by Guest » Mon Dec 06, 2004 8:12 pm
Nose ribs are more likely to break loose and fall over if they are not tight inside the skin, as in the early Series 1 models, but can occur in any Lazair that's been around awhile. Engine vibration causes them to vibrate back and forth. Originally, epoxy glue was used to attach the foam ribs into the aluminum U channel that's riveted to the spar web. When the rib moves, it stresses the foam at the edge of the glue line. The epoxy is much stronger than the foam, so the foam breaks, or shears, right along the glue line.
I did some tests way back in 1980 where I tried gluing foam ribs using different adhesives. The two extremes were epoxy and (adhesive type) RTV. When the rib that was rigidly glued in place with epoxy was pushed to the side, it would quickly fail along the glue line with not much force. When the same test was performed, this time glued with RTV adhesive, the rib could be pushed much harder - with so much deflection that the rib would flex over it's length quite a bit before the rib would begin to fail along the glue line. Because of my tests, the factory changed from epoxy glue to a somewhat flexible adhesive similar to (exactly like?) Liquid Nails Panel Adhesive. Dale Kramer thought it solved the epoxy problem at a much cheaper cost than RTV. I prefer the RTV, as the panel adhesive continues to harden over time until it's almost as hard as epoxy, but that's my opinion. The loads on the adhesive are low, as the strength of the foam is very low in shear and is the limiting factor.
A good fix that will keep stock ribs from working loose is to open the bottom of the spar, lift the skin up a couple of inches, and add a couple inch bead of RTV along the bottom of each rib, then close up the wing again. If the ribs don't move, they won't fail and fall over. If you ever have to open the wing up for repairs in the future, open it from the top. Obviously, the best time to do this is when you are doing a recover job.
On my Series 1, I just snapped each rid out of the channel by pushing it to the side, then re-glued it right back in. I then added the glue bead along the bottom of the ribs and closed up the spar. I've never had another rib failure in these wings in almost 25 years.
Chappy