The Rounded Rectangle cross-section has some additional behaviors that are worth noting. The Key Corner function of this cross-section will enforce the interpolated W lines nearest the intended corner to lock into that corner thus ensuring a sharp edge if desired. This also enforces an interpolated section line to intersect the rounded corner midpoint. Adding Skew or Keystone will alter the true corner radius and if set such that sharp corners are enforced, the radius will automatically return to zero. Users have the option to alter the symmetry among the top, bottom, left, and right sides of the cross-section so that corner radii may be set independently. Finally, users tend to use Rounded Rectangle for fuselage sections and have issues with VSPAERO VLM runs. The reason for this is that there are perfectly vertical sides in these sections that are collapsed to a single edge in the degenerate geometry which will crash the solver. For the purpose of VLM analyses, it is better practice to convert these sections to an ellipse so the cruciform plate is maintained without collapsing the sides. You may also try using a Superellipse with M and N set to 4.0 or a General Fuse section with all strengths set to approximately 1.5.
The Rounded Rectangle cross-section applies a rectangular section of set width and height and fillets the corners with a set radius. Additional parameters control this shape such as Keystone and Skew. Keystone will change the relative length of the top and bottom sides to make a trapezoidal or triangular shape. Skew will shift the top and bottom sides in opposite directions to make a rhomboidal profile. Vertical Skew behaves similarly for the left and right sides. Key Corner is an important feature that will force the interpolated W lines about the body to pass through the cross-section corners, ensuring a purely sharp edge if desired. Note that the Width and Height are the horizontal and vertical distances, respectively, between the side midpoints and are not the physical dimensions of your cross-section resulting from keystone or skew.
The Superellipse cross-section may be found in the Type dropdown menu. This cross-section provides additional control over the shape while limiting the number of driving parameters. For example, different combinations of M and N exponents can create circles, ellipses, rounded rectangle approximations, bell sections, and others. Altering the location of maximum width (MaxWLoc) along with different top and bottom M/N exponents further expand this cross-section’s capabilities. In many cases, a fuselage cross-section can be recreated or at least accurately approximated using the Superellipse type. For more complex sections, a General Fuse can be effective but the Edit Curve type will provide the most function and accuracy. Note that the MaxWLoc may be set beyond the prescribed height of the section, either positive or negative, where the physical distance between the topmost and bottommost points differs from the height which is the distance of the Y-intercept points of the cross-section profile.
The Ellipse cross-section may be found in the Type drop-down menu. An elliptical section has a specified height and width and will be located according to the XSec parameters at that station. An Ellipse of zero height or width will collapse the cross-section to a line which may be used to close a body. Setting both height and width to zero will collapse to a point. In this case, simply change the type to Point rather than setting the dimensions to zero.
The Circle cross-section may be found in the Type dropdown menu. The Circle cross-section places a circle of set diameter at the established location along a body or wing. Note that changing from a cross-section with different height and width to a circle will set the circle diameter to the average of these values. A Circle of zero diameter will collapse the cross-section to a point and can be used to close a component. In this case, it is better practice to simply change the section type to Point.
