formula required

[overwing]

Hi, can anybody give me a formula for mag track between two co-ord's. :confused:

[jabberwok]

It's not that simple...

It is an easy matter getting the true track between two co-ords but the magnetic track would depend upon how far apart the two points are.

If they were reasonably close together so that the Variation was very similar you'd be OK. If you are talking about points many hundreds of miles apart with huge Variation differences then you can only specify an initial heading from point A - it would not remain accurate for very long though.

[Chocks Wahay]

On a similar subject - can anyone come up with the formula to give a heading when you know the track and wind direction. Obviously not straightforward as you need to take into account track, wind direction, windspeed and airspeed.

Ta muchly

[Charley]

Try the Aviation Formulary page (http://www.best.com/~williams/avform.html) for loads and loads of goodies.

Hope this is of some use.

[overwing]

Thanks guys...Sorry for delay in replying. I've been at work. (Jabbrrwok) .. max distance is c.100nm so no problem with long routes. Can you give me a formula that will work. I've been to (charley) Avaition formulary but it is not giving accurate results. I might be entering the formula incorrectly (using Excel). If you can give me a worked example it might help. (Chocks Wahay) I've worked that one out before, I'll look it up and give it to you. Ta again.

[Charley]

Hi Overwing,

I've just checked it and it works okay, to within a degree or so. I checked it with a EGBB Cowly SID by plugging the Honiley and Cowly co-ordinates in.

There are three possible causes for the grief you might be getting. Firstly, the formula is a trigonometric great circle calculation therefore the result will be in degs True. This will account for small errors. Secondly, the author is from the States and therefore he treats western hemisphere longitudes as positive, whereas we Brits (and Irish?) would probably use a positive east and negative west. This would create somewhat large errors! :D

Finally, Excel is really picky with brackets. Depending on the arithmetic functions involved, you often need more than you would to write the formula.

For your viewing please I have created a small Excel workbook which works okay, I've even reversed the numbering convention for east/west co-ordinates to the 'correct' way (i.e. +ve E and N, -ve W and S). As I said I've checked that it works and, after applying the variation stated at the top of the instrument departure, the result was only a degree out. This should not be a big deal if you are only flying legs of less than 100nm.

Let me know if you want the spreadsheet emailing, you might have got it working yourself by now.

Cheers
Charley

[Charley]

chocks wahey!

The formula that you need is on the same page, look under the wind triangles section. There are also formulas for converting Mach numbers to TAS, depending on how fast you're planning to be going when you want to know the wind!

[overwing]

Charley, Please E-mail the spreadsheet to me. I cannot get the ans. within 10 deg. It may be the brackets or the ATAN2(y,x) v ATAN2(x,y)as Excel needs it. Ta in advance.

[Chocks Wahay]

Can't get the link to work to http://www.best.com/~williams/avform.html - is it down?

[Charley]

Chocks, have you had any joy? It's been okay for me, perhaps it's a proxy thing if you're trying from work.