It isn't that difficult and what to do has been
known for at least 35 years. I mentioned in my
vario talk in Canberra in 1980 that the
horizontal gust problem could be solved with an inertial platform.
How to do it at low cost has always been the problem.
You need to figure out where the glider is
pointing which is what the AHRS is for and then
where it is *going* which the accelerometers in
the AHRS will tell you after calibration.
This gives you the velocity vector of the glider
through the airmass it is flying in. GPS will
give you the velocities in 3D (E-W, N-S, up-down) relative to the Earth.
The difference is the 3D velocity of the airmass
relative to the Earth. The horizontal part of
this is the wind and the vertical part the
airmass vertical motion - aka netto vario.
This nice because the polar of the glider doesn't
matter (apart from knowing the mass) and bugs
have no effect. Neither does running the engine in a motorglider.
It is easy enough to build an artificial horizon
with the low cost MEMS gyros, accelerometers and
magnetometer packages. Many manufacturers are
doing this for the homebuilt experimental
market. i.e. Dynon , Grand Rapids etc.
Dynon use some very nice gyros from SPP which was
a collaboration between BAe and a Japanese
company which are a cut above the really low cost
gyros in things like the iPads. Even there they
had to use the rate of change of airspeed to stabilise the pitch attitude.
So what is a horizontal gust ? Yep, a rate of
change of airspeed. In turbulence you can see the
effects of the horizontal gusts on the Dynon on the displayed pitch attitude.
The problem is that a one degree pitch error on
your AHRS at 120 KTAS (a not unusual TAS in a
modern glider on a good day at altitude), by the
one in 60 rule causes a 2 knot offset on the
vario (this will change as the error wanders
around) . The same error on the horizontal
direction causes a 2 knot cross track error to
the horizontal wind calculation, which is
negligible. Hence my comment that the AHRS must
be at least a factor of 10 better for the vario
part than for the wind calculation. This accuracy
is what is called for in what are known as
"tactical grade" inertial platforms (there are
far better ones). This error of one to two
degrees is also negligible on an artificial
horizon for keeping the shiny side up. The 0.1
degree or so accuracy for a good vario (only 0.2
knots error) must be maintained during dynamic
manoeuvering. Tough to do with the "toy
helicopter" gyros as the guy from BAe who wrote a
review for Butterfly told me. He didn't believe it.
Our Dynamis sensor suite doesn't rely on the toy helicopter gyros.
More later.
Mike
.
.
At 10:19 AM 27/05/2014, you wrote:
Hi Matthew,
I'll see if I can find a reference.Â
Seems like Sam Gisiger from Triadis (Altair/Vega
fame) , who builds the Butterfly vario has
cracked the nut many manufacturers have been trying to for years.
He's a clever guy.
Cheers
Mark
On Tue, May 27, 2014 at 12:23 AM, Matthew
Scutter <<mailto:[email protected]>[email protected]> wrote:
>The algorithms we use are based on years
(literally) of research and part of a doctor thesis.
Was this published somewhere? It'd be interesting to read.
On 26 May 2014 10:12, "Mark Fisher"
<<mailto:[email protected]>[email protected]> wrote:
A response from Butterfly's Marc Forderer via Swift Avionics.
Dear Mike and Bernard
Both of you wrote us the same forum post. We
would prefer not getting involved into forums
discussions. Herâs the (short-ish) answer.
We are using AHRS computed data to determine
airflow around the aircraft in realtime. We do
this using our inertial platform inside our
sensor unit. The algorithms we use are based on
years (literally) of research and part of a
doctor thesis. The horizontal component is wind,
the vertical is what we call "vertical air massâ (VAM).
The advantage of this new computation method is
that horizontal energy addition (in gusts) does
not have influence on VAM readings.
We have a mixing method that allows the user to
blend in normal pressure sensor (TEK) based
Netto Varioâ signals into the VAM readiings.
If your installation is not perfect (= AHRS
wonât work very accurately) or if you prefer
not to use the new data but like your old
fashioned netto better (with gust induced energy
changes), then you can set it to 50% to mix it or even 0%.Â
The standard setup is 100%, that means 0
conventional and 100% AHRS based data. We are
very confident in this new technology and its
fun to use. We donât use just some Accel/Gyro
data and mix them in some way to tell people its
cool. Actually most users fly with 100%.
Instantaneous wind makes a huge difference to
your flying, even if not flying in mountainous
regions where the advantages are obvious. The
new wind indication helps you to locate thermals
or convergences in flatland areas as it reacts
to smallest local airflow/wind differences.
We have come a long way bringing this technology
to the market and making it available to many
pilots in different aircraft. We have had quite
some troubles in the beginning, but this is not
unusual. If you want to bring new technologies
to the market , you must know about the pain you will go through initially.
We are thrilled by the possibilities the system
offers and love flying with it, our customers do so, too.Â
Something you donât have to write in the forum
but its obvious:Â Old technology vario
manufacturers love to tell people that it does
not help you flying or does not work. Now that
its proven to work (ask people flying with it)
they love to tell people how difficult it is to
install it. I must admit that installation
requires good care and some hours worth of time.
But fly with one, you will see that its worth it.Â
Cheers
Marc
--
Mark Fisher
Managing Director
Swift Performance Equipment
Unit 2, 1472 Boundary Rd
Wacol 4076
Australia
Ph: Â <tel:%2B61%207%203879%203005>+61 7 3879 3005
Fax: +61Â 7 36076277
<http://www.spe.com.au/>www.spe.com.au
[]
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--
Mark Fisher
Managing Director
Swift Performance Equipment
Unit 2, 1472 Boundary Rd
Wacol 4076
Australia
Ph: Â +61 7 3879 3005
Fax: +61Â 7 36076277
<http://www.spe.com.au/>www.spe.com.au
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