I have problems specifying free rectangular loads on inclined surfaces. Do I have to use a different coordinate system for this?
It is not necessary to use a different coordinate system to specify the loads. The loads can be applied in local z-direction, if they are to act rectangular to the surface.
Please notice the plane of projection when specifying the loads. From the point that was specified in the section of the load position, a straight line is "constructed" perpendicular to the plane of load projection. If this line intersects with a surface contained in the "On Surface No." list, then the load is applied at the point of intersection. In this way, you can easily apply similar loads on many surfaces.
The boundaries of the surface load are where the grey dotted lines intersect with the surface. The surface load is also displayed in this way when it acts perpendicular to the surface.
For the graphical specification of the coordinates, a local coordinate system might of course be useful.