RE: Muffler in cowl for CG Chipmunk
I did that with my Chipmunk, and it works fine, BUT..........
you have to cut a pretty fair sized hole (mine is a 1"x 2" oval) in the bottom, and it helps to use some sort of an extension on the muffler. That 4-stroke exhaust is pretty hot and will easily melt the covering off the part of the fuse under the cowl if you let it impinge. Don't bother asking how I know all this, lol.
My Chipmunk is several years old and I ran the muffler outside til I recovered the plane and put on a new fiberglass cowl. It flew fine the other way but looks much cleaner with it enclosed.