Re: [Flightgear-devel] Yasim related Merge request

[Gary N. <gr...@gm...>]

On Thu, Apr 10, 2014 at 5:12 AM, Colin Howell <col...@gm...>wrote:

> On 2014-04-09 09:13:41, Vivian Meazza <viv...@li...> wrote:
>
> > On a different note - I see that we have introduced slat extension into
> the
> > equation for stall angle of attack, instead of the existing on/off
> version.
> > As I understand it when a slat extends at first it has no effect, then
> there
> > is a brief transition phase, and then it becomes fully effective at some
> > point in the extension. The exact points at which these phases occur
> depend
> > on all sorts of factors: design, AoA, velocity - best summarised as
> voodoo.
> > While not totally realistic, I think that in view of the uncertainties,
> > Andy's on/off implementation is better than adding a slat extension
> factor.
>
> That would be true if Andy's implementation was on/off, but it wasn't.
> It was always on. Just *having* the slats gave you the full benefit,
> and with no drag penalty besides.
>
> Assuming you're right about real slat behavior, then an on/off
> expression might look like:
>
> stallAlpha += (_slatPos > 0.5) * _slatAlpha;
>
> (for a critical point at half-travel) or some other boolean
> expression. Though I wonder if the real non-linearity is something
> rather less extreme than simple on-off. After all, if the slat was
> fully effective after, say, 60% of its travel, then most of the
> remaining travel would impose a significant extra drag cost while
> providing little benefit other than perhaps a larger safety margin
>
>

There are some aircraft where I would think slat behavior cannot be
entirely linear. For example, in the MD-80 series slats have two positions
slaved to flap deployment: mid-sealed for flaps up to 13 degrees, and fully
extended for flap extended from 15-40 degrees. I would expect that in
addition to stall alpha there would be some effect on wing camber and/or
chord, but unfortunately YASim doesn't model the later two possible
effects. In any case, if YASim modeled only on/off slat behavior, it would
break implementations like the MD-80's. The developer can always limit slat
behavior using property script or settings, but if slat behavior is limited
by the FDM, the developer loses a potential tool.

-Gary aka Buckaroo