A control parameter is a dimensionless number that stands for the state of a control device, e.g. the deflection of a control surface or the extension of a landing gear. Other parts of the dataset, such as configurations and aerodynamic increment maps, refer to a state only by this number. What the number means is defined once, at the control device itself: a control surface translates it into a deflection with the steps of its <path> (controlSurfacePathType), a landing gear into its extension and steering with the steps of its functions. A device with several functions, such as a landing gear that extends, steers and brakes, has a control parameter for each of them.
The steps of a control device follow these rules:
The steps are sorted by increasing control parameter, and no value occurs twice.
The values are arbitrary, but the range from -1 to 1 is recommended for a device that moves in both directions, such as an aileron, and the range from 0 to 1 for a device that moves in one direction, such as a flap.
The first and the last step limit the range. A control parameter outside it is invalid.
The value 0 lies within the range. The state at control parameter 0 is the undeflected state of the device, which forms the clean configuration of the aircraft, e.g. the one the aerodynamic map without increments refers to. A step at 0 is recommended, but not required.
Between two steps, the state is interpolated linearly.
The following path of an aileron covers the range from -1 to 1. At control parameter 0.5, the aileron is rotated by 10° about its hinge line, at -0.5 by -10°.
<trailingEdgeDevice uID="aileron">
...
<path>
...
<steps>
<step>
<controlParameter>-1</controlParameter>
<hingeLineRotation>-20</hingeLineRotation>
</step>
<step>
<controlParameter>0</controlParameter>
<hingeLineRotation>0</hingeLineRotation>
</step>
<step>
<controlParameter>1</controlParameter>
<hingeLineRotation>20</hingeLineRotation>
</step>
</steps>
</path>
</trailingEdgeDevice>Superposition of commands
A control device can receive several commands at the same time, e.g. from a <configurationDefinition> and from an explicit setting, directly or through one or more control distributors (controlDistributorType). Because a path is linear only between its steps, the result depends on where the commands are summed up, so this happens in a fixed order:
The command inputs of each control distributor are summed up to a total command input.
Each control distributor is evaluated with its total command input, which gives control parameters for its control devices.
The control parameters of each control device, from the control distributors and from direct commands, are summed up to a total control parameter.
Each control device is evaluated with its total control parameter, which gives its state.
A partial sum may leave the range of a control distributor or a control device. The totals evaluated in steps 2 and 4, however, lie within the range; otherwise the evaluation reports an error. For example, if a configuration sets the aileron of the example above to 0.3 and an explicit command adds 0.4, its total control parameter is 0.7, and its path gives a rotation of 14°.