By default, OpenVSP sets all airfoil sections to be vertical regardless of dihedral. This assumption works very well for relatively small dihedral angles and maintains surfaces as “watertight” for export to solvers or other tools. However, there are cases where it is preferred to have very large dihedral angles, such as with winglets, when vertical airfoils would collapse the wing section to a plate. By rotating the airfoils to match the dihedral angles, OpenVSP attempts to set the airfoils to be perpendicular to the wing rather than the horizontal and allow features such as winglets to have thickness corresponding to their design. Note that the entire wing will rotate airfoils with this setting and if there is a non-zero dihedral at the wing root, the surfaces will not be watertight. To avoid this, place a small wing section (span of ~0.05 ft to 0.1 ft) at the root with zero dihedral and then correct for this addition by changing the span, etc. of the next section. This enforces vertical airfoil sections at the root and will maintain a watertight surface in your model.