In article m Jim Wallis writes:
I think Ramy is correct in this. In his calculation he is basically
assuming that he is flying his true heading - that is, he has adjusted his
true course to compensate for wind. Because of this, all of the vectors
become co-linear.

- Jim




Ramy

To clarify more, my formula is not Wind =3D TAS-GS, it is HW component
=3D
=
TAS-GS. This is the true head wind component as I explained. XCSoar does
no=
t currently show the true head wind based on TAS-GS, instead it is
calculat=
ing it from the vector head wind which is not as accurate.=20

Ramy




If Ramy is flying due north at 40 kt, with a 10 kt wind from the east,
he will need to be crabbing into the wind to maintain his ground track.

With his true airspeed of 40 ktas, his true heading will be about
14.48 degrees, and his groundspeed will be about 38.73 kts. His true
course will be 0 degrees.

Relative to his true course, his headwind component is 0 kts.
Thus, TAS - GS = 40 - 38.73 = 1.27 kts.


If you convert that wind from the east into components towards his
nose and towards his right wing, then you get 2.5 kts on the nose, and
9.68 kts on the right wing. When you compute his resultant velocity
from 40 - 2.5 kts forward, and 9.68 kts sideways, you get the same
groundspeed as computed before, about 38.73 kts.


The basic problem is that it is generally meangless to compute TAS -
GS, as those are scalar magnitudes of vector values, and the vectors
are rarely colinear.


Alan