How does a glider thermal?

On Sep 2, 2:03 am, kestrel254 wrote:
On Sep 2, 7:28 am, Allen Smith wrote:

Frank,
thanks for all this information!
however I would like to know of the aerodynamic features of soaring,
what happens to the angle of attack relative wind etc... when
soaring

Allen
When a glider is thermalling it is circling in a bubble of rising air.
This tends to be shaped like a doughnut (vortex ring) with the centre
part going up and the outsides going down. This is why you often fly
through sink to get to the strongest lift. Quite often there is a
"whoosh" when you enter the core of the thermal and the airspeed will
show an increase. Sometimes it will push on one of the wings, changing
the angle of bank which you have to correct.
In the UK, the thermals are often smaller than the turning circles of
the glider so the vario reading for rate of climb will vary around the
circling turn that the glider makes in the thermal.
Changes in the angle of attack which a glider makes when try to keep
the speed constant and angles of attack to keep the angle of bank
constant will affect the rate of sink of the glider but to get the
best rate of climb, one will try and circle as near the core as
possible. By having the glider trimmed right and by having an audio
output on the (electronic) vario, one can safely circle a glider in a
thermal with other gliders and hardly need to look at the instrument
panel at all. A common mistake made by early soaring pilots is to fly
with their eyes glued to the vario and forget to lookout.
It is often easy to see where the core of a thermal is by watching the
relative position of the other gliders as you all circle round in the
lift. You position relative to theirs will change as you fly through
the sink on the outside of the thermal and the core.
HTH
George

Although the British Thermal model is a good thinking exercise, radar
and LIDAR measurements show it to be a bit more complex

http://cires.colorado.edu/~angevine/thermals_2006.pdf
http://lidar.ssec.wisc.edu/papers/akp_thes/node6.htm

Frank Whiteley