Just finished my first large structural piece using the laser

Same frustration on my end when I was cutting ribs for a 1.5 meter dirigible frame out of 3mm ply. I fought scorch marks for weeks before realizing my air assist nozzle was half clogged with residue. Once I cleaned it out, the edges came out pale and clean instead of that dark brown ring I kept getting, and the pieces stopped snapping when I pressed them into the joints. The other thing that helped a lot was putting a strip of low tack transfer tape on both faces of the sheet before cutting. It peels off and takes most of the smoke staining with it, so the surface stays crisp for gluing. Plain masking tape works in a pinch if you don't mind scraping a bit.

I also switched from one slow pass to two quicker passes. Less heat soaks into the wood that way, so the ribs keep thier stiffness instead of getting that cooked, crumbly feel along the cut line. My slots ended up snug with no sanding, wich made assembly go twice as fast. For the envelope, if you're thinking about heat-shrink film over fabric, test your adhesive on a scrap rib first. I had one brand of spray glue eat into the ply surface on my first attempt.

I went a slightly diferent direction with the ribs on my own airship build. Mine is a 2 meter Zeppelin-style frame and I cut the rings from 1.5mm poplar ply instead of balsa. The poplar held up much better under the beam and I got almost no charring even at settings I'd normally call aggressive for that thickness.

One thing that saved me alot of grief was raising the ply off the honeycomb bed on a couple of strips of scrap so the reflection from underneath couldn't scorch the back face. Flashback on thin stock used to leave a smoky ghost on one side of every rib, and that residue made glue joints feel gritty.

I also stopped trying to cut each rib as a single closed ring. Now I cut them in two half arcs with small scarf joints, which means the offcut falls away early and heat has less time to soak into the finished piece. The halves square up nicely against a former disc while the glue sets. For skinning, I'd suggest test-fitting the longerons with temporary pins before you commit to glue, beacuse tiny kerf differences add up over the full length of a hull. Mine needed a light sanding at three stations to keep the profile fair.

I actually disagree that optimizing the power and speed is the main fix, at least for very thin stock. When I tried cutting similar small ribs out of fine basswood, I found the laser wasn't the culprit at all, but how the ply was laid down. If you even have a tiny bit of surface grain or a slight warping across the sheet, the beam energy gets concentrated unevenly, causing localized burn spots regardless of your settings. It's a material prep issue more than a laser tuning issue. You have to make sure the material is perfectly flat and stable on the bed before you even fire it up. That made a huge difference for my delicate frame work.

I have been having trouble holding the little wood pieces steady while I cut them. If the balsa isn't locked down tight, the laser seems to move a little bit and it messes up the shape, even if the settings are perfect. I've tried using some small clamps, but the wood is so light it just bends instead of holding still. It makes the whole process slow down a lot.

I wonder if anyone uses a special kind of board or jig underneath the pieces to keep them totally flat and won't wobble during the cut? It would save me so much time.

One thing I learned cutting ring formers for my own hull frames is that grain direction matters more than people expect, because a rib cut across the grain will snap at the first hard landing even if the edge looks flawless. I orient the sheet so the long grain runs along each rib, which ussually means nesting the parts at odd angles instead of packing them tight. Also check wether your software applies kerf compensation, since a slot cut without it tends to come out loose by roughly the beam width on each side, and on thin ply that difference is the whole fit.