rtfm
Well-Known Member
Hi,
I've searched the forum for this question, and although I've found plenty of discussion about the relative merits of truss ribs vs plywood ribs - nowhere did I find any mention of how strong ribs actually have to be. It isn't that helpful to know that a truss rib will fail at (say) 120lbs if one has no idea what sort of breaking load is actually required.
If a rib sees a maximum of 40lbs, then designing a rib which fails at 120lbs seems pointless. It seems that some of the rib discussion takes on an element of a rib-strength competition. In equal-weight truss vs plywood ribs, the truss wins. But is this actually relevant to real-world scenarios?
I don't quite know how to calculate what duress a rib is placed under - but if one is talking about (in this particular example) a two-spar high wing with twin struts, it seems to me that the only part of the rib which is under any significant load which might cause it to fail is that section aft of the rear spar. And that section is taken up by the aileron or flap.
So, to my layman's way of looking at things, building ribs to withstand 120lbs of load seems like overkill. Why not build the rib out of foam and have done with it?
Always ready to take advice and instruction from those more technically minded than I am,
Regards,
Duncan
I've searched the forum for this question, and although I've found plenty of discussion about the relative merits of truss ribs vs plywood ribs - nowhere did I find any mention of how strong ribs actually have to be. It isn't that helpful to know that a truss rib will fail at (say) 120lbs if one has no idea what sort of breaking load is actually required.
If a rib sees a maximum of 40lbs, then designing a rib which fails at 120lbs seems pointless. It seems that some of the rib discussion takes on an element of a rib-strength competition. In equal-weight truss vs plywood ribs, the truss wins. But is this actually relevant to real-world scenarios?
I don't quite know how to calculate what duress a rib is placed under - but if one is talking about (in this particular example) a two-spar high wing with twin struts, it seems to me that the only part of the rib which is under any significant load which might cause it to fail is that section aft of the rear spar. And that section is taken up by the aileron or flap.
So, to my layman's way of looking at things, building ribs to withstand 120lbs of load seems like overkill. Why not build the rib out of foam and have done with it?
Always ready to take advice and instruction from those more technically minded than I am,
Regards,
Duncan
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