there is no contemporary quantitative data that I know of for aircraft behaviour at the edge of and beyond controlled flight. in the same way as there is none for eg. directonal stability. which is completely understandable - we are looking at the point where the nice simple equations describing the aircraft's behaviour break down into chaos. even today with the help of supercomputers accurately modelling behaviour just outside of the envelope is impossible.
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I don't know that Hitech does any subjective adjustments to make the aircraft fly as differently as they do but I'll take his version over a player's impressions of how they should "feel" to the player.
As far I know, HTC model the planes individually and calculate their behaviour at different conditions, then tabulate it and use interpolation for the real time game. They adjust their models to agree with the empirical data of official documents, but what happens at conditions other than the ones tested and reported in the documents is entirely up to their modeling.
You will not find any contemporary documents with accurate measurement of what happens during tails slides or accelerated stalls, only test pilot reports and impressions. Modelling is possible when the flow is coming from the right direction and in low angle of attack. Then, element by element calculations can work. But what do you do with a plane falling backwards? How much lift and drag does the wing element provide if the trailing edge is flying forward? What happens in high angle skids where the flow is going diagonally over the wing, thus the effective profiles of the wing along the flow lines are completely different than the "NACA" standards and change with skid angle and other parts of the plane are suddenly in the way (like fuselage).
The "normal flight conditions" modelling of most sims are pretty good. HTC has done a lot of work to achieve decent modeling even a little outside the "normal conditions", but extreme conditions are anyone's guess - and guess they must. The game is doing SOMETHING when you stall one wing while skidding in full engine torque with flaps out.
Now, my feeling is that HTC is doing the best unbiased model that they can and take what the calculation gives. There were cases in the past when they were not pleased with some of the results and they were changed. In 2002 AH1, the F6F had a nasty accelerated stall, while the F4U was a stable as a rock. After many complaints HTC had a FM revision and the behaviour has been switched. Was this the unbiased result of the improved mathematical modelling or was there some hand-made tweaks under the hood of the model engine?