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relative wind direction

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relative wind direction

Postby gkick » Sat Dec 02, 2017 11:04 pm

Hi,
regardless of basic or advanced or real weather fetch, is there a property which contains or calculates the angle, wind hits the aircraft based on wind direction and heading.
For instance heading 360 wind direction from 90 = wind from the right at a right angle 90 deg.
Now, if we change our heading to 30 deg, wind direction still is from 90, however the angle hitting the aircraft has changed. If such a property is not available, how could the relative value be derived by just using heading and wind direction in a fashion to produce the relative angle depending on whether we turn left or right.
Thanks
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Re: relative wind direction

Postby curt » Sat Dec 02, 2017 11:09 pm

The airplane doesn't really know about wind ... it reacts in the airmass (mostly) as if there is no wind. If you combine the aircraft (true) airspeed + heading with ground track and ground speed + wind direction and speed, it forms a vector triangle. If you know 2 of those, you can compute the 3rd. (Of course turbulence and gusts are a little different.)
Aerospace Engineering and Mechanics
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Re: relative wind direction

Postby gkick » Sun Dec 03, 2017 12:01 am

Thank you Curt. In that case since we have a property wind from and another one base wind from one could be tempted to assume, that the reciprocal of this property is the direction the wind (airmass) is blowing to.
Now if we want to create an instrument which points the nose at whatever heading and a needle pointing to the relative wind (air mass) angle, is there a simple solution using existing properties rather then coding in Nasal?
Because the wind from property appears to be a constant, only changes with specified layers.
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Re: relative wind direction

Postby Thorsten » Sun Dec 03, 2017 8:37 am

is there a property which contains or calculates the angle, wind hits the aircraft based on wind direction and heading.


No.

For instance heading 360 wind direction from 90 = wind from the right at a right angle 90 deg.


Only if the aircraft is not moving, otherwise the angle at which air hits the plane depends on airspeed.

how could the relative value be derived by just using heading and wind direction in a fashion to produce the relative angle depending on whether we turn left or right.


If the aircraft is not moving, by subtracting one from the other. If the aircraft is moving, by doing vector sums.

Because the wind from property appears to be a constant, only changes with specified layers.


Rest assured, it is only constant if you specifically ask the weather system to keep it constant. Otherwise it changes.
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Re: relative wind direction

Postby Alant » Sun Dec 03, 2017 10:39 am

Why do you need to know this?

Be assured that the FDM (JSBSim or YASim) does these calculations for you.

The airspeed and sideslip relative to the local air, the ground speed and ground track angle are all available in the property tree, as are the loacll wind speed and direction.

You can easily calculate related values such as drift angle from these properties.

Alan
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Re: relative wind direction

Postby gkick » Sun Dec 03, 2017 3:41 pm

Thanks all
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