Mechanical properties of ribs with no vs. fluted holes?

I want to understand what the effects of a flanged hole on a ribs mechanical properties are in comparison to having no hole in the rib at all.

The end ribs in our RV-10's horizontal stabilizer:


Looking at the ribs, I think that the large holes will significantly reduce the rib's tensile strength. I however believe that this is irrelevant, as it is still significantly higher than the forces which can be transferred by the rivets. Hence, the rivets are still the weak link. Is this correct?

My other assumption is, that the flanged edges of the lighting holes will increase the ribs stiffness. Is this correct and by how much?

I therefore believe that a rib with flanged lightning holes is superior to a rib without any holes. It also saves weight.

Am I on the right track? Are there any disadvantages in putting flanged holes in ribs? Is there possibly an easy to understand explanation somewhere on the internet? I searched, but probably didn't use the right terms.

Oliver

to the books

AC 43-13-1b and 8083-30. should give you a hint or two.

Billythekid said:

AC 43-13-1b and 8083-30. should give you a hint or two.

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I wanted to be spoon fed

Observe the truss structure of a wooden rib and then come back to us.

I can't get past the wooden floor in your shop to even seen the rib !!!

Interesting question. From a qualitative perspective, I may help to think of the rib as an I-beam. The flanges of the rib are analogous to the flanges of the I-beam. The flanges carry to load. the web (the vertical portion of the rib or I-beam) keep the flanges apart so they can do their job.

The web section is quite susceptible to buckling. The holes in the ribs remove unnecessary material (and provide a handy space to run conduit, push rods, etc.). The flanges around the lightening holes reduce the tendency to buckle.

In my -4 kit, the wing ribs came without holes, but with circular stiffening flanges distributed across the web. Drilling out the center section (hole) was the option of the build as an opportunity to save weight. I elected to drill mine out, but not everyone does. The plans admonished the builder not to cut a hole so large as to interfere with the flange (stiffening ring) geometry as it is important to the rib structure.

Holes are optional; stiffening rings are not (at least for RV ribs).

I hope this helps.

Dean

Billythekid said:

AC 43-13-1b and 8083-30. should give you a hint or two.

Click to expand...

Thank you, it didn't occur to me that the underlying physics of an aircraft structure would be explained in either of these documents. When I just looked at them, I indeed could not find an explanation in this regards in them. I'll spend more time on them tonight.

fr0gpil0t said:

I can't get past the wooden floor in your shop to even seen the rib !!!

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It is just cheap laminate. Turned out to be practical and nice looking.

Dean Pichon said:

[...] From a qualitative perspective, I may help to think of the rib as an I-beam. The flanges of the rib are analogous to the flanges of the I-beam. The flanges carry to load. the web (the vertical portion of the rib or I-beam) keep the flanges apart so they can do their job.

The web section is quite susceptible to buckling. The holes in the ribs remove unnecessary material (and provide a handy space to run conduit, push rods, etc.). The flanges around the lightening holes reduce the tendency to buckle. [...]

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This is also my general understanding - why are the holes however dimensioned the way they are? How much material, if any, must remain outside the flanged area? Is there an optimal angle for the flange? What about the size of the flange? How much stiffness will a flange actually add?

I guess you just provided me with the keyword 'web', which I needed to come up with more useful Google results. The ''Langley Memorial Aeronautical Laboratory' published in 1942 a report on 'The Strength And Stiffness Of Shear Webs With Round Lightrening Holes Having 45° Flanges': http://www.dtic.mil/dtic/tr/fulltext/u2/a801095.pdf
and
'Test Of Beams Having Webs With Large Circular Lightening Holes': http://www.dtic.mil/dtic/tr/fulltext/u2/a801086.pdf

I'll look at it later, it however appears as whether they had some of the same questions I had and will answer them in this report.

Wing

Keep up the good work and you will be flying soon.
Bob

Dean Pichon said:

Interesting question. From a qualitative perspective, I may help to think of the rib as an I-beam. The flanges of the rib are analogous to the flanges of the I-beam. The flanges carry to load. the web (the vertical portion of the rib or I-beam) keep the flanges apart so they can do their job.

Click to expand...

Partially correct... The web keeps the flanges apart so they can do their job, yes. But the web doesn't carry "no" load. It carries the shear load between the flanges (and between the spars). The shear load is a lot lower than the strength of a complete web, though, so removing some of the material does save weight. If you're going to remove material, you need something to reinforce the edge to prevent buckling, and that's where the crease around the lightening holes comes in.

Holes are optional; stiffening rings are not (at least for RV ribs).

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Stiffening rings are only necessary if there are lightening holes. The reason they're there in your ribs when you get them, is that all the ribs were produced in a press. The cost to add the rings while pressing is zero, and it gives eager builders the option of lightening their structure. But if you leave the web intact, you don't need them.