At 03:38 PM 23/05/2014, you wrote:
I believe the butterfly vario has an adjustment of how much you want
to 'mix in' the accelerator/gyro with the TE. I hear the recommended
setting has increased over time so they must be getting more confident.
Current the big advantage of these new varios is instantaneous wind
information. I can see that being very useful in the mountains.
As Jim says there is a number generated by the calibration process on
that vario as to whether the wind and airmass feature will even work.
If you can't get a high enough calibration number they are disabled.
They use a separate pointer for the gust resistant vario and it is
updated only once a second. I suspect it only works when flying in
straight lines.
Mixing crappy accelerometer/gyro data with "good" (but subject to
horizontal gusts) pressure TE gets you a less good pressure TE vario
with some attenuation of the horizontal gusts. We looked at that and
could get really good gust attenuation for very short period gusts (<
1 to 2 seconds) but the longer the gust went on the worse the gust
effect got. Might make the vario a little steadier.Again, easy enough
to get this to work in straight lines but didn't seem worth the
trouble because the real gust problem is those that last for 5 to 7
seconds which cause you to turn and fly through "sink". A false turn
costs around 30 to 60 seconds.
Years ago some people thought the problem was that they turned the
wrong way and Col Norman built a wing bending sensor to try to detect
on which side the thermal really was. Lots wrong with that including
that deflecting the ailerons will bend the wing (AFAIK there was no
control input compensation) and the scale of the lift bending the
wing - you may see a small bubble bending one wing but the larger
thermal may be elsewhere. Nothing came of that idea and I haven't
seen it revived after Col's death in a mid air. I think what was
really going on was horizontal gust encounters so there was no "right
way" to turn.
Wind calculations are done several ways now. It is better if you have
TAS information available. A couple are: You can measure drift in
thermals or difference between ground speed (from GPS) and TAS on
opposite sides of the thermal). In straight flight the difference
between GPS groundspeed and TAS easily gives the wind component in
the direction you are going. This is what you need for final glide
calculations. To get the wind direction as well in straight flight
you need a magnetic heading sensor as well as the TAS, ground speed
and track. This lets you complete the triangle of velocities to get
wind speed and direction. In any reasonable straight flight you
really only need a 2D compass. There is a little cheat possible in
gliders because we rarely fly in dead straight lines between
thermals. If a significant course correction is made you are sampling
the component in the direction of flight in two different directions
and hence you can get wind speed and direction as long as you have
TAS, groundspeed and ground track. You don't need a compass (magnetic
heading sensor ) for this. XCSoar does this. We had this in the B2000
13 years ago and called it the zig-zag wind.
I really wonder how useful second by second wind information is. When
heading for a ridge in the mountains you really want to know if the
wind is blowing at something like right angles to the face or not. I
suspect averaging the wind over 5 to 15 seconds is going to be more
useful to you and if you turn to avoid the ridge the zig zag wind
will give you that wind speed and direction anyway. It may be that
some enhanced meteorological awareness is possible in the vicinity of
thermals but I think a some better wind display will be required.
Maybe a 4 channel audio would help.
To be useful a 2D wind averaged over several tens of seconds to
within 10 degrees and a few knots is plenty good enough. For this,
the horizontal wind calculation can withstand inertial sensor errors
10 to 50 times the size of those you need for a good vario. You can
do that with the current MEMS gyros and accelerometers, aided by the
3D magnetometer(compass). How well anything using a compass will
work depends on where you are on Earth as to the characteristics of
the local magnetic field. You may need to do a recalibration when you
move some significant distance or install any new gear in the glider
or when the current draw changes. I've had a magnetometer on my bench
for 15 years. There just aren't many good places to put one in a
sailplane after considering the structure, wiring etc. The existing
compass will cause problems also and is usually already in the "best" place.
The same errors will make a *terrible* vario (vertical wind).
I suspect the real motivation behind the new generation inertial
varios is to do the horizontal gust elimination. i.e a total energy
vario that doesn't respond to horizontal gusts. You may get the
horizontal 2D wind as a side benefit. So far the first is only a
partial success, hence the playing up of the instant wind.
Mike
Borgelt Instruments - design & manufacture of quality soaring
instrumentation since 1978
www.borgeltinstruments.com
tel: 07 4635 5784 overseas: int+61-7-4635 5784
mob: 042835 5784 : int+61-42835 5784
P O Box 4607, Toowoomba East, QLD 4350, Australia
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