On Tuesday, March 13, 2012 7:51:26 PM UTC-7, Ramy wrote:
I gues this is where the confusion is. In your example, you indeed have 10 knots tail wind in the direction you heading. You assumed the wind is 90 degrees, but it is not since you drifting. Now if instead you crab to maintain heading, the tail wind component will change to head wind. So this is proving again that my formula is correct. This is the true head wind/ tail wind component as far as the glider see it, which is ultimately what impact your ground speed and glide perfromance over the terrain.

Ramy

To clarify more, my formula is not Wind = TAS-GS, it is HW component = TAS-GS. This is the true head wind component as I explained. XCSoar does not currently show the true head wind based on TAS-GS, instead it is calculating it from the vector head wind which is not as accurate.

Ramy