressure here, but that would be less precise. The neutral point is the center of pressure for all angle-of-attack-dependent lift forces, and it is those which determine pitch stability.

• Thank you for the response Peter, however wouldn’t keeping CP and CG result in a very unstable aircraft? I mean, isn’t it better for your CP to be aft of the CG so that the aircraft experiences a natural nose-down moment? Jul 11, 2018 at 0:35
• @shogunnyan: Any difference in the location between CP and CG will let the aircraft rotate. The aircraft is trimmed precisely to move the CP exactly below or above the CG. Jul 11, 2018 at 8:15
• i see! Ok! I understand now! Appreciate it Peter! Jul 11, 2018 at 8:31

The CP of the wing will, for most airplanes, still lie aft of the CG which is usually required to be longitudinally stable. Peter was addressing the whole airplane. The CP of the airplane as a whole (Lift, Weight, Drag, Thrust), will need to lie exactly in the CG or a moment will exist and rotation will take place. Peter chose to use the CP of the wing and the tail together, the total CP. (which indeed needs to be in the CG or pitch change will occur)