At 10:53 AM 18/08/2012, you wrote:
I think it is worth reminding users ESPECIALLY clubs re negative TE
setings. "100" is for not using TE probe (ie ele compensation) and
"0" TE compensation. If for some reason you during flight decide
you would like something negative for TE then when 302 is restarted
it will give problems like all but useless TE. If you as pilot use a
negative number you should restore TE to zero so as when the next
pilot who comes along and does not check the TE setting (ie left in
-ve) will not get totally frustrated. If you feel the TE is not
perfect then maybe check plumbing as I have seen a partly blocked TE
line causing real problems with 302 vario especially with a Winter
mechanical in the system. Talk to me if you wish to do a simple test
for partly blocked TE line.
Please try and discourage people fiddling with the TE and if they
are a fiddler then remind them about the -ve TE issue. And while on
varios the CNvario does have G sensing in various axis and a
magnetometer but these are not in use with current versions. Their
use is down the track and hopefully will be an improvement in gust
dampening in varios in the future.
Also just a reminder a partly blocked Pitot line will cause the mech
ASI to read something like 80 or 90 kts on tow (ie climb) and on
decent and landing read 20kts or even zero kts (esp with airbrakes
out). Suggest all put this in their mind OR when in doubt look at
horizon as that never lies!!!
Thats all
Ian McPhee
0428847642t.
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The best advice is to forget about electronic pitot/static
compensation and use a good TE probe. Under the right circumstances
electronic compensation can be made to work, sort of, reasonably, if
you are lucky and good but the fundamental problem is that you have 2
signals which can arrive at the sensors in the vario at different
times and this will result in all sorts of false temporary readings
on the vario. To avoid this it will be necessary to balance the pitot
- static systems of the glider so that the signals arrive at the
vario at the same time. This can be difficult as you need to install
a capacity in the pitot line usually and trim the volume. Not trivial
to do
properly.
The other problem is that the pitot and statics are far more
sensitive to pressure variations with sideslip than is a good TE
probe and may be located at quite different places on the glider.
Sometimes it is recommended to use a Prandtl probe (combined
pitot/static) mounted on the leading edge of the fin but if you are
going to do that you may as well use a TE probe mounted there.
Trimming the probe by using a little bit of pitot static may work
but you really need to understand what you are doing and it is better
to use a better probe or tune the existing one as there is usually a
second vario connected to the probe and if the probe is correct the
second vario will work properly also.
The problem with all TE systems is that they are sensitive to
airspeed changes (that's how they work and what they are designed to
counter) but airspeed can change because the pilot commands a pitch
up or down or because the air has turbulence. Turbulence in the
direction of flight is known as horizontal gusts. These cause TE
compensation signals that appear as fairly large readings on your
vario, depending on true airspeed squared in fact so that at 100
knots TAS, flying through the same gust will cause 4 times the vario
reading that it would at 50KIAS. The problem crept up on us as glider
cruise speeds went from 50 to 60 knots to 100 - 120 knots. The gusts
range from sub one second events to several seconds, they are the
ones that cause you to think there is a thermal there when there
isn't and you think you turned the wrong way but in reality there was
no correct way as there likely wasn't any significant vertical air
motion there. It doesn't take much of a gust. If you encounter one
where over 50 meters distance the wind in the direction of flight
changes by only 0.5 M/sec(about 1 knot) and you are flying at 100
knots TAS the vario will show a 5 knot reading even when there is no
vertical air motion. On a good day at 9 or 10 thousand feet your TAS
may easily be in excess of 120 knots which would result in a 7 knot
vario reading. You may have seen this after a low save just before
the final glide when you get conservative and take the thermal higher
than strictly necessary and burn off the safety margin only to find
what appears to be large lift and sink on the high speed glide.
Modern sensors provide a way to solve this problem (The SENSOR FUSION
VARIOMETER) and they are improving in performance all the time due to
the demand from vehicle stability systems and smartphones and
tablets. We are actively researching and test flying the solution
(which appears to work) with current sensors and when the system is
ready we'll implement it for production with the latest sensors. I'm
glad we didn't freeze the sensor design or deliver hardware with the
sensors installed, hoping that we'd solve the problem later. That
didn't work for the 302 and I see the designers are again promising
the same with their latest attempt.
The B500/600/800 are modular designs and should be able to be
upgraded fairly easily.
Mike
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