The ability to control most cross-section sizes by area and height-to-width ratio were added in OpenVSP version 3.26 to the existing width and height controls. Two of the available options are selected and fixed while the others are deactivated and solved to fit the constraints. This feature is particularly useful when designing flow-through components where deliberate, simple parametric variation of sectional area is desired. Many cross-section types take advantage of these new constraints including those more advanced or complex. For example, the radii of a rounded rectangle are taken into account when computing the area of such a section. In fact, because the cross-section curves are all integrable functions, the area may be computed for any arbitrary closed curve or iteratively solved to satisfy the constraints. This applies to Edit Curves as well.
A somewhat significant exception to the conversion accuracy of sections to Edit Curves is that of airfoils. While a NACA 4-series may be very closely approximated by an Edit Curve conversion, it is not exact. More complex airfoils such as NACA 6-series or custom AF/DAT airfoils will rarely be well approximated. Which is to be expected given that airfoils have very specific geometric definitions that are difficult to capture with longer, piecewise curve segments. Users are strongly discouraged from attempting to design airfoils with the Curve Editor.
This is not to say that converting an airfoil to an Edit Curve cannot be useful. In some cases, for example when blending a wing to a fuselage, an edit curve will result in a custom profile that will blend well to a surface. Another example is with the slight modification or reparameterization of a Wedge or biconvex airfoil type, which are formed from either straight or parabolic segments and will be exactly captured by an Edit Curve conversion.
Any cross-section type in OpenVSP may be converted to an Edit Curve by clicking the “Convert CEDIT” button next to Show and the type dropdown menu in the XSec tab (for bodies). Some cross-section types may be exactly recreated by the conversion process such as ellipses and rounded rectangles while other, more complex or mathematically strict, types may be approximated. Users should be aware of the limitations when converting different section types for modification. However, in most cases a close match to the desired shape may be achieved with a stock cross-section and then converted to an Edit Curve for further refinement without much effort making this capability exceptionally useful to the designer.
New Edit Curves may be initialized from the Curve tab where users may choose among a circle, ellipse, or rectangle. In practice, these are ways to initialize the three different curve types as Cubic Bezier, PCHIP Spline, or Linear, respectively. In each case, the new curve will be of unit coordinate height and width, ranging from -0.5 to 0.5 in both the horizontal and vertical axes. Note that initializing a new curve type does NOT alter the defined Scale XSec parameters controlling the height and width but rather creates a new curve in the coordinate space.
A background image may be inserted in the plot area in the Curve Editor to serve as a guide to the overlaid curve and control points. Activate the “Image” button to browse for an image file and then scale or fit the background as needed. If a background image is shown from an opposite view than OpenVSP’s convention, where U_0 starts on the right side of the plot and right side of the body, then the image may be flipped across the plotter Y axis.
Once a background image has been placed such that the desired width and height scaling fits the image, the background may be locked to the curve display using the “Lock Image” button. Once locked, the background and curve plot will zoom and pan in sync, enabling the user to closely focus on fitting corners and blends without sacrificing overall visibility.