Take him to the prop and ask why is it that—probably not your test plane, but certainly turboprops—feathering the prop reduces its thrust? And what happens when you feather them? Does it change the shape of the foil so that the top and bottom edges are the same length? Or does it just rotate them so that they have a zero angle of attack? The people who believe in the equal transit time theory seem to fully grasp how a fan works while struggling with how a wing works.