Login    Register
  • Author
    Message

Wind Vector

Postby JimB » Wed Oct 15, 2014 3:48 pm

When flying straight and level, it is not unusual for the wind vector to change direction significantly, say from the right of the aircraft to the left. And many times it will change back after a few minutes! Often the wind vector is a very different than forecast winds.

Often seems like heading and track information change. Hard to describe this problem but it didn't exist a few months ago and makes flying the plane difficult.

Only changes have been installation of torroids on some of the wiring.

So have been looking a magnetometer calibration. The correction lies within the wider set of lines. Is this acceptable?

Have also redone the TAS correction at cruise speed. If there is only a correction for cruie speed in the table, what happens at other speeds?

Anyone got ideas for what could be causing the wind vector to change?

Thanks

Jim Butcher
JimB
 
Posts: 72
Joined: Tue Mar 12, 2013 12:52 am

Re: Wind Vector

Postby GRT_Jeff » Sat Oct 18, 2014 5:18 am

The wider dashed lines are +/- 30 degrees, which is a large correction. Large corrections indicate strong interference and possibly a less reliable heading. It will still work if the interference is unchanging.

If control movements, turning lights on and off, transmitting on the radio, basically any activity other than changing the direction of the plane causes the Magnetometer Heading to change by more than a few degrees, then there's significant interference and will affect your heading and wind vector.

You need to have corrections for at least 2 speeds in the table. If your speed is above or below the correction table, a correction will be extrapolated from the closest two entries.
GRT_Jeff
 
Posts: 566
Joined: Tue Dec 11, 2012 12:11 am

Re: Wind Vector

Postby JimB » Mon Oct 27, 2014 2:27 pm

Thanks for the reply Jeff. I really appreciate that you answer questions here that require knowing how the code is written. Helps me understand how my EFIS operates:)

I did the magnetometer cal again yesterday and got a flatter correction curve. Still a sine curve but the max deviation is about 15-20 deg rather than 30 deg. Short test flight in turbulence seemed OK.

When I do a mag cal, I use the mag track from my GNS430 to align to north to start. However, there is limited room at my airport to do this so my north may be off a few degrees. How much does this affect mag cal?

Jim Butcher
JimB
 
Posts: 72
Joined: Tue Mar 12, 2013 12:52 am

Re: Wind Vector

Postby JimB » Mon Oct 27, 2014 3:52 pm

Jeff,

Couple more questions concerning mag cal. Does the rate of turn make a difference and does the rate need to be constant? I'm on a grass strip that isn't too flat so when I do the circle, I have to add power to go uphill and reduce power to go downhill at different places in the circle. I do the circle so it takes over a minute but some portions are at a faster rate than others.

The mag cal software says calibration complete probably when I reach 380 deg of rotation but it always surprises me and I think I need to go a little more. A little more rotation past that point isn't a problem is it?

Thanks

Jim Butcher
JimB
 
Posts: 72
Joined: Tue Mar 12, 2013 12:52 am

Re: Wind Vector

Postby GRT_Jeff » Fri Oct 31, 2014 6:57 am

If you don't start at north, the magnetometer calibration function will see that as an additional magnetic field error and try to correct it back, so you will have a constant heading error of a few degrees when you're done.

The rate doesn't need to be constant. The AHRS is trying to collect data points without too far of a gap between them. If you move too fast, you will skip some points and the AHRS will interpolate from the others. That's only a problem if there's a big change in the magnetic interference at those points. If you skip too many points, the AHRS invalidates the calibration.

The AHRS stops the calibration when it thinks it has all the data. If you keep rotating it has no effect on the calibration. After calibration is complete, the AHRS may reboot, and moving the plane during the early part of a reboot may throw off the alignment. If the AHRS comes up with a bad alignment after rebooting, you can fix that by rebooting the AHRS when the plane isn't moving.
GRT_Jeff
 
Posts: 566
Joined: Tue Dec 11, 2012 12:11 am

Return to Sport SX

Who is online

Users browsing this forum: No registered users and 1 guest

cron