Originally posted by Tinstaafl
I figured motion of the air mass won't be a problem since it's a parameter that must be entered into the sim.
That was why I asked what the prupose of the question was.
If Fergal is building the equations of motion for a sim then he/she should not be using 'rules of thumb'. You need to get the equations spot on or all kinds of nonsense will result. And every single parameter required is available, since you're running a 6dof simulation. So you can use the precise definition for every parameter, be it alpha, beta, whatever.
Having set a class of some 20 people to do pretty much similar to thay job (build a simple 3dof model with defined aerodynamics) and seen a good 75% of a fairly smart class get tripped up by various axis/EoM issues, including mis-calculation of alpha, with the resulting aircraft models being hideously unstable, close is simply not good enough for this kind of work.
Whereas rules of thumb may well be 'good enough' for derivation of the aerodynamic characteristics, depending on what you're after - a level 'D' qualified sim has a much tighter definition of 'good enough' compared to a fixed base visual systems test-bed, for example.
Waiting for guidance....