Chord is the straight-line distance from the leading edge to the trailing edge of an airfoil. On a rectangular wing, the chord is constant across the span. On tapered or delta wings, the chord varies — longer at the root, shorter at the tip.
The mean aerodynamic chord (MAC) is the chord-weighted average across the span, used as the reference length for aerodynamic calculations (pitching moment, Reynolds number, center of gravity position). For a rectangular wing, MAC equals the chord. For a tapered wing, MAC lies inboard of the geometric midpoint. For a delta wing, MAC is approximately two-thirds of the root chord.
Why chord matters for UAV design
Chord interacts with airspeed to determine the local Reynolds number (Re = V × c / ν), which in turn determines boundary layer behavior and airfoil performance. Small UAVs with short chords (50–150 mm) operate in the low-Reynolds-number regime (Re < 200,000) where airfoil selection is critical and surface finish matters more than at larger scales.
Chord also determines the internal volume available for structure — a longer chord provides more depth for a spar at a given airfoil thickness ratio, and more room for infill in 3D-printed wings. This is one reason delta planforms with their deep root chords can use simpler internal structure than high-aspect-ratio wings with narrow chords.
The center of gravity is conventionally expressed as a percentage of MAC measured from the leading edge — typically 25–30% MAC for stable flight. Moving the CG aft of this range reduces stability; moving it forward increases stability but requires more elevator deflection for trim, adding trim drag.
Related terms
- Span — the perpendicular wing dimension; together with chord, defines wing area
- Aspect Ratio — span divided by mean chord
- Reynolds Number — the flow parameter that chord length partially determines
- Center of Gravity — conventionally expressed as a percentage of MAC