When are thermals not circular and do thermal helpers assume thatthey are?

Thanks - now I see what you mean. My confusion was because XCSoar does a
pretty good job of predicting the wind drift (due to getting good
information from my CAI302) and so the flight track (rather than ground
track) is displayed while circling. This makes it pretty easy to get back
to the lift if, due to inattention or poor decisions, you get out of it.

Slightly off topic, but IIRC I can set up the software so that it accounts
for wind drift while circling and not in straight flight. I say this
because I use the track to stay in the best lift while wave flying.


"jfitch" wrote in message
...
On Sunday, August 11, 2013 10:19:40 AM UTC-7, Dan Marotta wrote:
What are you trying to say. Seems to me that the airmass is carrying the

thermal along and the glider with it. With that premise, circling in calm

air or in a wind makes no difference. I've thermalled straight up and
with

a 20+ kt drift down wind. I never had to hold heading into a wind to

maintain center in a thermal.



Or do I misunderstand what you're saying? If that's the case, please

elaborate.







...

On Saturday, August 10, 2013 8:43:05 AM UTC-7, Dan Marotta wrote:

Your glider is drifting with the wind, too...









Unfortunately, the earth, and the GPS system that tracks with it, is

not.....

It makes a difference if you are trying to relate your thermaling track to a
track projected to the ground. Some thermal assistants (like SYM) draw a
track on the ground color coded by climb rate. If you are drifting with the
wind, this track quickly drifts upwind relative to you and the airmass
(which are drifting downwind). Therefore the next time around your circle,
the track will not have much relevance. The only thing that matters (as you
have said) is the glider relative to the airmass, which the GPS cannot
accurately track and plot. That relationship has to be inertially derived,
or simple inferred to be static. The GPS plot is relative to the earth,
which can be thought of as moving underneath you at the speed of the wind.
Such a plot is of little value when thermaling though they can be useful for
ridge and wave, which are effectively anchored to the earth.

There have been some attempts to guess the drift of the thermal from an
assumed or calculated wind speed, but most thermals where I fly are not so
well behaved.