Eric Greenwell wrote:
Greg Arnold wrote:


However, this assumes that thermals are fixed with respect to the
airmass. In fact, to stay in a thermal you have to keep moving upwind
in relation to the airmass. This means that thermalling will give you
some extra distance toward your goal. So you want to spend more time
thermalling and less time cruising, which you get by increasing your
speed somewhat.

How much slower than the airmass do you think the thermals are moving? I
don't notice the wind determined by circling to be significantly
different than the wind determined by cruising, according to my
Cambridge 302.

I don't think computers are accurate enough to note the difference, and
the difference would be small in any event. Maybe a knot or two. I
don't know if the difference is significant enough to affect the way
that people fly, but my point was that it is not strictly true that we
want to ignore wind speed.

And also, why would you get blown out of a thermal? Since
you are circling in it, shouldn't you drift at it's speed, instead of
the wind speed?

No, because you aren't rising as quickly as the air, so you are sinking
into air that has left the ground more recently and that hasn't drifted
as far downwind. Also, see Todd's comment about wind shear.