On Sep 22, 3:07*am, Big Wings wrote:
"SeeYou" does it for you and is easy to use.

At 00:49 22 September 2008, GM wrote:



Hi all,

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- Hide quoted text -

- 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