I just set up a scenario in condor with my streak running xcsoar connected
to experiment with the glide bar behavior. The setup is as follows:
LS8, final Glide St. Croix to Puimoisson due north 10.2km distance with
50km/h headwind, 1352m MSL.
Switching MC values in XCSoar gives following results.
MC 0 = +11m above final glide
MC 0.1 = +8m
MC 0.2 = +3m
MC 0.3 – 0.6 = no glide bar
MC 0.7 = -596m (glide bar reappears)
MC 0.8 = -266m
MC 0.9 = -212m
MC 1 = -195m
I’m sure these calculations are technically correct but from a practical
point of view this is madness!
2011/11/22 Max Kellermann <[email protected]>
> This is the "I repeat myself over and over" thread!
>
> On 2011/11/22 20:51, David Reitter <[email protected]> wrote:
> > In my view, the problem at hand is that XCSoar seems to assume that
> > height is always gained by way of thermaling. That is obviously not
> > correct.
>
> No, you are wrong. XCSoar only assumes that height is gained by way
> of thermaling if you tell XCSoar to do so. If you don't tell XCSoar
> to do so, XCSoar will not assume it.
>
> If you set a positive MacCready value, you tell XCSoar that you want
> to gain height by circling thermals. If you don't plan to do that,
> don't set a positive MacCready value, because the whole point/basis of
> the MacCready theory is the assumption of future lift.
>
> All other reasons to set a positive MacCready value are kludges that
> don't fit into the theory, but XCSoar might need additional features
> to account for some of that. For example, head wind is already being
> considered, and you don't need to adjust the MacCready setting for
> that. You may use the "bugs" setting to degrade your polar, but don't
> tweak MacCready for that.
>
> > XCSoar 6.x does not make assumptions about finding a thermal or other
> lift source if MC>0, for purposes of reachability calculations, right? I
> would be very confused if it did. Please clarify. Recall that reaching
> some outlanding field is already "plan B", i.e., it's the plan in place for
> when the thermals die.
>
> If the outlanding field is reachable, no lift is assumed, because no
> lift is needed.
>
> If the outlanding field is not reachable, and you have a positive
> MacCready setting, then XCSoar will need to assume that you will be
> thermaling to ever reach that field, and it will include the wind
> drift while thermaling in its calculations.
>
> If you don't want XCSoar to assume you'll ever be thermalling, don't
> set a positive MacCready value. (Have I repeated this statement often
> enough in this email?)
>
> Max
>
>
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