ptxman,
I just paid the paltry $10 to get my Profili V2.0 upgraded to the full-version. Found out immediately that Stefano is now up to V2.14 with a truck-load of new features. OH MY GOODNESS!!!!!!!! Don't open this program unless you have several hours to spend playing with it!

Anyway, I took the RK40 airfoil and used the Xfoil processing features to thin it to 7.5% to simulate the Avionik D99 wing. Then I used the Xfoil TE thickness tool to increase the TE thickness to 0.5mm. I then made a version with an obscene 2mm TE. I had Xfoil calculate the polars at Re=500000 to see what would happen.

I'm sure that the Xfoil processing works a little differently than just physically truncating the airfoil, but it was interesting to see the calculated results, all the same. It seems you need a very blunt TE to seriously degrade the Cd.

An interesting side-observation is that the Cd becomes very high at very low Cl.

Here's the output:
-David