7. Symmetry

7.1. Symmetric components

Many components are symmetric, e.g. the left and the right side of a wing. Instead of storing both sides, such a component can be described once and mirrored with the @symmetry attribute:

<wing uID="mainWing" symmetry="x-z-plane">
    ...
</wing>

Symmetry attribute applied to a wing

The attribute can take the following values; the documentation of each element states which of them are allowed:

Value	Meaning
`x-y-plane`, `x-z-plane`, `y-z-plane`	The component is mirrored at this plane of the CPACS coordinate system, i.e. at a plane through its origin. Mirroring creates a second component: the component described by the data (defined side) is complemented by its mirror image (mirrored side), which is a component of its own.
`inherit`	The component takes the symmetry of its parent component (`<parentUID>`); a component without parent is not mirrored. This is the default, i.e. it also applies if the attribute is not set.
`none`	The component is not mirrored, even if its parent component is.

Components with a @symmetry attribute are e.g. wings, fuselages, ducts, engines, engine pylons, fuel tanks, landing gears, rotors and generic geometric components.

Because the symmetry is inherited along the <parentUID> hierarchy, it can be passed on, switched off and set anew. The following example shows four wings, each the parent of the next:

wing1 is mirrored at the x-z plane.
wing2 has wing1 as parent, but switches the symmetry off. It therefore exists only once.
wing3 has wing2 as parent and sets a new symmetry at the x-y plane.
wing4 has wing3 as parent and no @symmetry attribute. It inherits the symmetry of wing3 and is mirrored at the x-y plane as well.

<wings>
    <wing uID="wing1" symmetry="x-z-plane">
        <name>Wing 1</name>
        ...
    </wing>
    <wing uID="wing2" symmetry="none">
        <name>Wing 2</name>
        <parentUID>wing1</parentUID>
        ...
    </wing>
    <wing uID="wing3" symmetry="x-y-plane">
        <name>Wing 3</name>
        <parentUID>wing2</parentUID>
        ...
    </wing>
    <wing uID="wing4">
        <name>Wing 4</name>
        <parentUID>wing3</parentUID>
        ...
    </wing>
</wings>

Symmetry set, switched off and inherited along the parentUID hierarchy

Four wings seen from behind, defined sides in blue and mirrored sides in amber, with the x-z and x-y planes of the CPACS coordinate system: wing1 with symmetry x-z-plane lies in the x-y plane and is mirrored to the left; wing2 with symmetry none stands upright at its tip and has no mirrored side; wing3 with symmetry x-y-plane continues outwards from the top of wing2 and is mirrored below the x-y plane; wing4 without symmetry attribute goes down from the tip of wing3 to the x-y plane and, inheriting the symmetry of wing3, is mirrored below it

The figure shows the four wings from behind, with their translations and their rotations about the x-axis, which the excerpt omits. wing1 and its mirrored side lie in the x-y plane, wing2 stands upright at its tip and exists only once, and wing3 continues outwards from the top of wing2. The mirrored sides of wing3 and wing4 lie below the x-y plane, so that wing4 and its mirrored side form one upright strut. The planes are those of the CPACS coordinate system: wing3 is mirrored at the x-y plane through the origin, not at a plane through its parent.

7.2. References to symmetric components

Elements that reference a component by its @uID (stringUIDBaseType, e.g. <parentUID>) refer to both components created by a symmetry, i.e. to the defined and the mirrored side. The @symmetry attribute of the reference selects a single side instead:

Value	Referenced part of the component
`def`	defined side only
`symm`	mirrored side only
`full`	both sides, i.e. the defined and the mirrored component (default)

The attribute is only used in references to components with a @symmetry attribute.

Top view of a wing mirrored at the x-z plane with an engine pylon referencing it: with symmetry def the pylon is on the defined side only, with symm on the mirrored side only, with full on both sides

The figure shows an engine pylon whose <parentUID> references the main wing, mirrored at the x-z plane. With def, the pylon belongs to the defined side of the wing only, with symm to the mirrored side only, and with full to both sides.

<enginePylons>
    <enginePylon uID="pylon">
        <parentUID symmetry="symm">mainWing</parentUID>
        ...
    </enginePylon>
</enginePylons>

Engine pylon located only on the mirrored side of the wing

Older datasets often append the side to the referenced @uID instead, e.g. mainWing_symm for the mirrored side of the wing mainWing or engine1_def for the defined side of engine1. This is not intended and must not be used: the string does not match the @uID of the referenced element. Reference the @uID unchanged and select the side with the @symmetry attribute, as in the example above.

<enginePylons>
    <enginePylon uID="pylon">
        <parentUID>mainWing_symm</parentUID>
        ...
    </enginePylon>
</enginePylons>

Not intended: side appended to the referenced uID (do not use)