I have a staircase that’s a half circle. I have the “wall” of the staircase setup correctly but I need to cut the top half off (see bad clip art…) at a specific angle. I can’t seem to figure out how to run a line along the curve of the wall at the same time heading at an angle up the staircase so that I can cut off the top half.

The line that I have drawn in the middle “sort of” follows the arc but not enough for SU to figure out how to let me erase the top half of the wall.

Unfortunately not. The angle needs to be the same along the curved surface.
I forgot the plugin that included how to draw the 3D curve with constant angle from level A to level B on a virtual cylindrical surface. Something with shear in the name.

Your bottom part will be a trapezoid wound around a sector of a cylinder. So draw a trapezoid with dimensions H1 H2 and P (arc length). Now you can use Flowify or Shape Bender to wrap the trapezoid around your wall. Can also be done using the helix drawing plugin.

A very nice solution, but it will also be correct only if the curvature of the wall is a segment of a circle. A very nice solution, but it will also be correct only if the curvature of the wall is a segment of a circle. In the case of e.g. elliptical walls fail.

Only the copy-array part will fail for non cylindrical surfaces due to the absence of a common center.
The scaling part scales a constant pitch correctly for any 3D curve. Each segment wil be scaled the same.

As long as the segments have the same “width” because when you scale a circle to an ellipse it will change, or you scale an arc, or you use a spline and you don’t make the length of the segments equal… Many variables…

@wilschroter, even with different facet widths and for any curved surface (as long as common edges between adjacent facets are vertical) you can obtain the correct 3D curve with a constant pitch by applying the ‘Scale’ tool. See image below: