When the National Advisory Committee for Aeronautics (NACA)measured the lift and drag on airfoil models in the 1930s & 40sin their specially designed airfoil windtunnel, they made wingsthat spanned the entire section, with the wing tips butted againstthe two side-walls of the tunnel. This was done to ensure that theflow over each airfoil section of the wing was essentiallytwo-dimensional (no wing tip effects). Such an arrangementprevented measuring the lift and drag with a force balance. Instead, using a Pitot tube, the NACA obtained the drag bymeasuring the velocity distribution behind the wing in a planeperpendicular to the plane of the wing, i. e., the Pitot tube, located a fixed distance downstream of the wing, traversed theheight from the top to the bottom of the test section. Using acontrol volume approach, derive a formula for the drag per unitspan on the model as a function of the integral of the measuredvelocity distribution. For simplicity, assume incompressibleflow.