OpenVSP Subsurfaces
The guides below introduce Subsurfaces for OpenVSP components including how to add different types and how to interact with wing and propeller control surfaces.
Encyclopedia
Updated Sep 29, 2025
Introduction
Subsurfaces are lines, rectangles, or ellipses defined in the UW coordinates on an OpenVSP model surface. These features are honored by the CompGeom, CFD Mesh, and DegenGeom functions and may be used to model inlet/outlet boundary conditions, control surfaces, material properties, surface roughness, etc. This video provides an overview of the Subsurface panel in the OpenVSP GUI and demonstrates some basic use.
Subsurfaces have a variety of uses within OpenVSP, such as isolated regions in the Parasite Drag tool, and as tagged regions for external tools. They are also how Control Surfaces are applied for use in VSPAERO.
Brandon Litherland
Aerospace Engineer
Lines
Subsurface lines may be defined at either constant U or W locations on the surface of a component and will be placed for all W or U, respectively, for that location. Lines are a handy way of isolating or splitting regions of a component surface into sections by defining the relevant region either ahead (less than) or behind (greater than) the line location in the U or W directions. For Stacks or Fuselage components that wrap W around the component body, Lines may be used to define a face or region that is intended to be an inlet or outlet. This video tutorial demonstrates the Line subsurface type control and some applications.
Understand that the subsurface lines are straight in UW space but will bend and deflect on your OpenVSP components as the surface bends.
Brandon Litherland
Aerospace Engineer
Finite Lines
Subsurface Finite Lines are similar to subsurface Lines except that the start and end U and W locations may be controlled independently at any points on a component surface. Finite Lines are an excellent way to mark regions where you want to include additional feature lines or intersections, particularly when working with exported file formats. Note that Finite Lines do not have the ability to section off regions of the surface in the same way as other types because they typically will not completely enclose or split the surface. Similar to Lines, Finite Line subsurfaces are an interesting way of visualizing the surface Bezier curves defined by the component parameters.
Experiment with Finite Lines to look at how your OpenVSP component surfaces bend in space and see if you can identify regions where the surface kinks or shifts in unexpected ways. This could indicate where some extra Skinning work is due.
Brandon Litherland
Aerospace Engineer
Rectangles
Subsurface Rectangles are a closed boundary placed along UW coordinates on the surface of an OpenVSP component. Similar to other closed boundary subsurfaces, the shape is defined by a UW center location. UW lengths, and a theta rotation. These closed subsurface boundaries are useful for identifying regions for vents, hatches, doors, etc. in visualization or analyses.
Try to keep in mind that the rectangle is defined in UW space and will often be a completely different shape on the surface of your component. Experiment with different geometries to see this in action.
Brandon Litherland
Aerospace Engineer
Ellipse
Subsurface Ellipses are another closed boundary type that may be placed on the surface of an OpenVSP component. Similar to other closed subsurfaces, the boundary is defined by a UW center location, UW lengths, and a theta rotation. Ellipses, like other subsurface types, may be used to define isolated regions of the surface for export, analysis, or visualization.
Ellipses are unique in that they are also defined by the Num Points used to define the segments forming the ellipse. This means that they are technically N-sided polygons. You can use this to your advantage to create triangles, rectangles, pentagons, etc. on the surface of your component. If you wish to either bisect a corner of a polygon or make one side ‘normal’ in UW space, the quickest way to accomplish this is to use OpenVSP’s Smart Input feature and enter “180/N” in the Theta field where N is the Num Points used to define the shape.
Subsurface ellipses can quickly become a versatile tool in your arsenal when used cleverly. Try to see what different shapes can be created on the surface of your components.
Brandon Litherland
Aerospace Engineer
Control Surface Introduction
Control Surface subsurfaces are unique to the OpenVSP Wing and Prop (3.43) components and are used to define regions where ailerons, flaps, slats, etc. are placed in your model. The VSPAERO solver GUI and the API can also detect these types of subsurfaces and leverage them in vortex-lattice analyses. OpenVSP 3.45 and above support Control Surfaces in Thick mode as well. The region defined by a Control Surface may be tagged inside/outside as with any closed subsurface type and may also be defined on either the upper, lower, or both surfaces. Additionally, Control Surfaces may be tagged to attach at the leading edge rather than the trailing edge, if you wish.
Rather than define the subsurface with length and width in UW, Control Surfaces are defined in the chordwise direction as either a length or fraction of chord and in the spanwise direction by start/end U or Eta location. Note that by default, the Control Surface end angles will be automatically set to be aligned with the component X-axis.
The positive direction of control surface rotation will be about the +U axis along the wing. For a +U right-side wing, this means that trailing edge (TE) control surfaces will rotate TE down and leading edge (LE) control surfaces will rotate LE up on the RIGHT side and opposite this convention on the LEFT under positive rotation. It’s worth briefly noting here that the deflection setting in VSPAERO will therefore rotate the control surface in opposite directions like an aileron unless the gain for one surface is set to be negative e.g., VSPAERO > Control Grouping tab > Deflection Gain per Surface. Setting one of the gains to be negative will cause a negative rotation about the axis when a deflection is applied in VSPAERO.
Even if you don't intend to use the VSPAERO solver, Control Surfaces on your model will help you understand the size of these features and keep scale and potential controllability in mind.
Brandon Litherland
Aerospace Engineer
Control Surface Refinement
The chordwise length and end angles may be further refined for Control Surfaces (CS). The chordwise length may be set as constant for the span of the surface or may have different lengths depending on your application. The end angles may also be set manually and either set to be equal or different than each other. By default, the angles are defined to align with the component X-axis. The leading or trailing edge is the reference for these angles. For example, an angle setting of 90° will result in the CS end to be normal to the trailing or leading edge of the wing. This is very easily visualized with highly swept wings. By leveraging unique lengths and angles at the start and end of a CS, users are capable of defining a variety of interesting deflection surfaces for analysis, export, or visualization.
Users should also be aware that the Num Points in the Surface End Angle box sets the number of points used to spline the UW curve for the CS. Very low numbers of points will result in a curved or “wiggled” CS boundary in some cases but you’ll find that the default value of 15 will result in a generally straight side. Highly variable or stretched surfaces such as a heavily swept vertical tail root fairing or tip will potentially lead to odd deflections in the CS boundary. In these cases, try adjusting the length or Num Points to clean up the edges.
Control Surfaces are meant to be as representative as possible in UW space but may not always be exactly straight for highly variable geometries. In such cases, try different combinations of settings to see if you can't straighten things out.
Brandon Litherland
Aerospace Engineer