Originally Posted by Madratter
I'm going to go out on a limb here but I have a background in physics and I am a software engineer. My guess is that the time step they are using in their physics model is not sufficiently small when dealing with the forces generated by larger airplanes. That is only one possibility but that is my best guess based on what I am seeing. Things remain fine when flying with the large model until some large force is generated by a maneuver that overwhelms the time step they are using. On top of that, based on what I am seeing, I believe their drag model has issues as well. At any rate, whether those things are bugs or problems with the physics engine is a matter of semantics. What is absolutely clear and demonstrable is that if you scale planes up, bad stuff can happen.
Time Step: It actually goes the other way. With larger planes, the rates are slower (not as nimble) and you can run at slower physics update rates, e.g. ~30 Hz might work for a jet transport in Microsoft Flight Sim. But when going smaller, the rates are higher and things can blow up unless the rate is increased. We run FS One at 300 Hz on physics side. So when we scale up to larger size, this 300 Hz is really overkill, i.e. things *actually* get easier.
MSFS ~30 Hz Ref: Michael Zyskowski, AIAA Paper 2003-5818 (August 2003)