Having constructed the initial wing shape as a stack of airfoils, the 2D flow around an airfoil can tell us much about the 3D flow around a finite wing. In particular, exploring first in 2D the mapping from shape to flow to performance and its inverse tells us much about the roles that thickness and camber play in attaining sought-after performance. A rapid, special-purpose tool for airfoil analysis greatly aids the aerodynamic designer if results can be run in seconds on a laptop computer. This chapter describes one such tool, MSES, a surrogate model to the Reynolds-averaged Navier–Stokes (RANS) methodology, which very rapidly solves the steady Euler equations coupled to the integral boundary-layer equations. As a rule, a RANS code is too slow for routine design work and has not yet shown any accuracy advantages over the much faster zonal approaches. However, it is more robust with respect to Mach number and flow separation and can compute the entire shock stall phenomenon, as we saw in the steady-flow example in Chapter 6. Examples are given showing MSES applied to airfoil designs in both direct and inverse modes. MSES together with RANS completes the computational fluid dynamics tool kit needed for the applications in the remaining chapters.