Hello, I have been programming a doublet lattice code for nonplanar surfaces with zero thickness. I have some questions about LANDAHL's formulation. 1) How the dihedral angle is measured? This question may appear silly, but please explain. Suppose to have a wing with positive dihedral (let's say 3DEG). Suppose that the plane y-z is a symmetry plane. In the negative portion, is the dihedral still 3 DEG (in the mathematical formulation of the kernel) or 180-3 DEG (measured from + y)? 2) Connected to 1) how do I impose the symmetry boundary conditions? I was doing so by using a symmetric doublet for each sending panel, but then I did not know how to consider the dihedral (see previous question). Anybody knows?