Hi Kenny,

1) ...um... maybe - not sure I understand the question! If you have a look at my spreadsheet (in the group's files section) you'll see that the load reacted on each rib is the load on an 12"(?) span of wing - in other words, the load from half the distance to the next rib either side of the one in question (I think the ribs are about 24" apart.)

Spreadsheet: f1.grp.yahoofs.com/v1/8PNTT7Jb49QCkLNff5...201%20Feb%202012.xls

Ideally the upper cap (on the test stand) should be in pure tension and the lower one in pure compression, with neither experiencing any vertical sheer... which is exactly what the 2" wide straps were doing! Not ideal, but the best I could manage/think of.

2) The way I spread the load over the back half of the rib was a bit of a compromise (or fudge!) - the article I read on rib testing (link below) seemed to suggest that the load in that series of tests was hung out of a single point towards the farthest extremity of the rib. That didn't seem right to me and I felt that the load would more probably be spread over the entire length of the rib (but would it be a linear spread or not??)

f1.grp.yahoofs.com/v1/8PNTT-UU64UCkLNfa-...4RttZ4rQ/RibTest.pdf

Now, the only way I could hang the load off a single point at the end of the rig was to use a G-clamp, but given it had to hold over 32kg the clamp would have had to be tightened so much it would have crushed the foam core. So I compromised and spread the load over the rear half only. Even doing this was not easy, as the 2" wide straps did not conform perfectly to the curved cap strip, and transferred their load instead at the much narrower edge of the strap where it contacted the cap strip. However, I don't think that this was hugely significant.

More significant was the fact that the strap would have crushed the cap strip flange unless I supported it with a small strip of wood held lightly in place by two small clamps (just visable in the photos.) Although this strip of wood could be said to be adding extra strength to the cap strip, preventing it from buckling, the rib ultimately failed in twist (the upper and lower cap strips departed from the vertical plane) - the small clamps were only just tight enough to stop the strip of wood from falling out, so I don't think its effect was significant.

I still have to re-test the root rib to destruction once I get a chance to re-glue it to a new strip of wood for mounting on the test stand, so if you have any suggestions for loading it this time, let me know!

It's actually quite difficult to spread the load evenly along the cap strip as the cap strip is a constantly changing curve. I tried narrower straps (several individual straps independantly attached to the water barral) but it proved difficult to divide the load equally among the straps, so I ended up using a single 2" wide strap wrapped several times over the rib and to the barrel. At least this way I could roughly equalise the load on each length of the strap by sliding them around a little as I increased the load.

Please sing out if you have any bright ideas,

Phil.

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From: This email address is being protected from spambots. You need JavaScript enabled to view it. [This email address is being protected from spambots. You need JavaScript enabled to view it.] On Behalf Of Kenny Andersen
Sent: 04 March 2012 12:57
To: This email address is being protected from spambots. You need JavaScript enabled to view it.
Subject: [Carbondragonbuildersandpilots] rib testing



Phil,

I've been thinking about your testing just a bit and I have a couple of questions


1) did you put the total air load at that station as the load?

2) Why did you put the load only on the back half of the rib? (equivalent moment, or just being conservative?).




Depending on how you answer those questions your test may be pretty conservative -- especially it look like it failed at the upper center joint which would have been experiencing more moment than expected (if the load was uniform across the rib. Also, if you applied the total load only to the back half, the moment would have been MUCH higher than it would be installed on the aircraft.




Just thinking... I think those conservatisms may mean a fair amount of weight in the end And I'm guessing the wood ribs probably wouldn't carry half that load! It would be interesting to compare an original loaded in the same way. I do recommend applying the load more uniformly over the rib though -- it would b a much better indication fo failure.