Should composite ribs be made in the same way as metal ones?
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:
- Carry lift forces from the nose and rear part of the airfoil to the spar.
- Keep the wing skin in shape and prevent buckling.
- Support bearings for control rods, cables or actuators.
- 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.