Chord (aeronautics)
Chord is the width of an aerofoil section from leading to trailing edge.
In aeronautics, a chord is an imaginary straight line that connects the leading edge and trailing edge of an airfoil cross-section, running parallel to the airflow. Its length is simply the distance between those two edges. For the main chord, the point on the leading edge is often taken as the spot with the smallest radius. For turbine airfoils, the chord can be defined by the line between where the front and rear of a 2D blade section would rest on a flat surface if placed convex-side up. Since wings, horizontal and vertical stabilizers, and propeller or rotor blades are all based on airfoil sections, the term chord (or chord length) also describes their width. For a wing, stabilizer, or propeller, chord is measured from leading to trailing edge in the airflow direction. The term also applies to the width of flaps, ailerons, and rudders.
Many wings aren’t rectangular, so their chord varies along the span. The chord is usually largest where the wing meets the fuselage (the root chord) and shrinks toward the tip (the tip chord). Most jet aircraft use tapered, swept wings. To get a single comparable figure for different wing shapes, the mean aerodynamic chord (MAC) is used, though it’s complex to calculate. The MAC is important for calculating pitching moments. A chord can also be defined for compressor and turbine airfoils in gas turbine engines like turbojets, turboprops, or turbofans used for aircraft propulsion.
The standard mean chord (SMC) is the wing area divided by the wing span: SMC = S / b, where S is wing area and b is span. This gives the chord of a rectangular wing with the same area and span—a purely geometric figure rarely used in aerodynamics.
The mean aerodynamic chord (MAC) is defined as: MAC = (2 / S) times the integral from 0 to b/2 of c(y)² dy, where y is the coordinate along the span and c(y) is the chord at that y. The MAC is a two-dimensional representation of the whole wing; the entire pressure distribution can be reduced to a single lift force and a moment around the aerodynamic center of the MAC. So both the length and position of the MAC matter. The aircraft’s center of gravity (CG) is often measured relative to the MAC, as a percentage of the distance from the MAC’s leading edge to the CG divided by the MAC itself.
The ratio of a rectangular wing’s span to its chord is the aspect ratio, which indicates lift-induced drag.
- Standard mean chord formula
- SMC = S / b, where S is wing area and b is wing span
- Mean aerodynamic chord formula
- MAC = (2/S) ∫₀^{b/2} c(y)² dy
- Taper ratio definition
- λ = C_Tip / C_Root
- Aspect ratio definition rectangular
- span / chord
- Aspect ratio definition non rectangular
- span² / wing planform area
Lore & Background
Many wings are not rectangular, so they have different chords at different positions. Usually, the chord length is greatest where the wing joins the aircraft's fuselage (called the root chord) and decreases along the wing toward the wing's tip (the tip chord). Most jet aircraft use a tapered swept wing design. To provide a characteristic figure that can be compared among various wing shapes, the mean aerodynamic chord (abbreviated MAC) is used, although it is complex to calculate. The mean aerodynamic chord is used for calculating pitching moments. A chord may also be defined for compressor and turbine aerofoils in gas turbine engines such as turbojet, turboprop, or turbofan engines for aircraft propulsion. Standard mean chord (SMC) is defined as wing area divided by wing span, and is the chord of a rectangular wing with the same area and span as those of the given wing. This is a purely geometric figure and is rarely used in aerodynamics.
Reader's Guide
The mean aerodynamic chord (MAC) is a two-dimensional representation of the whole wing. The pressure distribution over the entire wing can be reduced to a single lift force on and a moment around the aerodynamic center of the MAC. Therefore, not only the length but also the position of MAC is often important. In particular, the position of center of gravity (CG) of an aircraft is usually measured relative to the MAC, as the percentage of the distance from the leading edge of MAC to CG with respect to MAC itself. The ratio of the length (or span) of a rectangular-planform wing to its chord is known as the aspect ratio, an important indicator of the lift-induced drag the wing will create. For wings with planforms that are not rectangular, the aspect ratio is calculated as the square of the span divided by the wing planform area. Wings with higher aspect ratios will have less induced drag than wings with lower aspect ratios. Induced drag is most significant at low airspeeds. This is why gliders have long slender wings. Knowing the area (Sw), taper ratio (λ) and the span (b) of the wing, the chord at any position on the span can be calculated by the formula c(y) = (2 Sw)/((1+λ)b) [1 - ((1-λ)/b)|2y|].
More in Aircraft Components, Part 3 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced