OpenVOGEL/User's guide/Guide 2 Part 1

Native Fuselage Modeling

In this tutorial we are going to create and edit a fuselage. Tucan creates the surface of the fuselage through a spline interpolation of cross sections inserted along the longitudinal local Z axis. Each cross section is contained in the local XY plane and consists of a number of nodes that are linearly interpolated to form a closed loop.

In this tutorial we are going to design a cylindrical fuselage with conical edges and the following characteristics:

Length	Diameter
8m	1m

Creating and editing a fuselage

Before you starting the fuselage modeling, you will need to have in mind what cross sections are fundamental to describe the shape of your design. You will also need a sketch with the dimensions of the model.

Adding a fuselage

Tucan normally starts up with an empty work-space. To introduce a fuselage (or any other component) go to the Model tab of the main ribbon, click on Add and then pick Fuselage in the pop-up window. In the drop-down menu choose the Fuselage - 0 item that has just been created and then click Edit. You will see that a new pop-up window named Fuselage Editor immediately appears in the main screen. This window is the one we will work with, as it lets you create, remove and adapt any cross section.

Adding a cross section

The first step to generate the fuselage is to generate our primitive cross sections. Click Add at the top left corner and notice that Section 0 is inserted in the top left list box. Select this section using the left button and adjust its longitudinal position using the numeric box beneath. It is a good practice to locate Section 0 at the nose of the fuselage along the local Z axis (i.e. at the local origin with coordinates [0,0,0]). However, this is not a requirement and can be done differently. For our simple model we repeat this procedure until we have four cross sections available in the list (Section 0,1,2 and 3). These sections comprise our fuselage backbone. Sections 0 and 3 are respectively the front and back end of our design, so if we have Section 0 at Z=0 we shall put Section 3 at a position equal to our fuselage length Z=8.

Adding nodes

After we have created our primitive sections, we select Section 0. Let us create a node by clicking Add at the center left side of the window. We notice then that a node is represented at the right side of the window, and added to the Cross section list box. This node is node number 1 and its coordinates are [0,0] in the local XY plane. If we wish to change the coordinates then we have to select the node from the drawing and move it by keeping the left mouse button down, or we simply enter the values under the Current node location label. Since this is the nose of the airplane we are not going to need more nodes here, so we keep coordinates as they are ([0,0]). We continue the same procedure with Section 3 and create again a single node (1:[0,0]), since this is the back of the fuselage.

In Tucan, the front and back fuselage ends can be declared either as a single node or as a vertical two nodes segment (both nodes along the local Y axis).

Now let's select Section 1 and see how to handle more nodes. Sections 1 and 2 will actually give shape to our fuselage.

Cross section Shape Editing

Now that we have shown how to create a cross section and how to add nodes in it we are going to continue with our fuselage build up. We select Section 1 (Z=1) and add two nodes, one after the other. Since the diameter of our fuselage is 1m we set the coordinates to be:

Node 1	Node 2
[0,0.5]	[0,-0.5]

It is important that the nodes are entered in a clockwise order for the kernel to compute the normal vectors in the correct direction!

Let us now add a few more nodes. We select Node 1 and add a node. This new node is Node 2 is located at the middle of the former two nodes. We select Node 2 and set the coordinates to be [0.5,0]. Now that we have an outline in our section we go on creating new nodes and positioning them at the desired coordinates in order to form a circle.

It is important that the first and the last nodes are located along the Y axis for the fuselage to be fully closed.

The same procedure is to be performed for Section 2 (Z=7).

Fuselage Refinement

Before we finish our fuselage design, we will try and make the shape of the fuselage look a bit smoother. In order to do this we select Section 2 and add a cross section and then do the same with Section 0.

When a cross section is selected and we choose to add (insert) a new cross section, Tucan positions this new surface after the selected one and the new shape is obtained from the interpolation of the two original cross sections (the selected one and the one originally after it). This feature is useful in saving time for the design procedure and also to easily smooth the edges.

Having this this in mind, we position the later two created section as follows:

Section 1	Section 4
Z=0.2	Z=7.8

Spline interpolation

By default, Tucan will use linear interpolation between cross sections. However, at each cross sections we can choose to maintain continuity on the surface curvature by using spline interpolation. To do this you have to select the section and uncheck the option broken edge.

Fuselage mesh

The last step in our design is to choose the number of Longitudinal and Cross panels. This option is to be done according the size of our fuselage and the precision we require in our calculation. For this tutorial we set:

• Longitudinal refinement: 15
• Cross refinement: 10

Take into account that when lifting surfaces are anchored to the fuselage, the fuselage will automatically consume the same number of chord wise panels in the longitudinal direction along the anchor line. Therefore, the mesh in this portion of fuselage cannot be enhanced without changing the wing mesh.

Anchoring

In the Fuselage Editor window we can choose to anchor the lifting surfaces to the fuselage. If our model has wings that we already have designed, in the box Anchor to: appear all the available surfaces that can be anchored to the fuselage. If we double-click upon these options Tucan will extend the surface of the fuselage to form a connection between the fuselage and the lifting surfaces.

It is mandatory after we make changes to the anchored lifting surfaces to perform a fuselage re meshing. This procedure is not done automatically to avoid a possible misbehavior.

Take into account that Tucan cannot handle anchors that overlap in the global X direction.

Importing a fuselage

Beside the native fuselages, Tucan is also able of importing fuselages from input files. The current version supports two kind of import file formats: native or STL. The native format is very easy to create and deal with, but it is not a recognized standard. It has been designed to transport OpenVOGEL mesh data. The STL file format is a very well know file format that can transport triangular meshes. It is very inefficient in terms of data compression, but it is easy to read and write. Most 3D design tools include an STL imprt/export function, including OpenVSP. There is where I would like to focus for now.

Importing from OpenVSP

OpenVSP has the advantage of being incredibly popular among aerospace students, and there are a lot of models available in their own VSP format. It is not the idea of OpenVOGEL to mimic OpenVSP, and importing their native files might be a laborious task. So in order to provide a minimum of interoperability, OpenVOGEL is able to read STL files generate by OpenVSP and vise versa. To import from OpenVSP:

• In OpenVSP: select the component you want to export, go to File > Export, select Stereolith (.stl) and pick a target file name.
• In OpenVOGEL: add a new imported surface, click Edit, pick the STL file format in the open file dialog, search for the file and click Ok.

The next image is an example of a Cessna 182 fuselage imported and simulated in OpenVOGEL.

Fuselage model imported from OpenVSP.