LAK 17 (18m) polar as measured by the German Idaflieg is he
http://www.lak.lt/LAK17.HTM
Also, Dick Johnson measured the LAK 17 polar in the July 2001 issue of
Soaring.
Alan Irving wrote:
 Can anyone supply a polar curve for a current 18m sailplane?
  Ideally the Ventus 2cxT, although Schempp-Hirth polars
 seem to be unobtainable.
 I already have the Dick Johnson review of the ASH-26E,
 so will use the polar from that if all else fails.
  That said, if anyone has a curve for a DG-800/808
 or LS6c-18 I would be interested to see it.
 Thanks
 Alan