Best Wing System: An Exact Solution of the Prandtl's Problem

In 1924, Ludwig Prandtl published a fundamental paper in which he showed that the lifting system with minimum induced drag is a box wing. The results were obtained through an approximate procedure. In this chapter we obtain an exact solution of Prandtl's problem. In particular, we prove that the lift on the horizontal wings results from the superposition of a constant and an elliptical distribution and, on the vertical wings, it is butterfly shaped, as already shown by Prandtl. The discrepancies between the two solutions are discussed.