NOVA Without the aerodynamic forces of lift and drag, aircraft would be unable to fly, wind turbines would not spin, and countless other machines that we depend on every day would not work.

When a plane is in the air, drag is a retarding force that equals the thrust of the craft, or the propelling force. Airfoil models are also unit tested in wind tunnels.

What separates an airfoil from a flat sheet of paper or balsa wood is the shape of its nose. Once a flat sheet’s angle of attack exceeds a few degrees, it stalls.