Brent,

 

I don't even pretend to understand this airfoil stuff.  But I think you make a good point, the "questionable" airfoil only exists at the tip, one inch inboard we have a hybrid section where no test data exists.  I'm not sure you can interpolate airfoil performance between two sections.

 

During a phone conversation with Tim Ong a few months ago, he mentioned that the graphs assume an infinite length airfoil and don't take into account some of the things you mentioned like wing tip vortices.  He said, "It just does do what the graph shows in the real world".

 

So the airfoil that most closely matches the 64212 is most affected by vortices and other factors not accounted for in the graphs.  As you move inboard and the wing more closely resembles an "infinite" section, you are now dealing with a very different section than the 64212.

 

The reason the FAA makes you fly the tests is the lab data doesn't always accurately predict real world characteristics.  The data is a good starting point, that's all.

 

Mike