Viscosity is fundamental. A two dimensional airfoil inside inviscid fluid has zero drag (maybe you expected that) and indeterminate lift (that for me was a surp

Viscosity is fundamental. A two dimensional airfoil inside inviscid fluid has zero drag (maybe you expected that) and indeterminate lift (that for me was a surprise), unless you force the stagnation point to be on the trailing edge. This mathematical condition is known as Kutta condition, and the physical process that allows it is viscosity. I know the explanation is somewhat cryptic but it's a starting point.