I was suggesting using the box-section tubing for the straight sections of the rib structure only - not the curved, top and bottom surface rib caps. The box-section tubing would be protected from damage as they are inside the wing and not liable to get whacked. The top and bottom caps would be made out of the .092" x .220" rod which is flexable and more than strong enough to take a little punishment from ground handling.

Actually using the .092" x .220" rod for the rib caps may not work after all. I knocked up a root-rib out of a straight grained white deal wood (cut to 1/4" x 5/16" and 5/16" x 5/16" as per the plans) to investigate the bending properties of wood (this is purely a test piece - very rough and ready.) I drew out the profile and truss structure of the rib on paper and taped it to a board. I then hammered in pins at various locations along the rib cap lines and placed the wooden cap strips and truss struts in place. Sadly the rib cap strips do not bend naturally to conform perfectly to the rib profile as drawn - they are out by as much as 2mm in places (see photos in my photos directory.) I'm guessing that some steaming is needed to shape the ribs perfectly. I'm also guessing that it is not unreasonable to assume that any other material with uniform stiffness will bend in much the same way. So, unless 2mm is an insignificant error then we have to find some way of holding the carbon rod in shape (perhaps with a CF cloth flange) while it cures.

My plan for cutting out the nose rib shapes from the carbon/foan sandwich was to use a table fret-saw (a machine for cutting out very fine detail woodwork) to roughly cut out the shapes and then to sand them down to the exact profile using a high speed rotating spindle sander (like a Dremmel tool). This way I hope to avoid damaging either the fibers or the epoxy at the micro scale. If this method works for the nose ribs then it may as well be used for the tail ribs also - certainly a lot easier than cutting out and gluing a lot of fiddly bits of rod and tubing. Indeed, you could still use the .092"x.220" rod for the rib caps, sandwiched between the two layers of cloth. It will certainly be necessary to have something like this along the front of the rib where it is bonded to the spar and to the rear edge where other bits get attached.

The square section CF tubing stringer should be well protected behind (and bonded to) the CF leading edge skin. The only big question is how many layers of cloth to use for the skin. If the stringer can be dispensed with altogether then so much the better!

Phil.

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Kenny wrote:
I'm not really getting where you are going with the rectangular box section. The section will not conform to anything - it is going to be pretty robust. Better to use a strip of fabric, which will go all the way across the top and on to the side by .25 - .37 of an inch. You will likely need to do some experimenting if you vary very much from the tested configuration in the test pdf. The reason the pultrusion crushed so easily is that is really unidirectional. The only strength it has other than along its length is the strength of the epoxy. I would fear it in a location (upper or lower surface) where it would easily get whacked. The fabric has strength in both directions, so in this case would be a better choice and more robust for impact damage.

With regard to the front ribs and false ribs, a trade-off study is needed. Your idea on this one shows some merit, but you'd have to figure out how you cut them out without damaging the fibers and separating the sheet from the foam. It's common to use the filler as you are suggesting.

For the leading edge I would say the same comments exist - the pultrusion is a bad choice since it is horrible for impact damage. It's not load-carrying per se, it is there to support the ribs and stiffener leading edge (I presume). Actually, I don't think it does so much. First, it's very near the neutral bending axis of the spar. I suppose that since it is forward most, it could be acting as a cap for bending about the Z axis? Again, I'm a stress analyst not a loads guy. Especially in the case that you use composite skins, it may well be that this could be eliminated. Hmmm. Just thinking out-loud again: If we look at the stiffness difference between the [usual] wood stringer and the graphite skins, I'm guessing we could toss the wood stringer altogether, since, in fact, the leading edge would be picking up the entire load along the leading edge. That would simplify construction and get back a bit of the weight of the carbon skin. If that works out, then going to the carbon skins might actually be pretty close to weight neutral. It will very likely be much stiffer as well.