Hi,then yes, you agree, there is a residual moment ( x being not zero) when Cm_de = 0.Now why the other surfaces (flaps end spoilers) do not show also a residual moment when their Cm's is zero too ?Troublesome isn't it?

Yes :) But I wouldn’t call it troubling.This is CLAlpha and CmAlpha doing their job...as should be expected.And it’s reassuring to see how CLelevator, CLhstab, CLflaps, CLetc contribute to total body lift – that’s an accurate flight model.It’s not true that flaps and spoilers behave differently…they have the same effect.Ailerons however don’t have a CL coefficient...being generally (not absolutely) symmetrically opposite their contribution to body lift would be very small. As far as I know CLaireron is not a conventional coefficient…anyway.It is a ‘residual moment’…still I think there must be a better way to describe it.The word ‘Moment’ sort of confuses this effect with the product of Cm coefficients - those are clearly direct moments.As opposed to this effect of changing total body lift. (...product of CL coefficients)Changing the sum of body lift will cause a change in AOA....It's an indirect result.

I think I've lost the thread of the argument about "residual" pitching moments.The lift coefficient from the elevator is given by CL_de * elevator angleThe pitching moment coefficient from the elevator is given by CM_de * elevator angleThe are both zero when elevator angle = zeroWhat is the evidence for the existence of this "residual" moment?Changing the angle of attack changes the lift: not the other way round.

That's all true, but..Removing the moments for the elevator (elev/hStab/elevTrim Cm=0) causes the elevator to act simply as a device to adjust total body lift.As you move such an elevator the aircraft will react slowly to the change in total lift you’ve introduced.You might say the pitch change is sort of indirect…it’s a feedback through CLalphpa and CMalpha…searching for a new equilibrium.Calling it a "residual elevator moment" was a poor description on my part.It leads to misconceptions…it's more of a feedback control from the sim.To me it does indicate total body lift does generate a moment….so it can’t be acting at the CG… but at a genuine aerodynamic center.Mgh that was your premise back on page 2 or so ;)The back of the envelope style might use ¼ mean chord (typical for a wing in isolation).Conceivable a coefficient like CM₀, for instance, might be used to refine the location to account for the fuselage and such. Maybe have a look at elevCM=0 and see if you come to a different conclusion.