On Wednesday, January 22, 2014 8:36:38 AM UTC-8, Dave Nadler wrote:
On Tuesday, January 21, 2014 7:14:33 PM UTC-5, son_of_flubber wrote:

Get a high tow into still air...



The likelihood of measuring meaningful data yourself is ZERO.

Johnson published measurements are in some cases badly off,

as the CG was not controlled during testing, and the datapoints

and reduction inadequate. It is REALLY HARD to do this properly

and only Idaflieg (?sp) measurements are generally useful.



Choose a similar polar and dial in a few bugs until final

glides generally match...



Hope that helps,

Best Regards, Dave

This is imprecise at best for a whole bunch of reasons relating to measurement errors and differences between the configuration of your glider and whatever measured or analytical approach was used to make the polar you get from whatever source.

For a modern glider you really care about a speed just around (preferably above) best L/D and one that is in the middle of your most typical cruise speed.

Fly some long final glides (still air is best of course) one of these two speeds and see if you gain or lose on the arrival altitude. Move the point down of you are consistently losing, move the point up if you are consistently gaining. It is possible that you will adjust the points in opposite directions. Remember to note your wing loading.

Simpler is to pick your most common glide speed and use the bugs adjustment.. There are quadratic formulas widely available to make polars from three data points.

9B

Then you will have a polar that works for you.