On Calculation of Induced Drag and Conditions Downstream of a Lifting Wing

Consideration of certain subtleties connected with the calculation of the induced drag of a wing in terms of conditions far behind it. These subtleties cast light on some interesting facts concerning the flow downstream of a lifting wing system and arise from the fact that, for a lightly loaded wing, the drag is a second-order quantity. Considering specifically a wing of elliptic lift distribution, it is shown that the induced angle far downstream cancels out of the calculation and that the net pressure contribution is a force directed upstream.