LX-Polar Program
GM
does anybody have a usable, step-by-step decription of the LX-Polar
program? The instructions that came with the manual of my LX-20-2000
are pretty useless. I have a polar curve for my glider and I am trying
to calculate the factors a, b and c used to define the polar.
Thanks,
Uli Neumann
Tuno
The instructions you're looking for cannot be found. (At least I've
never been able to find them, looking on and off since 2004.)
It's been a while since I played with the program but if you're
patient and play with the import/export feature and the .txt files it
uses, you can gain some insight into how it works.
I wish LX would devote a few more hours to rounding out the
functionality of that program. It seems it was dropped on the floor
about 15% short of being where it was intended.
Not that I've ever done that myself :)
-ted/2NO
Ian Kennedy
http://www.naviter.si/download/other-tools/index.php?Itemid=27
This gives you the Programme.( Polar.exe )
Then go to
http://data.naviter.si/docs/polar.pdf
for a guide to usage.
Cheers
Ian K
"Tuno" <tedcw...@gmail.com> wrote in message
news:942b4550-b4aa-423e...@k7g2000hsd.googlegroups.com...
Big Wings
ablac...@comcast.net
>
> - Show quoted text -
Uli,
I am assuming you want to use the polar for use in a computer for
final glide and other flight performance calculations. If so, here are
some hopefully useful and not too pedantic thoughts.
If you are working from a measured polar you need to pick three points
off the curve. Choose well if you intend to get good advice from your
computer. I recommend one point at best L/D, one at your typical
cruise speed, and one at a fast final glide speed. I use 60, 85 and
114 knots on my ASW-27B - but I fly in the west. Since you are fitting
a curve to three points the curve will vary a bit from the actual
polar at points away from the ones you pick.
You can use the LX program or solve the equations yourself (or both as
a check). Basically, you are solving for the quadratic polar equation
Y=aX^2+bX+c, where Y is sink rate in knots and X is airspeed in knots
(you can use other units, depending on what your computer uses). With
three points (call them: Xone,Yone; Xtwo,Ytwo; Xthree,Ythree) you have
three equations in three unknowns and the solution is:
a=((Ythree-Ytwo)/((Xthree-Xtwo)*(Xthree-Xone)))-((Yone-Ytwo)/((Xone-
Xtwo)*(Xthree-Xone)))
b=(Yone-Ytwo+a*(Xtwo^2-Xone^2))/(Xone-Xtwo)
c=Yone-a*(Xone^2)-b*Xone
This is the basic format for use in a spreadsheet, but you have to
make the references point to actual cells.
You should also know the wing loading of the polar you are using to
get the points so you can adjust for the actual wing loading of your
specific glider. If R is the ratio of the weight you want to the
weight the polar was measured at (typically what you want is the
flying weight dry for YOUR glider with you and everything you
typically carry). The adjustment is simple to go from old (wing
loading for the measured polar) to new (wing loading for your glider):
a(new)=a(old)/R^0.5
b(new)=b(old)
c(new)=c(old)*R^0.5
Of course in the real world gliders don't always perform to the
measured or factory calculated polar for a whole bunch of reasons so
use your computer's advice with care.
Hope that's helpful.
9B
matt...@gmail.com
Uli,
Basically you just have to invoke the program. It expects input in km/
hr and m/s, so if you are using American
units you'll have to convert them first. I had a devil of a time
figuring this out with the LX5000
in our club 304C; the values for the coefficients didn't make sense.
Finally I determined that
they output the sink rate in m/s when fed the speed in 100s of km/hr
(hectakilometers?). 9B's
comments about wing loading, actual experienced polar, etc. all apply
too
Armed with that information I found that the 304 entry programmed into
the unit must be the old
Glasfluegel 304 from the 1970's with about a 36:1 glide ratio. The
304C is closer to 42:1 and of
course has a different wing loading (especially with me in it), so the
glideslope info the unit spits out
is not very useful for flying fast. It was very disappointing to
arrive at the finish line about 1000' high
after my PDA cable went inop the last day at R5N, causing me to have
to navigate the old fashioned way.
Now that I know what the coefficients are, all I have to do is
convince the unit to use them!
Matt (sometimes AI)
GM
Hi all,
thanks to all that responded. The problem was simply in the
installation of LXe on my Laptop. For some reason or another, I could
not insert the measured data pairs into the coordinate system. I wiped
LXe from the computer , downloaded the latest version from the LX-Nav
site and re-installed it. Now it works like a charm.
I also got a short version step-by-step from Dr. Michael Seischab at
LX-Navigation.
Here is how to use LX-Polar:
1 if you have a polar curve for your glider (measured or calculated),
make sure the speed is in km/h and the sink is in m/s
2- open LX-Polar
3- under tools, pick 'Insert'. The pointer changes from the arrow to a
hand
4 - move the hand over the coordinate system to the X-Y position. The
exact X-Y position is shown below the coordinate system
5- left-click to insert the point
6- repeat for each point beginning at # 3
7- if you misplaced a point, use the move function to correct the
placement or the delete function to erase a point.
8- enter as many points as you like.
9- when done, hit F9 or select 'calcuate' from the Tools drop-down
menue. The result are the three factors a, b and c displayed in the
coordiante system and on the left side. The program also shows the
max. L/D at what speed and sink rate.
On my LX-20, I can enter these three points under the User-Polar. I
will have to experiment a bit with this new polar before setting out
on that record breaking flight ;-)
Again, many thanks for all the responses.
Uli
Libelle 'GM'