When NACA measured the lift and drag on airfoil models in the 1930s and 1940s in their specially designed airfoil wind tunnel at Langley, they made wings that spanned the entire test section, with the wing tips butted against the two side-walls of the tunnel. This was done to ensure that the flow over each airfoil section of the wing was essentially two-dimensional. Such an arrangement prevented measuring the lift and drag with a force balance. Instead, using a Pitot tube, NACA obtained the drag by measuring the velocity distribution behind the wing in a plane perpendicular to the plane of the wing. Using a control volume approach, derive a formula for the drag per unit span on the model as a function of the integral of the measured velocity distribution. For simplicity, assume incompressible flow.