A rib is a structural member oriented along the chord direction (perpendicular to the spar). Ribs define and maintain the airfoil cross-section, prevent the wing skin from buckling, and transfer distributed aerodynamic pressure loads to the spar.
Rib spacing is a design trade-off: closer spacing provides better skin support (preventing buckling and maintaining airfoil accuracy) but adds weight and assembly time. Typical rib spacing ranges from 50–100 mm for small model-scale wings to 200–500 mm for tactical UAVs.
Rib construction methods in UAVs:
- Laser-cut plywood or balsa — light, cheap, precise. The traditional model aircraft method. A laser cutter produces a complete rib set from a 3 mm plywood sheet in minutes.
- 3D-printed — can be printed integrally with the skin, eliminating assembly. Alternatively, printed as separate pieces that slide onto the spar.
- CNC-cut foam — ribs can be integral with a foam wing core, cut by hot-wire or CNC router.
- Sheet aluminum — stamped or CNC-routed. Used in larger tactical and MALE UAVs.
In fully 3D-printed wings, the distinction between rib and infill blurs: the internal lattice structure (gyroid, rectilinear, etc.) serves the rib’s function of supporting the skin and transferring loads, without being a discrete chordwise member. This is one reason printed wings can use fewer or no separate ribs — the infill does the work.