# Creating control surfaces

The APM Preprocessor has the ability to create control surfaces by deforming the mesh. To create a control surface add the following lines to the end of the **.conf** file.

```
[control_surface]
    x1 = 0.0000
    y1 = 0.0000
    z1 = 0.0000
    x2 = 0.0000
    y2 = 0.0000
    z2 = 0.0000
    x3 = 0.0000
    y3 = 0.0000
    z3 = 0.0000
    deflection = 0.0000
    differential_deflection = 0
    mirror = 1
```

The three vectors with components `x1`, `y1`, `z1`,`x2`,`y2`,`z2`,`x3`,`y3`,`z3` define the mid-plane of the control surface. The hinge line is defined as `x2-x1`, `y2-y1`, `z2-z1`. The image below shows the three vectors and the resulting control surface mid-plane.

![Definition of a control surface](/files/-MVXbYIuqxVpeZZPpFDB)

{% hint style="info" %}
Control surfaces should always be created on the left side of the $$x-z$$symmetry plane (negative $$y$$-direction). The APM Preprocessor will automatically mirror the control surfaces across the $$x-z$$symmetry plane.
{% endhint %}

Deflection specifies the amount of control surface deflection in degrees and differential specifies whether mirrored control surfaces deflect in the opposite direction.


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