WIND

[gillesbuzare]

Is it Possible to display on map the wind vector when flying ?
with head or back and cross wind.
Thanks

[kitercuda]

It is possible, but how to know it?

[bitbrusher]

Hi
You can find some background on how to calculate it here
http://en.wikipedia.org/wiki/Wind_triangle
and here
http://en.wikipedia.org/wiki/E6B

Although external data, like true airspeed and heading, are demanded from the pilot for the calculations.

Cheers

bitbrusher

[kitercuda]

Triangle is very easy, but how to know current strength and direction of wind?

[bitbrusher]

Petr
The E6B page show the equations to calculate the already known delta a and Vg (half way down the page).
You'll have to extract w and Vw from inside those two eqs.

This will lead to a "non linear" system of two equations two unknowns.
The complication is the sin/cos reverse calculations that you eventually will have to deal.

I will try to rewrite those equations in terms of w and Vw as unknowns.
Let's see if it will turn your like easyer.

Just a remark: the pilot will still have to input Va (true airspeed) and mag heading and the calculations are only valid IF any of these two don't change during the flight phase.

Cheers

bitbrusher

[bitbrusher]

Hi Petr

If I didn't screw anything up, you should use the following formulas.

Vw = sqrt( Va*Va + Vg*Vg - 2*Va*Vg cos(d-a))
wind direction(w) = d + arcsin( Va * sin(d-a) / Vw )

watch out for division by zero.

where
Vg = FiF ground speed
d = FiF TRK mag (I realy mean TRK - screen center just above compass)
Va = user provided TRUE airspeed (pressure and density compensated)
a = user provided compass aircraft heading

Hope this helps.

Cheers

bitbrusher

[marpdiaz]

Hi bitbrusher and Petr

It sounds nice but, of course, TAS must be entered correctly.
Most of us have Indicated Air Speed only which means to know the equipment specific calibration table to find the Calibrated Air Speed. Then we must provide the temperature and pressure altitude to find TAS.

typical pilot input would be:

Outside Air Temperature,
IAS + (previously loaded lookup table)
Pressure Altitude,
and
HDG

many of these are already in digital form somewhere inside our panels but it seems too much to read and fill during normal flight (except perhaps in tedious airway flight .)

nice flights,

Marcos

[bitbrusher]

Hi Marcos

Now I am confused!
Do you think it is not worth working on this?

Cheers

bitbrusher

[marpdiaz]

Hi bitbrusher

I think it would be ok feeding the values during cruise but pretty difficult in any other phase of the flight.

The error of using GPS altitude instead of pressure altitude may be acceptable. so we end up with:

OAT - thats easy
IAS + (preloaded corrections, if significant) - guess thats easy too, depending on the IAS scale in analog instruments.
HDG - I foresee some problems getting the correct HDG from the compass/gyro. here a few degrees
are important, specially when we are in cruise speed.

I am unsure it can be made robust and still provide reliable readings for most users. It would be nice to collect real data sets (plus coordinates) and compare the results with the winds aloft chart.

best,

Marcos

[bitbrusher]

Hi Marcos

Based on the "seeing is believing" principle I ran some calculations.
Here is what I found:

Code: Select all

 
Conditions: Va=118.5, Vg=120.0, TRK=  0.0

HDG Error  Speed       From    StreamWind     CrossWind
  0 =>        1.5      180.0         1.5       -0.0
  1 =>        2.6      233.7         1.5        2.1
  2 =>        4.4      249.2         1.6        4.1
  3 =>        6.4      255.0         1.7        6.2
  4 =>        8.5      257.8         1.8        8.3
  5 =>       10.5      259.3         2.0       10.3
  6 =>       12.6      260.2         2.1       12.4
  7 =>       14.6      260.6         2.4       14.4
  8 =>       16.7      260.9         2.7       16.5
  9 =>       18.8      260.9         3.0       18.5
 10 =>       20.8      260.9         3.3       20.6



Here I similated a 1.5Kts tail wind for one acft heading north.
If a HDG reading error of only 10 degrees is embedded, the calculation will inform a 21Kt wing from west.
Note the fake crosswind component inserted by the HDG error.
Way too wrong!

This confirms your forecast.
The calculation is too sensitive to user input.

As always: garbage in, garbage out!

Cheers

bitbrusher