A spar is the main load-bearing beam inside a wing, oriented along the span. It resists the bending loads produced by lift — the wing wants to fold upward under aerodynamic load, and the spar is what prevents it. Most wings have one or two spars, positioned at 20–30% and 60–70% of the chord from the leading edge.
A spar consists of:
- Spar caps (upper and lower flanges) — carry the bending loads in tension and compression. The caps are where most of the spar’s material should be concentrated for structural efficiency, as material far from the neutral axis contributes most to bending stiffness.
- Spar web — the vertical plate connecting the caps. Carries shear loads (the vertical component of aerodynamic force) and prevents the caps from buckling toward each other.
Spar design is where the aspect ratio trade-off manifests structurally. A wing with twice the span has four times the root bending moment (for the same wing loading), requiring a spar that is roughly four times as stiff — which means heavier, more expensive, and more precisely manufactured. This is the fundamental reason that expendable and cost-sensitive UAVs favor short spans.
Spar materials by UAV class:
- Balsa or plywood — model aircraft and micro UAVs. Adequate strength-to-weight at very low cost.
- 3D-printed thermoplastic — small to medium UAVs. Infill density in the spar region can be increased to 40–60% while the surrounding skin uses 15–20%.
- Carbon fiber tube or pultruded rod — the standard for small to tactical UAVs. Off-the-shelf carbon tubes provide excellent stiffness-to-weight ratio at low cost.
- Machined aluminum — tactical to MALE UAVs. Higher labor cost but well-understood design methods.
- Carbon fiber composite layup — MALE and HALE UAVs. Maximum stiffness-to-weight ratio, but requires skilled labor, tooling, and quality control.
In some expendable delta-wing designs, there is no discrete spar at all — the thick root section and stiff wing skin carry bending loads as a distributed structure, eliminating the spar as a separate component and simplifying manufacturing.