http://www.5c1.net/Glider Performance Airspeeds.htm

After eighteen years administering FAA practical tests, I have
concluded that a lot of glider pilots and flight instructors are not entirely
comfortable with the relationship between performance airspeeds and polar
curves. If graphs look like way too much math for you, maybe the following
explanation will help.

If the four gliders start together and proceed in the same direction at their
respective speeds and sink rates, a side view will look like this. After one
minute, their positions relative to their starting points will be shown by the
distance graph (for simplicity we will let one knot = 100 feet per minute - the
error is trivial). That means that the blue glider has flown 2600 feet
horizontally and 180 feet vertically. Similarly, the green glider has flown
3100' horizontally and 160' vertically, the yellow glider 4200' horizontally and
190' vertically, and the red glider 6200' horizontally and 320' vertically.

Now if we connect the gliders with a smooth curve and change the distance scales
to speed scales, we have just drawn a polar curve for the Schweizer SGS 1-26.