# Why wing ribs have diagonal struts

Why do ribs have diagonal struts,why not vertical as seen in airfoils of Citabria airplane?

The diagonal struts are best to transfer shear. Imagine an upward pointing load at the nose. In order to get that load transferred to the fuselage, it first needs to be transferred to the wing spar. Imagine now that you hold the rib firmly in your right hand where the spar is and press the nose upward with your left hand. With parallel struts, the rib will have much less stiffness and the nose load will deform the rib easily. With the diagonal struts, the rib is much stiffer when that particular nose load is applied.

• In #1 the struts are strong in compression, but the joints are weak in torsion, so triangles are used. In #2, the joints are strong in torsion (in the rib plane). – amI Sep 22 '18 at 4:47
• I have a follow up question @PeterKampf. So if parallel struts are better for bending stress, and the diagonal struts better for shear stress, would some aircraft combine the two or would that create issues with weight? Would those new honeycomb structures they use on some planes be a compromise between the two? – Jihyun Sep 23 '18 at 2:53
• @Jihyun: Many aircraft do this already. Ribs in large aircraft have round holes so the remaining structure will keep the upper and lower flange of the rib apart (good for bending) and transfer shear as well (good for transferring lift). Those holes will save some mass, but with larger aircraft there is no way around a heavier structure. – Peter Kämpf Sep 26 '18 at 18:52
• @Jihyun: Regarding honeycombs: They are used for three functions: To transfer shear between the top and bottom layer, to keep them apart (good for bending) and to prevent them from buckling. What kind of load those panels can transfer is determined by the fiber direction in those layers. – Peter Kämpf Sep 26 '18 at 18:53
• I see. Thanks for the thorough answer! – Jihyun Sep 27 '18 at 1:08

This isn't really aviation-specific, it's just good structural engineering practice. Triangles in structures are strong because they don't bend: whichever way you apply load to the triangle, the load is along the members. With a rectangle, any load that's not perfectly aligned to the structure will bend the joints and squash the rectangle into a diamond shape, which will then squash flat easily. You don't want any rectangles in your structure anywhere.

• @DavidTeahay What is that link supposed to relate to? – Dan Hulme Sep 21 '18 at 17:41
• That the wing ribs of some planes(maybe few) is actually rectangular and not in triangles(truss)....is this a design flaw or can I just go with that kind of wing rib in the design of my own wings? @Dan Hulme – David Teahay Sep 22 '18 at 3:22
• @David Teahay: You could of course design your wing using rectangles, but it would probably be heavier for the same structural strength. – jamesqf Sep 22 '18 at 4:20
• @DavidTeahay Those google results don't show that at all. The pictures there show two forms of rib: ribs constructed of triangular shapes (such as your new picture 1) and solid ribs with holes in the middle to reduce weight (such as your new picture 2, though more of the examples have circular holes). The latter will generally be heavier for the same strength, but might be easier to manufacture. If you try to build a truss without triangles, you'll need so much reinforcement at the joints it'll end up heavier than a solid rib anyway. – Dan Hulme Sep 22 '18 at 8:56
• russellw.com/logbook/citabria_wings/LH%20tip%20inward.jpg – David Teahay Sep 22 '18 at 9:15

It's a truss. Ideally, all of the beam elements in a truss are in either tension or compression, without major bending loads. This offers the lightest weight for given load.

This same design is commonly seen in the open in steel bridges, construction cranes, and spacecraft. Triangles that a truss consist of cannot be bent without changing the elements' lengths. If all of the struts were vertical, the structure would consist of rectangles, which can be easily deformed by bending the joints.

It's to make the wing stiff and strong in torsion. For torsion stiffness, you want two things. First and foremost, you want as much material on the outermost fibre as possible. The further the material from the neutral axis, the more it contributes to torsion stiffness due to the moment arm from the neutral axis.

Secondly, you want the cross-section to remain undeformed. Consider for example a matchbox 'shell', which is in theory very stiff for torsion. However, if the forces do not enter the matchbox exactly right, the open ends will deform into a diamond shape and will twist easily. This shearing into a diamond shape can be prevented by placing baffles in the matchbox.

This is in fact the very reason wings have ribs: so that the wings maintain their cross-section by transferring shear loads due to torsion from and to the top and bottom skins of the airfoil. In terms of stiffness, thin plates would be ideal, but these are prone to buckling so a strut structure is used instead. The best way to transfer shear loads with struts is to use diagonals.

So, in short: the diagonal struts transfer shear loads to increase torsion strength and stiffness of the wing.

• You think ribs are mainly for the prevention of buckling? Then the vertical struts in the first picture would work just as well. I'm afraid you are not really answering the question at all. – Peter Kämpf Jul 26 '18 at 17:39
• @PeterKämpf Not at all. Stiffness and buckling are separate phenomena. I'm saying the diagonal struts are added for torsional stiffness to increase the eigenfrequencies, similar to adding baffles to a tube structure. – Sanchises Jul 26 '18 at 18:13
• No, for torsional shear the ribs should be built similar to the wing skin - after all, they are part of the torsion box. With those puny struts they are certainly not up to the task. In other words, the rib shown is from somewhere internal while an end rib (which is part of the torsion box) would look much beefier. – Peter Kämpf Jul 26 '18 at 20:46
• I understand your second point but not the first. what do you mean by "outermost fibre" and what is "neutral axis"?....are you talking about the neutral point of an airplane? @Sanchises Pls elaborate – David Teahay Sep 24 '18 at 9:28
• @DavidTeahay The 'neutral axis' is the line along which the cross-section of a structural member rotates if a pure torsion is applied (for a round tube, this would be exactly in the middle). With 'outermost fibre' I mean to say that you want as much material as possible as far away from this neutral axis. Material further away from the neutral axis contributes more to torsion stiffness than material near the neutral axis. Think of the difference of twisting a string or a tube of equal mass: you will find that the string is easier to twist because the material is closer to the neutral axis. – Sanchises Sep 24 '18 at 9:36

A top picture of number 2 might have been a bit better, but notice the "puny" cross struts run both ways, much like in the wing of an old time biplane. Their function would be help prevent the wing from collapsing BACKWARDS as a result of excess airspeed. (spokes in wheel rims may also seem puny, but the rim cannot come out of round because the spokes, 90 degrees from the load, cannot be pulled apart).

This seems to be a bit of sensibly added insurance, but no, they probably would not add to the torsional stiffness of the wing.

Regarding the design of the ribs themselves, as mentioned, 1 is a classic truss. 2 has the center of the rectangular rib filled in with metal and the center circular portion and edges made as an I beam for strength. 1 of wood, 2 of stamped metal.

As far as making a stiffer wing torsionally, might consider replacing aluminum skin with plywood or some type of composite material. Others may know how better to do it within the wing.