## Introduction

Snap-To is an interactive collision detection function that enables components to be placed at a fixed, normal distance from another set of components or to check a model for clearance or collision. In this tutorial, the Snap-To function is introduced and demonstrated to provide a general sense of operation.

## Introduction

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.

## Introduction

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.

## Introduction

Subsurface lines may be defined at either 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.

## Introduction

Control Surface subsurfaces are unique to the OpenVSP Wing component 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. 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 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. 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.