Skin is the outer surface of a wing or fuselage — the material that contacts the airflow and defines the aerodynamic shape. Skin may be purely cosmetic (a non-structural covering over an internal skeleton) or structural (carrying shear and torsion loads as part of a semi-monocoque structure).
Non-structural skin is stretched or adhered over ribs and spars and carries no significant load. Examples: heat-shrink film (Monokote, Oracover) on model aircraft and small UAVs; thin fiberglass over a foam core.
Structural skin carries a significant fraction of the wing’s torsion and shear loads. The skin acts as a thin-walled tube or box around the spar, forming a torque box that resists twisting under aerodynamic load. This semi-monocoque approach is used in most UAVs above the hobby scale. Structural skin must resist:
- Shear from aerodynamic forces
- Torsion from offset lift and control surface hinge moments
- Panel buckling under compressive loads (the upper skin of a wing in positive-g flight is in compression)
Skin material choice depends on scale and budget:
- Film or fabric — lightest, non-structural. Sub-2 kg models and micro UAVs.
- Fiberglass over foam — moderate weight, good shape fidelity, semi-structural. The standard for many small to medium UAVs and expendable drones.
- Carbon fiber composite — highest stiffness-to-weight ratio, fully structural. Tactical through HALE UAVs.
- 3D-printed shell — the outer perimeters (wall/shell count) of a printed wing serve as skin. Increasing shell count from 2 to 4 perimeters can improve torsional stiffness by 40–60% with modest weight increase, because the added material is at maximum distance from the neutral axis.
Skin surface quality directly affects aerodynamic performance. At low Reynolds numbers, surface roughness from FDM layer lines, fiberglass weave, or panel joints can increase parasitic drag by 5–15%. Whether this justifies the cost of sanding and painting depends on the mission duration and efficiency requirements.