Positioning

General

The positon description is decribed by the following two components:

• AirData: Position specifications for any submodels uses an airData model to extract values for atmospheric properties from the atmosphere model in a specific location. The separation of atmosphere and airData model enables the usage of multiple airData models at different positions, speeds or acceleration while relying on the same atmosphere. This approach is needed for simulations with multiple vehicles in a single experiment. Since many component models use an outer airData model, it is always recommended to place and inner airData model on an experiment model.
• WorldRepresentation: The world representation defines if for the coordinates they are given in Cartesian coordinates x,y,z (���FlatEarth���) which is the default or if they are given in for a flat earth in north, east, down coordinates or a spheric description of the globe is used and coordinates are converted accordingly. Since many component models use an outer worldRepresentation model, it is always recommended to place and inner worldRepresentation model on an experiment model.

Coordinate system

The choice of the coordinate system is up to the user. There is only one assumption that the coordinate system must fulfill, it has to be a right handed one. A typical choice is a weight-and-balance coordinate system (This is sometimes also called the aerodynamic coordinate system. In order to avoid confusing it with the wind coordinate system used in flight dynamics we use the name weight-and-balance system in the documentation). This coordinate system has the origin at the nose of the aircraft (or slightly in front of it). The x-axis points backwards along the center line of the fuselage. The y-axis points towards the starboard ("right") wing tip. The z-axis points upwards. See Figure 1 for an illustration of such a coordinate system.

Figure 1 Weight and balance coordinate system

Position

All components that contain volumes can be positioned relative to the reference frame. The position is set by the parameter position (or similar) and can be set in the parameter dialog or via a frame connector by setting useFrame=true. The default value for position is [0,0,0] i.e. at the origin of the reference frame.

As described in the connecting principles, you should always build up alternating flow model-volume connections (a "staggered grid"). Therefore, if a volume connector is exposed on one end of a component (unfilled connector), the component will also prescribe the position of the connection set. If a flow model connector is exposed on one end of a component (filled connector), then the component will read the position of the connection set.

Simple component models clearly falling into one of these catergories indeed always behave this way (for instance, a multi-port volume will always prescribe position, and a static pipe, a pipe without dynamic balance equations, will always read position).

For an example parameter dialog to set position, see Figure 2. This is taken from a simple tank. Here, we can prescribe the position of the tank itself, and its ports (as the tank has finite dimensions). Additionally, we can configure whether the positions of the ports are relative to the position, or absolute.

Figure 2 Positions section in tank parameter dialog

Many sophisticated components additionally allow the user to configure whether volumes (or flow models) shall be exposed on the ports. Look for the parameter tab "Volume positioning" to find the corresponding variables. Components that support this type of positioning include:

• Components in Volumes
• Components in Sources
• Components in Valves
• Component Ejectors
• Components in Pipes
• Component Pump

If volumes are switched off on a given port, then the positions of these ports (or of the entire component if no volumes are contained at all) are determined by the position of the connectors of the other components. Hence all connectors of these components need to be connected to another component in order for the positions to be fully specified.

Connectors

The connectors of the components gives information whether the position is set or received. If the square background of the connection is light grey the position is set. If the background is black then the position is received.

Figure 3 - Component with connectors that receive position.

Figure 4 - Component with connector where position is prescribed by the user.