Jim -- space does not permit a full reply, if you email me at I can send you a fuller set of notes ... but there is a fundamental thing about "polar plots" that most people don't get: the two axes on a polar plot must be HORIZONTAL and VERTICAL speeds, so that the axes are orthogonal, that is the very definition of what the plot means, and it is key to the use of the plot to find best L/D etc by graphical methods.

Because the L/D of sailplanes are high the error that results from the common lazy use of the glider's airspeed for the horizontal axis aren't too big, but it's mathematically incorrect.