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Re: pressure distribution on a wing and rib loads 11 Dec 2013 01:37 #651

Phil, All,
That makes sense; I would have expected the spar to be around 30%. For sure the control surfaces must be included in that length and in calculating the loads on the rib â€" you can toss the first calculations.

I think you will find that there are 3 ways to get a light-weight rib
1) Truss like the original plans call for
2) Foam with wood
3) Foam with composite
No matter what you do, you still have to react the shear load caused by the summation of the air-loads. The truss does that through the axial loads in the truss members and the foam ribs do that through shear in the foam and axial loads in the rib-caps. You are right; the loads aren’t really that high. There are a few things going on here. One is that this is a very light-weight aircraft, so the wing-loading is low, and the second is that the wing depth is fairly deep for the cord width, so, there is a lot of depth to react the total load, which is low. Keeping that in mind we can talk about the options
1) The truss isn’t such a bad deal (see spar test sheet) with regard to weight. Additionally they are open which allows systems to pass through easily. The biggest problem is that they are time-consuming to make.
2) If you look at the test data in the Adobe document you see that is doesn’t take much wood to make a decent rib cap. Also, I mildly confused that they didn’t do the obvious test, which was .75 foam and 1/16 spar caps, which should result in a load of 131*.75 = 98 Lb, which should be the same weight as the truss, but 50% stronger and MUCH easier to make. 1/16 in is just .062 thick wood; that isn’t much. We could say that all we need is the same strength, rather than something that is 50% stronger, so theoretically, you could make the ribs ½ inch wide out of foam with 1/16 inch caps for around 2.5 oz each and the expected failure load would be 131/2=65.5 Lb. There may be some limits though, with regard to handling, but since the wing is going to be less than 40 Lbs, a 60 Lb point load doesn’t seem SO much. Builder makes the call.
3) Foam ribs with Composite caps don’t seem to be any better than the wood caps and they weigh about the same, although they said that they think they sanded through the fiberglass ply. The test data definitely looks bad for the composite caps (that would have been a good one to redo). Hmmm, I don’t know if it is worth the hassle. In the case of the carbon rods in the spar caps, the allowable is SO, high, that is it worth screwing with the composites to get the extra strength. When you roll over the composite for the caps â€" especially on the upper surface, they will tend to fold over and will not carry load, but will act to stabilize the upper surface of the cap (which is OK). I think there is some potential here, and it would be worth pursuing. If I were going this direction I would stick with the .75 foam, and wrap the fiberglass over the sides about .25 inch. In case where the builder lives in a really humid area, then it they may be motivated to go with all-composite construction
General comments: I think on the foam ribs I would also put a .5 inch ring on either side of the hole (kind of like those hole reinforcements that we used to use back when we made paper reports, onl I would use a light-weight (maybe 2 oz) fabric. I don’t see a compelling reason to go with something other than wood or wet fabric.

I’m not really getting where you are going with the rectangular box section. The section will not conform to anything â€" 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 â€" 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.

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