RE: [Jsbsim-devel] Calculating forces and moments

[Jon B. <js...@ha...>]

> > I think that is true, but can you explain more what you mean?
>
> yes...   what I meant is that there is no set limit on the number of
> coefficient that might be specified within an axis group.

True.

> For example if I had an aircraft with a canard and a horizontal tail and if I
> could specify the delta e of the canard from the control surfaces, I
> can list another coefficient to calculate the force due to the canard.

Yes.

> Also could you tell me if there is a mistake in the configuration
> files of the c-182 and the c-172p?  At Cmalpha, one of the factors the
> coefficient is multiplied with is alpha.  That doesn't make sense to
> me.

Here is what is in the file:

<COEFFICIENT NAME="Cmalpha" TYPE="VALUE">
  Pitch_moment_due_to_alpha
  aero/qbar-psf | metrics/Sw-sqft | metrics/cbarw-ft | aero/alpha-rad
  -1.8
</COEFFICIENT>

There are really several ways to do this. If we have a curve that shows Cm as a function
of alpha (that is, alpha is along the X axis of the curve, and total Cm is on the Y axis),
then there is no need for the Cmo definition, and the Cmalpha definition would not include
alpha-rad.

However, what we have is the slope of the Cm curve due to a change in alpha, dCm/da. So,
our equation for total Cm is:

Cm = Cm0 + qSc(dCm/da)a

where "a" is alpha, "q" is qbar, "S" is wing area, "c" is chord.

That is how Cm is defined in the C172p (and other) files (of course, there are also other
contributions to Cm).

Does that help?

Jon