Here's something to chew on:

Don't hand launch gliders and most small rubber models use thin, undercambered airfoils ? If a Clark Y or USA-35B is better, why don't the highly competitive HLG guys use the thicker airfoils ? And how did full scale designers of the WWI era get started using thin airfoils in the first place ? What about the Wright Brothers, didn't they use a wind tunnel to develop their thin airfoils ?

Hint: These questions will lead you to an important insight for your WWI models.

Dick Fischer