Structural Items

Structural Items are used to represent internal equipment. These can be located anywhere on the airframe and can have location, mass and inertia information added to them. The mass and location information is used to automatically calculate the resultant aircraft’s centre of gravity. Any mass and inertia values are transposed accordingly.

Aerodynamic Items are added to the model to represent objects in the airflow. Those may be the whole airframe, or individual components such as the fuselage, wing, or vertical tail. These can be used to add additional components to existing airframes, including wing tip tanks or radomes.

Aerodynamic Items build upon Structural Items, adding aerodynamic reference information and aerodynamic coefficients to the model. The coefficient data is dimensionalised based on the selected reference data and transposed to the aircraft’s defined reference location. This includes the automatic calculation of moments due to forces away from the reference location.

The resultant total forces and moments are non-dimensionalised based upon the airframe’s reference information, to provide the user with the total coefficients. As a result, it is not necessary for the user to write code to transpose the forces or to scale the coefficients – for example the empennage – to the wing.

When working with code, aerodynamic data needs to be built up for the whole aircraft using formulae and tables, and the generation of this data can be an expensive process.

Aerodynamic Strip Theory enables the three-dimensional dynamic characteristics of the aircraft to be established using basic 2-D airfoil characteristics. However, implementing this requires the inclusion of all the aircraft geometry and the integration of the contributions into the overall airframe to be calculated – making the coding of such a solution hugely time-consuming and expensive.

With the the J2 Elements plug-in and its built-in Stripped Items, all the code is written for you and only the geometry and airfoil data are required, significantly simplifying the problem and speeding up the model development.

These are used to represent moving surfaces, from standard control surfaces and high lift devices to speedbrakes and undercarriages. Dynamic Items extend the Aerodynamic Item, adding Deflections into the model parameters that can be used as variables in tables and calculations.

Most aircraft have some form of propulsion, and these are represented by the Propulsion Item. Propulsion Items extend the Aerodynamic Items by adding thrust as well as thrust location and orientation. Multiple Propulsion Items can be added to the model to represent engines at any location.

The total thrust and any resulting moments about the centre of gravity are automatically calculated by the software, so there is no need to add in extra equations or code. This also means that engine out scenarios can automatically be built in and the resulting asymmetry is automatically calculated. The aerodynamic contributions due to nacelles, pylons and windmill drag can be added in too.

Helicopter Rotor Dynamics are a complex problem to solve, adding additional degrees of freedom to a model. A simplified Momentum Theory approach can generate some results, but this can fall down when considering such scenarios as hover and autorotation.

With a Rotary Assembly Item added using the J2 Rotary plug-in, it is possible to add a full Blade Element Rotor Model (BERM) into the aircraft model.

The final Structural Items available within J2 Builder are Force Items. These are used to represent anything that adds a dimensional force to the airframe – this might be undercarriage or skids, or other external items such as a slung load or towed item. The Force Item extends the Structural Item by adding body axis forces and moments to be applied directly to the airframe at the specified location.

From each of the different items available, the model is assembled as a series of parent and child components. This is done to replicate the aircraft layout and to make navigation and reference information easier. Children inherit reference location from their parents and so when the parent is moved so the whole assembly is moved with it and thus avoiding items floating out in space.

The navigation of the model is simple as each item is self contained and any contributions from a given component are located on the item.

For further information on what j2 Builder can offer, see:

• J2 Builder
• Hierarchical Model Structure
• Parameter Types
• Links and Controls
• Delta Models
• Model Build Process
• Adding Data