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