My suggestion is that the simulator fidelity is questionable only outside the phase space captured by the stability derivatives.
If you have the relevant dependent variables in the multi-dimensional matrices, just because you are outside the normal operating envelope, does not necessarily mean the simulator will respond inaccurately. The test phase space should obviously be larger than the operating envelope.
However, I agree you need to be far more conscientious when extrapolation is the name of the game.
Interpolation is easy.
In actual fact it is quite easy during wind tunnel testing to acquire stability derivatives outside the normal operating envelope of the aircraft.
The characteristics for a commercial aircraft may be fairly benign as the operating envelope is fairly small.
I would hazard a guess that in the 737 aero model, outside the tested phase space, the stability derivatives are merely set to the values at the extreme ends of the test matrix.
In general for more manoeuverable aircraft with greater control power and authority the derivatives become highly nonlinear.
When you start solving the equations of motion, associated with such systems is the problem of sensitivity to initial conditions - just like any nonlinear system.
Normally the pitching moment is the hardest to model.
I spent years attempting to
extrapolate F-18HARV performance at high rates of pitch using the existing quasi-steady aero models.
In the end, I had to move away from stability derivative methods to nonlinear indicial response methods.
Unsteady effects are one case where even
interpolation may yield incorrect results because in the case of unsteady effects, the instantaneous aerodynamic loading is actually a function of the whole past history of motion.
Its only my opinion, but Mach Tuck is a relatively simple problem to model. Whilst it is obviously a transonic effect, and the transonic regime is notoriously difficult to model, I would be surprised if Boeing had not tested slightly beyond Vne.
I'd have faith in the model until at least slightly beyond Vne.
Because, of course, there are probably very few people, if any, that are able conclusively to substantiate the corollary! If there are, I'd like to know what they were doing at the time...
