Sub Tab

Contents

Every component (Geom) in OpenVSP is edited in a window whose first tabs are the same for all of them: Gen, XForm, Mass and Sub. The Sub tab holds the Geom's sub-surfaces. Blank, Hinge, Human, Mesh, NGon Mesh, Point Cloud, Routing and Wireframe have no Sub tab.

Sub-Surfaces

A sub-surface marks a region, or a line, on a Geom's surface. It does not change the shape; it labels part of it so that analyses can treat that part apart:

• CompGeom and the mesh exports tag the triangles in the region, so their area is reported separately and the exported mesh carries the tag.
• VSPAERO deflects control surfaces (the Control_Surf type only); each symmetric copy is a separate control surface there.
• Parasite Drag can give the region its own treatment, or no drag.
• CFD Mesh and FEA Mesh imprint its boundary into the mesh, when their Intersect Sub-Surfaces option is on (the default); FEA Structure can turn it into beam elements or remove its shell elements.
• STEP and IGES export can split the surface along it, and DegenGeom reports it.

Sub-surfaces are defined in the surface's own parameters, U along the Geom and W around it, each from 0 to 1 (see Attachment Coordinates for where they start). So a sub-surface stays on the same part of the surface as the Geom changes shape, and a rectangle in U and W need not look rectangular on the surface (Figure 1). Each region tags the side of its boundary chosen by Tag, Inside or Outside; for a Line, Test Greater tags the side of larger U (or W) and Less the other.

Figure 1. A Line at U 0.2, a 0.2 by 0.2 Rectangle turned by Theta 30, and an Ellipse 0.15 by 0.15, in U and W (left) and on a pod (right).

Choose the Surface before clicking Add: ALL puts the sub-surface on each of the Geom's surfaces (every blade of a propeller, for example), Surf_i on that one only (Figure 2). The choice applies when the sub-surface is made; the Sub tab cannot change it afterward. A sub-surface on a surface is on all of that surface's symmetric copies.

Figure 2. The top of the Sub tab of the default Propeller, holding a Line added with Surface ALL and a Rectangle added with Surface Surf_1. The SURF column shows the surface each sub-surface is on.

Each sub-surface has its own name, which becomes part of its tag, and its own attributes (see Attributes).

Sub-Surface Types

The Rectangle and Ellipse lengths and Theta act in the U-W plane, so on a long body a "square" of 0.2 by 0.2 is much longer along the body than around it, and Theta is not an angle on the surface.

A Line's value is a fraction of the surface's U (or W) range by default, [0, 1]. With [0, N] it counts the surface's sections instead, so a whole number puts the line exactly on a cross section: on a Fuselage, U 2 is its third cross section; on a Wing the root and tip caps count as sections, so U 1 is the root airfoil and U 2 the end of the first panel. W counts the four quarters of each section; on a Wing W 2 is the leading edge, and a Line of constant W there splits the upper surface from the lower (see Attachment Coordinates for where W starts on other Geoms). Use [0, N] whenever a line belongs on a section: it stays on that section when sections are added behind it, where a [0, 1] value moves to a new place among the sections.

An Ellipse is drawn as a polygon of Num Points corners, so a small number gives a polygon: 3 a triangle, 6 a hexagon (Figure 3). The first corner lies at Theta from the U direction, so at Theta 0 a corner points along U. A Theta of 180/N degrees, for N points, turns the polygon by half a side, so that a side faces along U instead; the value boxes take arithmetic, so 180/6 can be typed as it is. Give the U and W lengths equal values for a regular polygon in U and W; as with the Rectangle, it is regular on the surface only where U and W are scaled alike.

Figure 3. Ellipse sub-surfaces of Num Points 3 and 6, each at Theta 0 and at Theta 180/N, in U and W (left) and on a pod (right).

An XSec_Curve sub-surface starts as a circle of diameter 1, which spans the whole surface; set its size first. Its width and height are fractions of the U and W ranges, from 0 to 1.

An Intersect sub-surface is computed when Intersect is clicked, from the Geom chosen in Intersect Geom, and kept as an edit curve. When the two Geoms meet along more than one curve, Curve Index chooses which of the Num Curves to use. It does not follow later changes to either Geom; click Intersect again after changing them. Between clicks the curve is an ordinary XSec_Curve outline: edit it, or change its type, to tidy or simplify the footprint.

Control Surface

The Control_Surf type is offered on Wing and Propeller Geoms (Figure 4). It marks a region bounded by the trailing edge (or, with Leading Edge, the leading edge), two spanwise stations and a hinge line, on the Upper, Lower or Both surfaces. Model a flap, aileron, elevator or rudder with it rather than with a Rectangle: a Rectangle can be made to look like one, but its sides follow U and W rather than a depth from the edge, and VSPAERO deflects only a Control_Surf.

Figure 4. The Sub tab's panel for the default control surface on the default wing: Start U 0.4 to End U 0.6, a constant depth of 0.25 of the chord, on Both surfaces. Controls that the present choices do not use are greyed.

• The spanwise stations are Start U and End U, or, with Use Eta for Spanwise Coordinate, Start Eta and End Eta, fractions of the span from root to tip (of the radius, $r/R$, on a propeller; see Spanwise Coordinate Eta). U includes a wing's root and tip caps (see Attachment Coordinates), so the default U of 0.4 to 0.6 on a one-panel wing is 20% to 80% of the span (Figure 5).
• The chordwise depth is given at the start and at the end, as a fraction of the local chord (L/C) or as a length (L); Constant makes the end equal to the start. The hinge line passes through the points on the surface that lie that straight-line distance from the edge, on each surface.
• The sides follow the wing's sections unless Surface End Angle sets the angle each side makes with the edge, in degrees; with both angles set, Same Angle makes the end angle equal to the start. With an angle not set, its slider shows the angle the section makes with the edge.
• Each edge, the two sides and the hinge line, is a smooth curve through four points: its ends and its thirds. The depth and the end angles are met exactly at those points and blended between them. Num Points is the number of points each edge is drawn with, and so how closely the tagged region follows that curve; raise it when the outline must be smooth, on a long control surface or with end angles set.

Figure 5. The default control surface on the default wing: Start U 0.4 to End U 0.6, 25% of the chord deep, on both surfaces. The hinge line is dashed.

Hinge Line and Deflection

The 3D view draws each control surface's hinge line, from the Start end to the End end, with a curved arrow around it for a positive deflection. A positive deflection turns the surface right-handed about that line, from Start toward End. On a wing whose Start is at the root, that turns a trailing-edge surface down and a leading-edge surface up.

A symmetric copy is a mirror image, so its hinge line also runs from root to tip, and the same positive deflection turns its trailing edge up (Figure 6). VSPAERO deflects each copy by the control group's deflection times that copy's gain. Its automatic grouping puts both copies in one group with a gain of 1 each, which deflects them as ailerons; for a flap or an elevator, which deflect together, set the gain of the copy to -1.

Figure 6. The default control surface on the default wing and its symmetric copy, each given the same positive deflection, seen from behind with $+Y$ to the right. The hinge lines run from Start to End; the curved arrows show the positive sense; dashed outlines are the surfaces at rest.

Scripting

Every control on this tab is a Parm, set and read with SetParmVal and GetParmVal by name and group; GetParmActiveFlag tells whether a control is active or greyed, as of the last Update. Sub-surfaces are made with AddSubSurf( geom_id, type, surfindex ), whose surface index defaults to 0, the first surface; pass -1 for all of them, as ALL on the Sub tab does. GetSubSurfIDVec lists a Geom's sub-surfaces, GetSubSurfParmIDs gives a sub-surface's Parms, and IntersectSubSurf does what the Intersect button does, with the Geom set by SetIntersectSubSurfGeomID.