At 03:06 30 September 2007, Eric Greenwell wrote:
Dan G wrote:

BTW, on the gust issue, there is another solution
(though not nearly
as elegant as the rotating TE probe) - accelerometers.


I think I understand how accelerometers could distinguish
between a
horizontal gust and the vertically moving air in a
thermal, but I don't
understand how aligning the probe with the horizontal
gust will reduce
it's effect on the vario. Wouldn't that accentuate,
rather than
attenuate, the gust effect?

--
Eric Greenwell - Washington State, USA

Not at all, at least not if it is the 2 or 3 way probe
and this is why. It is first and foremost the static
system that is particularly sensitive to gusting. When
a horizontal gust hits the side of a plane, it creates
positive pressure on one side of the static inputs
(whether fuse or probe mounted), throwing the reading
way out of whack. With the static on a rotating probe,
it can align itself to always face directly into the
wind and alleviate the big errors in static reading
that horizontal gusts ( or other disturbances in yaw)
create. It does not hurt to have the TE also rotate
with it, and does not hurt to have the dynamic rotate
as well (with rotating dynamic, a cup shaped Kiel tip
is not required to maintain accuracy-less drag) but
dynamic and TE are not as sensitive to misalignments
in airflow as the static system is. This probe system
will provide accurate readings during both slipping
maneuvers (like if you mildly slip while thermalling
ala Holighaus) as well as gusting incidents. BTW, the
'fancy' 2 way probe Dr Smolik sells is only 80 Euros,
and the 3 way probe only 99. This video is the 3 way
probe:
http://youtube.com/watch?v=OpN9sYHF_yc , and this link
shows drawings and prices for the whole line:
http://www.eaglebrandproducts.com/TE...rices_2006.pdf
I am only assuming here, but I would imagine these
probes to be much cheaper than an accelerometer compensated
computerized vario system (not to mention compatible
with most anyone's current setup) but perhaps maybe
not as effective in all situations. It is a simple
and elegant aerodynamic solution to an old problem,
that I'm sure will be seen on many more gliders in
the near future. Again, I will share the data tables
Ludek provides, once he sends them to me.

Paul Hanson

PS. I have no financial involvement with these probes,
I just think they are very cool.
PPS. there is a reason that NASA and most other flight
test groups use static/dynamic (they don't mess with
TE) probes that rotate (in 2 axis) to keep readings
accurate while evaluating aircraft.