In the real world of building wings we always have to truncate the trailing edge of flying surfaces to some amount relative to the typical airfoil theoretical end point coordinate (x=100, y=0). Yet we use the polar data which assumes this perfect profile. It seems that in 'airfoil intensive' pursuits like racing, sailplanes, high performance etc, every effort is made to minimize truncation & extend the profile as much as possible & of course keep it rigid. The thinner the airfoil profile, the harder this is to achieve assuming a minimum acceptable TE thickness.

My question is, where on the airfoil polars would this truncation show up as a degradation? An overall inneficiency loss spread out over the Cl/Cd curve? Or more weighted at high AOA & therefore reduced Cl max? See what Im getting at? Do fat airfoils suffer less than thin airfoils? Is there a magic rule of thumb like "if you achieve less than 95% of the theoretical chord, the polars cannot be trusted & you need to use a fudge factor"? Assuming a truncation is inevitable, would it be better to radius the edge or leave it square?