How do I cut a semicircle into a curved surface? How can I cut a semi-circle out of a curved surface, such as this:



As you can see from the picture, the circled area is one I'm having trouble with. I can't seem to find a way to cut a hole into a curved surface without messing up the topology. Could someone help me with this?
Cutting it without messing up the topology (ngons, tris, unwanted distortions, etc.) will be hard.
However, for your specific case I think you might have better luck building the shape by extruding/filling.
For example:

Duplicate a cylinder:


Select some of the edges along the top/bottom of the cylinder and press W&gt;Bridge edge loops:


Repeat for the other side(s), then add a solidify and a subserf modifier: