Eric Greenwell a écrit :

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.

My Lx5000 seems to find a slightly higher wind speed in straight flight than in circling (about 1/4). But it depends on the settings (I use combination of drift and speeds) and there may be an error in Vi as measured by the computer, or in conversion to Vp, that would induce an error in the wind in straight flight, whereas the wind in climb is not subject to these errors.

Anyway, as Todd told it's not worth any correction to Mac Cready setting.

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?

THat's perfectly true if the thermal is vertical, which should be the case if it is not triggered by a ground feature, and with no significant wind shear. In the other case, as you climb slower than the air in the thermal, you will get under it and have to correct upwind to find it again (another solution for us impure pilots is start the engine to climb again in the original part of the thermal without ajust our circling ;-) )

Denis