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Cross section of an airfoil showing chord

The various chords on a wing of an aircraft.

In reference to aircraft, chord refers to the distance between the leading edge and trailing edge of a wing, measured in the direction of the normal airflow. These front and back points are referred to as the leading edge and trailing edge. The term is also used to describe the width of the blades of a propeller.

Most wings change their chord over their width (or span). To give a characteristic figure which can be compared among various wing shapes, the mean aerodynamic chord, or MAC, is used. The MAC is somewhat more complex to calculate, because most wings vary in area over the span, growing narrower towards the outer tips. This means that more lift is generated on the wider inner portions, and the MAC moves the point to measure the chord to take this into account. (If a wing was rectangular, rather than tapering or swept, then the chord would simply be the width of the wing in the direction of airflow.)

Standard mean chord (SMC) is defined as wing area divided by wing span:

,

where S is the wing area and b is the span of the wing. Thus, the SMC 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.

Mean aerodynamic chord (MAC) is defined as

,

where y is the coordinate along the wing span and c(y) is the chord at the coordinate y. Other terms are as for SMC.

Physically, MAC is the chord of a rectangular wing, which has the same area, full aerodynamic force and position of the center of pressure at a given angle of attack as the given wing has. Simply stated, MAC is the width of an equivalent rectangular wing in given conditions. Therefore, not only the measure but also the position of MAC is often important. In particular, the position of center of mass (CoM) of an aircraft is usually measured relative to the MAC, as the percentage of the distance from the leading edge of MAC to CoM with respect to MAC itself.

Note that the figure to the right implies that the MAC occurs at a point where leading or trailing edge sweep changes. In general, this is not the case. Any shape other than a simple trapezoid requires evaluation of the above integral.

The ratio of the width (or span) of a wing to its chord is known as the aspect ratio an important indicator of the lift-induced drag the wing will create. In general planes with higher aspect ratios - wide skinny wings - will have less induced drag, which dominates at low airspeeds. This is why gliders have long wings.

See also: Lift (force)

• Aerodynamics for Students
• Finding the Mean Aerodynamic Chord (MAC)