I'm surprised by some of the airfoil thicknesses stated here. I heard biplanes often have thinner airfoils. But are they really as thin as 9%? A rough measure of a BJ Craft Encourage showed ~13% at the root to ~11% at the tips. Am I reading this correctly, or are there other factors that make thinner more desirable?
What would the expected effect be of thicker airfoils on the biplane?