On Wednesday, March 14, 2012 5:39:40 AM UTC-7, T8 wrote:
On Mar 14, 4:21*am, (Alan) wrote:
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

It isn't meaningless from the point of view of the glider, but I agree
that the math is sloppy.

Consider that a 90 degree cross wind relative to course track degrades
the glide to goal performance of the glider much the same as an
actual headwind. 1.27 kts in your example above. As a pilot, I can
deal with the mathematical sloppiness for information that aids
situational awareness.

-Evan Ludeman / T8

Not only it isn't meaningless, but it is very meaningful. The difference between your true airspeed and ground speed (1.27 kts in this example) has direct effect on your glide over terrain or final glide performance, and your task speed. Perhaps calling it HW component is not mathematically accurate, call it quarterly headwind or whatever, but this number is very important, especially when you point more into the wind. The current method some flight computers are using to derive the headwind from the vector wind is far less accurate as I noticed in a recent wave flight. Without circling or changing heading, even the 302 was significantly lagging in it's vector wind estimation, and as a result in the HW/TW calculation, while subtracting GS from TAS gave much more accurate and instantaneous HW/TW. The result error was in a magnitude of 20 knots.

Ramy

Ramy