Should composite ribs be made in the same way as metal ones?

Question

A lot of metal ribs are made from bending or stamping sheet metals. So we see shapes which have lots of discontinuities in the flange area for ease in manufacturing. My question is - is it the most ideal shape for a rib keeping manufacturing ease and stringer requirements aside? In an aircraft with carbon fibre sandwich skin, would ribs with complete flanges be more suitable?

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Answer 1

Should composite ribs be made in the same way as metal ones?

No

Doing so would forfeit some of the benefits of composites, but certification authorities and their regulations might force you to.

To start, it is important to understand what the rib is for. Most have several purposes:

1. Carry lift forces from the nose and rear part of the airfoil to the spar.
2. Keep the wing skin in shape and prevent buckling.
3. Support bearings for control rods, cables or actuators.
4. Introduce point loads into the wing, say at an engine mount or a landing gear leg attachment.

1 and 4 mean that the rib is like a small spar in itself: The upper and lower flange are loaded in tension rsp. compression and the rib area itself carries shear forces. This translates into unidirectional fibers along the flange and ±45° woven fibers of lower stiffness for the area in between.

Generally, it is better to reduce wall thickness instead of using cutouts to reduce weight, but the resulting thickness could make the rib prone to buckling. Therefore, it can be optimal to use a corrugation or sandwich in the shear loaded parts and 3 could make cutouts necessary. Fuel passages in a wet wing should be kept small to avoid excessive sloshing. As a rule of thumb, add as much fiber mass around the cutout as has been removed.

Discontinuities in the flange area would be a stupid mistake in composite construction since the wing skin is most likely a sandwich panel without stringers running across.

And now we need to placate the authorities: This could mean that the rib is not bonded, but riveted to the wing skin in order to allow later removal for inspection or repair. Never mind that it will be easier to repair a composite structure by grinding away the damaged parts and replacing them with an adequate layup, but a century of bad experiences with metals have condensed into some very specific rules which demand that you use composites as black aluminium. Next, instead of a sandwich the wing skin might consist of a solid sheet stiffened by discrete stringers. In that case, you would first bond the unidirectional fibers on the inner skin before adding the stringers and the rib area with those cutouts you show in your metal rib picture.

Answer 2

The cuts in the metal ribs serve several purposes:

1. They accommodate the need of the metal to bunch up or wrinkle as the flange is formed around a curved form block (because the edge of the metal is being forced to shorten in the forming process). The relief cutouts make the rib easiest to form, at the cost of some strength.
2. They provide a secondary use in making room for stringers, lateral stiffeners, or machined flanges in a milled wing plank.
3. They provide a fuel path if the wing is "wet" so fuel in the outboard end can flow to the inboard end past the ribs.

If you don't need the stringer cutouts and it's not a wet wing, an alternative to the flange relief cutouts is to form indentations along a continuous flange, called flutes, to take up the excess metal that is created in the forming process. You can avoid cutouts or flutes completely in a metal rib, but you have to form the rib in a dead soft annealed state, and heat treat it after forming.

For a composite rib made in a mould, you would only use cutouts or slots along the flange to accommodate lateral stiffeners in the composite skin, or for fuel passage in a wet wing. A composite skin that is flat on its inside surface would have no need for flange relief cutouts for that purpose, and if there isn't any fuel, it wouldn't need fuel openings either.