CPACS root element
The Common Parametric Aircraft Configuration Scheme (CPACS) is an XML-based data format for describing aircraft configurations and their corresponding data.
This XML-Schema document (XSD) serves two purposes: (1) it defines the CPACS data structure used in the XML file (e.g., aircraft.xml) and (2) it provides the corresponding documentation (see picture below). An XML processor (e.g., Tixi or XML tools in Eclipse) parses the XSD and XML files and validates whether the data set defined by the user (or tool) conforms to the given structure defined by the schema.
This documentation explains the elements defined in CPACS and its corresponding data types. Data types can either be simple types (string, double, boolean, etc.) or complex types (definition of attributes and sub-elements to build a hierarchical structure). In addition, the sequence of the elements and their occurrence is documented.
To link the XML file to the XSD file, the header of the XML file should specify the path of the schema file. An example could look like this:
<cpacs xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="pathToSchemaFile/cpacs_schema.xsd">
CPACS is an open source project published by the German Aerospace Center (DLR e.V.). For further information please visit www.cpacs.de.
CPACS data is modeled in a hierarchical structure whose underlying concept follows a top-down description of a system-of-systems which decomposes a generic concept (e.g., an aircraft or rotorcraft) into a more detailed description of its components. This originates from the conceptual and preliminary design of aircraft, where the level of detail is initially low and continues to increase as the design process progresses.
For some concepts within CPACS, however, a bottom-up approach is applied where the components are first defined in detail (sometimes referred to as library) and then linked within an instantiated higher-level concept. This is advantageous when used multiple times within complex systems, such as engines, which only have to be defined once in order to be referenced several times on the aircraft. The combination of these two methodologies is known as middle-out approach and enables the goal to fully parametrize aeronautical systems.
Coordinate systems are a regular cause for ambiguous interpretation of data. In CPACS, the reference coordinate system is the CPACS-coordinate system. This coordinate system is used for most of the data. A single exception is made in order to keep aerodynamic data in an aerodynamic coordinate system. The following paragraphs outline the determination to known coordinate systems.
The CPACS coordinate system is the coordinate system identified by TIGL, CPACS's geometric library. It is a right-handed coordinate system. If an aircraft is defined in the CPACS coordinate system it will usually follow the directions listed in the table below.
Therefore, the CPACS coordinate system can be confused with the body-fixed coordinate system. While often the CPACS coordinate system and the body-fixed coordinate system overlap, this must not always be true. Several definitions for body-fixed coordinate systems exist (x-axis through nose and tail, x-axis perpendicular to nose plane). For non-symmetric aircraft, body-fixed coordinate systems become even more complicated. Hence, analysis tools should stick to the CPACS-Coordinate system. It remains to the designer to model the geometry accordingly.
The CPACS coordinate system does not rotate with flow. Hence, aerodynamic calculations do rotate their flow relative to the CPACS-coordinate system. If not stated explicitly different, e.g. for target lift-coefficients, results are returned in the CPACS coordinate system, i.e. the cfx-coefficient is parallel to the CPACS x-Coordinate, regardless of the way the geometry is defined.
The following table gives a "best-practice" advice on how to locate a geometry within CPACS. Different approaches are, of course, valid as well.
|x||tailwards||from nose to tail|
|y||spanwise||from symmetry plane to the right wingtip|
|z||upwards||from landing gear to tip of vertical tailplane|
The following figures show an example of a geometry that is aligned with the CPACS coordinate system, i.e. the body-fixed coordinate system corresponds to the CPACS coordinate system.
The aerodynamic analysis is relative to the CPACS coordinate system. That is, the angle of attack is represented by the dashed orange line. Results of the aerodynamic calculation are given in the CPACS coordinate system.
The following figures give an example of a geometry that is not defined in alignment with the CPACS coordinate system. It is a valid CPACS file, but only used in this example for demonstrative purposes.
The body axes and the CPACS coordinate system do not align. That is, the origin of the geometry is not at CPACS (0,0,0) but at a point in positive x- and z-direction.
Again, the aerodynamic analysis is relative to the CPACS coordinate system. That is, the angle of attack is represented by the dashed orange line. Results of the aerodynamic calculation are given in the CPACS coordinate system.
There are no explicit attributes describing units in CPACS. The general convention is that all values must be given in the following SI-units:
or in derived units, e.g.:
The only non SI unit used throughout CPACS is the angle in degrees [°]. For the sake of an intuitive use the angles are given in degrees rather than in radian [rad].
To provide a better overview, it is possible to split up a CPACS dataset into several files. This can be done by inserting an <externaldata> node at an arbitrary position into the datatset. This node contains a <path> node with a URI to the external file(s), followed by one or more <filename> nodes, containing each a name of a file to be included at that position. Below, an example of such external data is given:
<?xml version="1.0" encoding="utf-8"?> <cpacs xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="pathToSchemaFile/cpacs_schema.xsd"> <vehicles> <profiles> <wingAirfoils> <externaldata> <path>file:://airfoils</path> <filename>NACA0010.xml</filename> <filename>NACA2412.xml</filename> </externaldata> <airfoil uID="NACA0012"> <name>NACA 0012 Airfoil</name> <pointList>...</pointList> </airfoil> </wingAirfoils> </profiles> </vehicles> <cpacs>
Such an external file would look like:
<?xml version="1.0" encoding="utf-8"?> <airfoil uID="NACA0010"> <name>NACA 0010 Airfoil</name> <pointList>...</pointList> </airfoil>
The file would be included completely, except for its title line <?xml version="1.0" encoding="utf-8"?> . This concept can also be used recursively (external files of external files), then it is important to prevent circle connections (file "A" loading file "B" loading file "C" loading again file "A" ...).
For path URI addresses, the trailing file separator "/" may be omitted. Below, some examples for path URIs are given:
A CPACS dataset with external files, being loaded by a special library like the TIVA XML Interface TIXI, shall collect all its external datafiles and build up a single tree from them. A validation against this schema is only possible for such a single tree file; the <externaldata>nodes are not recognized by it. To preserve the information, necessary to split the file up into external files again later, externaldata information is maintained within three attributes of the former external top node:
The single tree for the example above would look like:
<?xml version="1.0" encoding="utf-8"?> <cpacs xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="pathToSchemaFile/cpacs_schema.xsd"> <vehicles> <profiles> <wingAirfoils> <airfoil uID="NACA0010" externalFileName="NACA0010.xml" externalDataDirectory="file://airfoils" externalDataNodePath="/cpacs/vehicles/profiles/wingAirfoils"> <name>NACA 0010 Airfoil</name> <pointList>...</pointList> </airfoil> ... <airfoil uID="NACA0012"> <name>NACA 0012 Airfoil</name> <pointList>...</pointList> </airfoil> </wingAirfoils> </profiles> </vehicles> <cpacs>
The CPACS-dataset often uses references between nodes. Typically, these references define connections between elements which are located somewhere else in the hierarchical dataset (e.g. a wing is connected to a fuselage; a specific engine is connected to a pylon; etc.). These connections are defined by unique identifiers (uID) which are specified as attributes. Thus, there are elements which can be referenced via a uID attribute, e.g. a fuselage:
as well as elements which refer to the former, e.g. a wing pointing to its geometrical parent:
<wing uID="e382bf5j"> <name>ATTAS main wing</name> <parentUID isLink="True">ATTAS_fuselage</parentUID> ...
Since uIDs are only used to link nodes within the XML file, no naming convention is required. UIDs, however, must be unique! Although a common practice for naming uIDs is their position in the data hierarchy (e.g. uID="mainWingSection3"), uIDs as shown in the above example are absolutely valid as well. It is therefore recommended to use the name element to convey human-readable meanings.
Sometimes it might be useful to specify a part of the aircraft as symmetric instead of holding all the data twice in nearly identical form in the dataset (e.g. left and right wing are usually identical, except for the sign of the y-coordinate). Hence, some parts offer the option to set a symmetry attribute for them, like:
This attribute explains that the whole part with all its subnodes is symmetric to the given plane. Possible planes are:
All nodes, e.g. parentUID, in CPACS that refer to a component that holds symmetry attribute, e.g. wing, have to carry the symmetry attribute as well.
The symmetry attribute may take three values: symm, def, full:
<wing uID="ATTAS_main_wing" symmetry="x-z-plane"> ... <segments> <segment uID="ATTAS_main_wing_innersegment"> ...
In the example above, to refer to the "other" side of the wing on must use the definition as such:
<loadcase> ... <segments> <segment> <segmentUID isLink="True" symmetry="symm">ATTAS_main_wing_inner_segment</segmentUID> <strip>...
For large data sets (e.g. increments of aerodynamic coefficients due to control surface deflections) it is advantageous to map them via vectors and arrays instead of using a sequence of nodes for each data value. Therefore vectors and arrays are defined as semicolon-separated lists in CPACS. Via the documentation (derived from the XSD) of the corresponding nodes it has to be checked whether it is a vector or an array.
The vector is meant as a one-dimensional-array. In such a node, the values are given in a semicolon separated list:
As for vectors, multi-dimensional arrays provide values in a semicolon separated list. An array is always preceded by a sequence of vectors, containing the dimensions and index values. Which vectors of an array are dimensioning is specified in the respective documentation of the array.
<altitude>1000.;2000.;3000.</altitude> <!-- vector element --> <incrementMaps> <incrementMap uID="incMap_b3ac2"> <controlSurfaceUID>InnerWingFlap</controlSurfaceUID> <controlParameters>-1;-0.5;0;1</controlParameters> <!-- vector element --> <!-- array of dimension length(altitude) x length(controlParameters): --> <dcl>11.;12.;13.;14.;21.;22.;23.;24.;31.;32.;33.;34.</dcl>
|Control parameter = -1||Control parameter = -0.5||Control parameter = 0||Control parameter = 1|
|Altitude = 1000m||11.||12.||13.||14.|
|Altitude = 2000m||21.||22.||23.||24.|
|Altitude = 3000m||31.||32.||33.||34.|
Control parameters are abstract parameters, linking a generic floating point value to a certain status of a control device (e.g. control surface, landing gear, suction system, brake parachute, ...). For control surfaces, such a data pair (control parameter and control surface deflection status) is called a <step> and the ordered list of all steps for a control surface forms its deflection <path>.
The control parameter values for each step are arbitrary floating point values. However, it is strongly recommended to use values between -1. and +1., or between 0. And +1. (depending on the type of control surface). The smallest and the largest value implicitly define the maximum deflection limits. It is mandatory, that the value “0.” is within the specified range, as this value is treated as undeflected and used to specify a “clean” aircraft configuration (e.g. used in the clean aero performance map). It is recommended, but not mandatory to specify a <step> with a <controlParameter> of 0. Consequently, no <controlParameter> must be used twice within a single <path> definition. Deflection values between two specified steps are handled by linear interpolation.
The following example shows the usage of control parameters within a control surface deflection path definition:
<controlSurfaces> <trailingEdgeDevices> <trailingEdgeDevice uID="InnerWingFlap"> ... <path> ... <steps> <step> <controlParameter>-1</controlParameter> <hingeLineRotation>-20.</hingeLineRotation> </step> <step> <controlParameter>-0.5</controlParameter> <hingeLineRotation>-10.</hingeLineRotation> </step> <step> <controlParameter>0</controlParameter> <hingeLineRotation>0.</hingeLineRotation> </step> <step> <controlParameter>1</controlParameter> <hingeLineRotation>5.</hingeLineRotation> </step> </steps> ...
At some places in CPACS, an atmosphere has to be selected (e.g. for connecting an altitude with a certain pressure or density). Currently, CPACS does only support a single atmospheric model: The ICAO Standard Atmosphere (ISA) from 1993 (see ICAO Doc 7488/3 'MANUAL OF THE ICAO STANDARD ATMOSPHERE', third edition, 1993) It covers temperature, pressure, density, speed of sound, dynamic viscosity and kinematic viscosity with respect to altitude. In CPACS, 'altitude' means what is called 'geopotential altitude' (H) in the ISA reference document and is given in [m]. For details, see ISA manual, section 2.3, page E-viii f. ISA covers a range from -5000 m to 80000 m.
Temperature offsets are introduced on top of the definitions in the ISA manual (which does not cover such variations). The offset model is based upon the idea that the pressure at a fixed geopotential altitude is independent from temperature offset (pressure altitude). The temperature offset changes only the density (following rho = p / Gas Constant / T) (and viscosity, of course)
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CPACS 2.3 is the fourth public release of CPACS. Major changes include:
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