302 wind calculation

On Mar 29, 1:25*pm, Darryl Ramm wrote:
On Mar 29, 9:55*am, mattm wrote:



On Mar 29, 12:31*pm, T8 wrote:

On Mar 29, 11:29 am, Darryl Ramm *wrote:

For the 180 degree wrong problem, any system that uses track and TAS/
GS difference calculations risks being 180 degrees wrong it it just
has that difference on two reciprocal tracks. If you run straight down
a ridge and do a rapid turn back the other way and fly the same track
back the other way there are two perfectly equivalent trigonometric
solutions which will give the wind from either direction. The flight
computer has absolutely no idea what direction you are crabbing the
glider into the wind.

I understand the problem of very limited information. *This is a
different problem :-).

In this case picture wind from 360, trip out on ridge with track 270,
303 shows wind from 360 @ 12, relative wind 90 degrees, arrow points
left (i.e. "wind blowing on glider" from right). *On the reverse trip,
having failed to get a wind update, the
computer now shows track 90, wind from 360 @ 12, relative wind 90
degrees, arrow points left. *See the issue? *Even in the absence of
updated wind, the relative wind should be showing 270 based on old
wind, new track. *I verified this behavior again yesterday, turning
through 45 degrees (slowly), watching track change, seeing relative
wind stay constant despite changing track until the device updated the
wind. [edit: I managed to mung the tracks up the first time].

Ignoring the 180 degree wrong wind issue, if you do do +/- 30 degree
or more track changes (not track reversals) do you get wind updates
that agree with a PDA software. What if you reset that PDA calculated
wind before each test does the wind calculated then agree?

No. *On "typical" thermal soaring days with winds under 15, but with a
shear I *know* is there, I won't get a zigzag update on the 303 unless
I alter track +/- 45, and sometimes not even then. *A single 360 turn
always works, but who wants to make such drastic maneuvers? *+/- 30 is
vastly more acceptable and that would do the trick on the older CAI
system. *My old PDA software was WP Pro 9.11. *In general, the wind on
the PDA was preferable to the 303, but less useful because I do my
final glides on the 303 (too many things to go wrong in WP, too hard
to read the PDA at high speed). *XCSoar has the nifty feature that you
can do wind calc by either circling, zigzag or both. *In zigzag only,
it out performed the 303 in yesterday's somewhat unusual conditions
(repeatedly transiting a known shear layer).

"relative wind" that is relative to track (not heading).

Understood, yes.

-Evan Ludeman / T8

Hum, interesting. *A number of flight computers have a mag compass
attachment which allows computing the wind vector in straight flight.
I had
thought the 302 had one of these, too, but now I see that it doesn't!
That
would solve this whole issue. *Did the LNav have that?

-- Matt

A compass could clearly help disambiguate the reverse track problem I
described earlier.

Some computers have a fluxgate magentometer 'compass", the LX 7000
series is the one I see mentioned most. *The main desire to have a
fluxgate magnetometer is to calculate wind without turning at all, I
don't know if any systems fold them into disambiguation track/TAS
calculations.

As mentioned earlier I've heard very few comments overall and some
negative/neutral comments about how good the LX wind calculations are
compared to standard non-fluxgate systems. I have never flown with the
LX 7000 series.

I have some concerns that trying to do magnetometer based wind
calculations with a 2D/dual fluxgate sensor (i.e. a basic compass) is
open to aircraft pitch/magnetic inclination coupling. Modern fluxgate
sensors used as a part of AHRS systems will be three axis and include
MEMS accelerometers for pitch/inclination coupling correction
(although they are still prone to acceleration/turn induced errors,
but the AHRS system can at least work out if that is going on).

I suspect many fluxgate "compasses" used in gliding computers are
simple 2D type and I wonder if this is an issue. The LX also seems to
try to detect steady straight flight (using GPS and airspeed data?)
and I am curious how much of a problem this is in practice both either
from producing inclination/pitch calculation errors or having the LX
try to prevent errors and refuse to do the calculation when the flight
is not steady (I believe it warns you when this happens). Anybody
flown with the LX series with magnetometer and want to comment on how
good the wind calcs are using he "COMPASS" setting?

Cambridge has never offered a fluxgate sensor on its flight computers
AFAIK.

Darryl

I used to fly a plane that had an LX5000 installed in it, and I
connected
my PDA running SoarPilot to the unit. It had the fluxgate compass I
believe. The wind computations were always reasonable as far as I
could tell, but I never had a chance to run a ridge with it. It
probably was
smart enough to disregard the compass when circling or accelerating,
and actually was probably using the circling drift algorithm as well.
I believe the LX160 smart vario computes wind just from circling,
when it computes it at all. It doesn't send wind data on the NMEA
stream.

-- Matt