At 11:11 AM 5/09/2011, you wrote:
With the general progress away from flow based variometers (electric
and otherwise) where doing away with the problems of flasks seems
beneficial, surely the next step is to get rid of the flexible
plastic plumbing from the TE head on the fin LE to instrument
panel. It can't help to have the flexible plumbing moving during
gusts and pull ups as the movement can induce unhelpful volume
changes in the line, albeit that they might be small, which alters
the signal. With power and data wiring to the fin, the pressure
transducer unit could sit in the fin in a waterproof accessible
location. Then the pressure transducer would see raw data from the
TE head better but then perhaps the signal filtering may be more
challenging. Or does the long plastic pneumatic line act as a
signal damper in a useful way?
Just a query for a slow Monday morning. Maybe someone out there has
attempted this approach?
Roger Druce
This is certainly possible to do. Could even do it wireless nowadays
with a fatter section on the fin TE probe, a small solar cell and a
lithium battery.(you read it here first). Maybe a section of
fiberglass wrapped around the fin LE
There are several problems with the fin TE mount. You can get over
the tubing problem with hard nylon pressure tube in the fuselage,
tied down properly. It is in our installation guidelines I think.
The real problem is that it is around a meter above the panel. As the
g loads change this introduces changing pressure signals. Think about
it. When you are pulling 2 g the pressure gradient in the fin is
twice what it is in the atmosphere outside so going from 1 g to 2 g
the bottom of the fin thinks it has gone down one meter. Take the g
off and it has gone back up one meter.
So a sudden increase in g will show momentary sink on the vario,
removing the g will show lift. The quicker you do this and the
greater the g the larger is the pressure transient on the vario.
Moving the sensor to the fin will help here. NOTE: changes in g cause
momentary vario transients. Steady g won't.
This is a simple treatment. It is more complex than this in reality.
Regardless of where the TE probe is mounted there are still other problems:
The induced drag effect. Surprisingly, maybe, this is greater at low
speeds than at high speeds. At best L/D induced drag = profile drag
so doubling the G load will double the sink rate due to induced drag.
NOTE: Here steady g causes the extra sink or lift on the vario.
Horizontal gusts. In thermals we fly in in a turbulent atmosphere.
We're looking for vertical air motion going upwards. There's also
lots of air motion horizontally. The effect of small horizontal
gradients in the atmosphere is surprisingly large on TE systems OF
ANY KIND and depends on the TRUE air speed SQUARED of the glider
penetrating the gust. Aerokurier ran an article in 1990 on this
called. "your vario tells lies". Several pages of the physics and
mathematics of this. Unfortunately the author got the frame of
reference wrong and came to the wrong conclusion. He thought it
depended on TAS not the square of it.
This can be fixed by not having TE at all. Unfortunately the vario
then also becomes useless.
I've got an article on our website www.borgeltinstruments.com about this.
Note that the last two problems aren't fixed by mounting the sensor on the fin.
In my opinion the horizontal gust problem is the last interesting
instrumentation problem in sailplanes. A solution is in sight.
Mike
Borgelt Instruments - manufacturers of quality soaring instruments since 1978
phone Int'l + 61 746 355784
fax Int'l + 61 746 358796
cellphone Int'l + 61 428 355784
email: mborg...@borgeltinstruments.com
website: www.borgeltinstruments.com
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