1. The problem statement, all variables and given/known data From my notes: "In an irrotational flow, Γ = 0 for any curve lying wholly within the fluid. But circulation around a wing (airflow) is possible! Why?" 3. The attempt at a solution The obvious answer is that the air around the wing isn't irrotational. But that seems a bit too simple: they're implying that there's a possible contradiction here. I suspect that the answer is in Bernoulli's or Euler's equations, which I've heard are linked to why planes fly. But that's well ahead in my notes and I don't understand any of that yet.