Calculating Headwind/Tailwind component

[Ramy]

Most flight computers nowadays calculate wind vector, but only few calculate headwind/tailwind component, which, in my opinion, is almost equally important, since it tells you if your glide over terrain/final glide is going to suffer or benefit from the wind component, as well as help you estimate your task speed. When the vector wind is cross, it is hard to estimate if it results in HW or TW component (as it is factor of wind speed, direction and glider speed).
Thankfully, some flight computers such as 302 provide TAS, so my solution was always to subtract TAS from ground speed to determine HW/TW component, but I am surprised that not all flight computers use this simple calculation to provide instantaneous HW/TW component. After discussing it with some, it was suggested that my assumption is wrong, and that HW != TAS-GS when cross wind is presented. I am not sure why though.
Thoughts?

Thanks,

Ramy

[Max Kellermann]

Ramy > wrote:
> Thankfully, some flight computers such as 302 provide TAS, so my solution was always to subtract TAS from ground speed to determine HW/TW component, but I am surprised that not all flight computers use this simple calculation to provide instantaneous HW/TW component. After discussing it with some, it was suggested that my assumption is wrong, and that HW != TAS-GS when cross wind is presented. I am not sure why though.

This formula is correct:

Wind = TAS - GS

... but only if you use vectors. If you use only the magnitude, you're
wrong, because you're ignoring the direction of these vectors.

Taking an extreme example: your airspeed is 40kt, wind is 30kt from
the right (i.e. no head wind). You would think that your ground speed
is 40kt because you have no head wind. But since the cross wind
displaces you, your ground speed is really 50 kt (according to
Pythagoras: square root of 30^2 + 40^2).

Btw. XCSoar shows head wind component in an InfoBox. The true one.
And if you have airspeed input (e.g. with a CAI302), you get wind in
straight glide nearly instantly.

Max

[Ramy]

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

[Ramy]

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

[Jim Wallis]

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
>

[[email protected]]

Hi Ramy,

See http://williams.best.vwh.net/avform.htm#Wind
This may clarify it for you.

Your formula is only correct if the wind is parallel to your ground track. It also approximates the correct value when wind speed is much smaller than airspeed.

[Ramy]

On Wednesday, March 14, 2012 12:15:16 AM UTC-7, Jim Wallis wrote:
> 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
> >

Precisely Jim. The formula is based on the glider track relative to the ground. Subtracting GS from TAS (assuming your flight computer provides both) will always give you a precise instantaneous head wind/ Tail wind component in the direction you heading (not the direction your nose is pointing) which is really what matters to your glide performance over the ground, and to your ground speed. The formulas that other mentioned is only relevant if you wanted to calculate the head wind in the direction your nose is pointing, but there is no real value in it.

Ramy

[Alan[_6_]]

In article > 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

[Evan Ludeman[_4_]]

On Mar 14, 4:21*am, (Alan) wrote:
> In article > 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

[Ramy]

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

[Evan Ludeman[_4_]]

On Mar 14, 12:04*pm, Ramy > wrote:

> Not only it isn't meaningless, but it is very meaningful.

[snip]

I agree, perhaps I was just soft pedaling.

There's much to be improved in wind calculation using GPS and TAS even
without resorting to magnetic compass inputs. I've been working on
better algorithms, which I test using recorded nmea from actual
flying, including ridge, wave & thermals. I find that the 302
component wind works really well. It's robust, reliable and rapid.
The vector wind, well, not so much. I have an algorithm in manual
spreadsheet form that flat out eats the 302's lunch for vector wind
using the same nmea and I've promised (but not yet delivered) a brief
to JW on this (it's better than XCS' wind calculation too). If he
likes it, perhaps we can make some improvements in XCS.

Another thing that may happen sooner is using the component wind to
sanity check XCS' vector wind and report a figure of merit of some
sort.

-Evan Ludeman / T8

[Roel Baardman]

I recall seeing an OSTIV paper, by someone from New-Zealand if I'm correct, where
the wind vector was derived from just the pitot and GPS. Using multiple pitot/GPS
combinations, the most likely wind direction was estimated.
The paper used data from the perlan project as an example. The more deviations in
course, the better the result was. Appeared quite promising to me.

Roel

[Ramy]

On Wednesday, March 14, 2012 10:44:06 AM UTC-7, Roel Baardman wrote:
> I recall seeing an OSTIV paper, by someone from New-Zealand if I'm correct, where
> the wind vector was derived from just the pitot and GPS. Using multiple pitot/GPS
> combinations, the most likely wind direction was estimated.
> The paper used data from the perlan project as an example. The more deviations in
> course, the better the result was. Appeared quite promising to me.
>
> Roel

Thanks for all the explanations and opinions. I understand that more complex math and data is required to calculate wind vectors precisely. However I am looking for a conclusion about the value of the simple math of TAS-GS, and why it is not suitable as a valuable information in a flight computer. I have been using this for years to determine if and by what magnitude the wind is helping my final glide and my arrival time, or working against me. No complex math, just common sense. Perhaps calling it head wind component is not accurate, so I am open for better definition. Regardless how we call it, I have no doubt it is very valuable, more accurate and more instantaneous then the often inaccurate calculated wind vector, and thus should be included in flight computers. I believe LK8000 calculates HW, I am curious to hear if it is derived from wind vector or TAS-GS.

Ramy

[JJ Sinclair[_2_]]

On Mar 14, 5:28*pm, Ramy > wrote:
> On Wednesday, March 14, 2012 10:44:06 AM UTC-7, Roel Baardman wrote:
> > I recall seeing an OSTIV paper, by someone from New-Zealand if I'm correct, where
> > the wind vector was derived from just the pitot and GPS. Using multiple pitot/GPS
> > combinations, the most likely wind direction was estimated.
> > The paper used data from the perlan project as an example. The more deviations in
> > course, the better the result was. Appeared quite promising to me.
>
> > Roel
>
> Thanks for all the explanations and opinions. I understand that more complex math and data is required to calculate wind vectors precisely. However I am looking for a conclusion about the value of the simple math of TAS-GS, and why it is not suitable as a valuable information in a flight computer. I have been using this for years to determine if and by what magnitude the wind is helping my final glide and my arrival time, or working against me. No complex math, just common sense. Perhaps calling it head wind component is not accurate, so I am open for better definition. Regardless how we call it, I have no doubt it is very valuable, more accurate and more instantaneous then the often inaccurate calculated wind vector, and thus should be included in flight computers. I believe LK8000 calculates HW, I am curious to hear if it is derived from wind vector or TAS-GS.
>
> Ramy

In a no-wind situation, TAS = GS, so TAS - GS = head (or tail wind)
component. What's the proplem? Remember that TAS will decrease as you
get lower and the GS is tied to the track you were making good at the
time you read your GS.
JJ
USAF Nav/bomb School 1960 & 1961