Thorsten wrote in Sun Jul 30, 2017 4:57 pm:Maybe Thorsten can explain this as I can't.

Generically they're transient effects, the airflow adapting to a change and generating forces in the process. For instance one driving effect for pitching moment due to alpha dot is delayed downwash.

Thanks for the explanation of the aero effects - which is almost how I understood it; but it's the mathematics that I was struggling with. However after one of those magic sleeps[1] I think I now almost understand what the derivative is.

For a long time I've understood that the derivatives are second order functions[2]. My realisation this morning is that all of the coefficients must be differential equations - because of the nomenclature in the Boeing document., and what's happening with these equations is to include values into the coefficients based on an acceleration or rate (e.g. q or alpha dot). But I'm not exactly sure of how all of this works mathematically.

Bomber wrote in Sun Jul 30, 2017 5:59 pm:Well I agree up until separation at which point downwash wont effect the h-stab and the AoA seen by the h-stab airfoil will become the same as seen by the front wing

Depends on what you mean by separation; if you're talking about flow separation; the resulting turbulent air is going to largely destroy the effects of the htail, but the downwash is still there, just turbulent.

It's also wrong to add downwash onto the alpha(frl[3]) for the tail and expect the result to be correct; I'm sure I've drawn this out once already, it included vector maths, alphadot, q, and quite a few other terms.

Bomber wrote in Sun Jul 30, 2017 6:02 pm:Richard wrote in Sun Jul 30, 2017 2:23 pm:Also needs to be considered are the derivatives, pitch damping due to q dot (CMq), change in pitch moment due to alpha dot. Last time I looked at one of your models you didn't have any derivatives

They are there, just that I have them effecting the Angle of Attack.

Unfortunately that won't work and can lead to a twitchy model; pitch damping (due to pitch rate) needs to use the pitch rate (Q); without this the effects will be instantaneous. You need the aerodynamic derivatives.

Bomber wrote in Sun Jul 30, 2017 6:12 pm:dany93 wrote in Sun Jul 30, 2017 6:02 pm:The point I would make Dany is that the h-stab can be stalled out..and it will do so at a defined value of fuselage AoA depending upon it's incidence......agreed ?

And that if the elevator is deflected such that it's trailing edge is lowered.... the h-stab airfoil will stall at a lower fuselage AoA.

That seems wrong to me. If we're talking about an all moving surface then lowering the trailing edge will increase the incidence of the surface. For a surface with a moving tail edge section (i.e.elevator) the shape of the airfoil changes, but we don't normally model that, instead the entire horizontal tail would be at a certain incidence, which defines the amount of airflow, and the available pitching moment becomes a function of this - and with the conventional way of treating an airplane as an entire set of geometry [4] this will be correct[5] for each given alpha (frl) - because the measurements are taken (or calculated) in the freestream.

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[1] woke up this morning and it all became much clearer

[2] although I probably still don't properly understand what a second order function actually is

[3] fuselage reference line, or could be wing design plane; i.e. the alpha that is used for the wings.

[4] from which the forces and therefore the coefficients are derived.

[5] subjective to some sort of error, either interpolation or measurement.