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Re: pressure distribution on a wing and rib loads 11 Dec 2013 01:32 #648

The flexibility of the wood or composite comes from the thinness. Try bending 1/16 or 3/32 by .75 wood, and you’ll see that it’s pretty easy. I guess you are talking about the vertical portion in the front and aft of the rear. I’m still not keen on them for that. Hmmm, how about just using the fiberglass and wrap the front and back the same as the top and bottom so you end up with a C-channel around the foam rib. Even in the case of the spar pultrusions, they are wrapped (that’s the recommendation for the manufacturer), so you should wrap them anyway, and now it’s heavy and over-strength (I think). What you want is just enough, maybe a wee bit extra, and that’s it.

You want some flexibility in the upper and lower caps or you will get some pull-off pre-stress between the rib and the cap, and it will want to separate. So, the wood is flexible, and the wet lay-up fabric is flexible so they would be much better in this app. If you do use something I think you should be looking for an actual layup verses a pultrusion.

I do like your plan for the ribs â€" I think that should work out OK

I’m thinking right now that that leading-edge stringer can go, IF, the GR/EP skins are used for the LE. I’ll have to do some modeling to confirm.

--- In This email address is being protected from spambots. You need JavaScript enabled to view it., "Philip Lardner" wrote:
>
> 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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