ClippedCub, any idea if they are forming shocks on the tail at this speed?
And what balance limits do they use of the elevator? Original P51 figures?
Was going to keep this to myself so as not to clutter up the discussion, but the elevator has been reshaped for high speed effectiveness. It's more bulbous. This is to get it out of the boundary layer of the horizontal stabilizer and would have been selected by his aero guys. There is a basis for this from the old NACA reports that I was going to dig through again, but if a shock did form, it will form on the deflected elevator itself and not on the hinge line or ahead.
The tail is at a much lower thickness-to-chord than the wing, so the tail wouldn't have a shock. But shock waves are sensitive to curvature too, and the bulbous elevator will cause a re-accleration of the flow. Doubt there was a shock here, but in the interest of thoroughness, it's on my list to check, if for nothing else, to determine the margin.
Don't know anything about the balance specs, but it's not unusual for the guys do flutter flight tests when needed.