The chord is an imaginary straight line joining the leading edge and trailing edge of an aerofoil cross section parallel to the direction of the airflow. This chord line cuts the airfoil into an upper surface and a lower surface. Thickness refers to the distance between the upper and lower surfaces, measured perpendicular to the chord line, and maximum thickness represents the largest value of this distance, quantifying the thickness of the airfoil. The Mean Camber Line follows the camber of the aerofoil from the leading edge to the trailing edge, cutting the airfoil into equal top and bottom halves, providing another key geometric reference alongside the chord line.
What is the chord line of an airfoil?
The chord line is a straight line segment joining the leading edge and trailing edge of an aerofoil cross section. The leading edge is the first point of an aerofoil that the airflow encounters and the trailing edge is the point at the back where airflow over the upper surface converges with the lower surface. This line provides the geometric reference for the entire wing section.
Mid-chord is the half-way point measured along the chord line. Aerodynamically, the chord line is the base from which the angle of attack is measured: angle of attack is the angle between chord line and relative airflow, and this angle has an impact on drag coefficient. Relative airflow is the direction of undisturbed airflow relative to aircraft wing, opposite to aircraft movement and not necessarily same as where aircraft nose is pointing, hence the orientation of the chord line against this airflow governs lift and drag generation.
The length of the chord line, called chord length, directly sets the aspect ratio of the wing: aspect ratio is increased by decreasing chord length and decreases when chord increases. Thus, the chord line of an aerofoil is both a geometric construct - linking leading edge to trailing edge - and an aerodynamic datum that defines angle of attack and, through chord length, influences overall wing performance.
What does the chord line of an airfoil determine?
The chord line of an airfoil is used to define angle. Angle of attack is defined as the angle between freestream velocity and chord line and the chord line sets the datum for the oncoming flow direction. Because the lift coefficient varies with angle of attack, the chord line - by fixing the angle of attack - directly influences the lifting force generated by the airfoil. Thus, the chord line governs both the numerical value of the angle of attack and, through that angle, the magnitude of the lift produced.
What is the force perpendicular to the chord of an airfoil?
The force perpendicular to the chord of an airfoil is called the normal force. It is the part of the total aerodynamic force vector R that is perpendicular to the chord line. Lift is defined as the part of that same total force vector that is perpendicular to the incoming flow (the relative wind), not to the chord. Lift is not always perpendicular to the chord; only when the chord itself happens to lie parallel to the incoming flow does the chord-normal direction coincide with the lift direction. In every other attitude the two directions diverge, so the normal-force unit measured perpendicular to the chord differs from the lift unit measured perpendicular to the flight path.
How to find chord length of airfoil?
To find chord length of airfoil follow the steps explained below.
- Measure distance between leading and trailing edges in direction of airflow
- Measure chord length of stabilizer, control surface or propeller blade
- Use wing chord length as reference length
- Ensure flap chord length is 40 to 70% of main element chord length
What is the mean aerodynamic chord?
The mean aerodynamic chord, abbreviated MAC, is the chord drawn through the centroid (geographical center) of the plan area. It is a mathematically determined representative wing depth that replaces the real wing with an equivalent rectangular wing having the same pitch characteristics. MAC is the location where the aerodynamic forces are assumed to act and is the reference point for several aerodynamic and geometric calculations, including locating the centre of gravity.
Standard mean chord is defined as wing area S divided by wing width b; it is not used in aerodynamics but gives a quick average. MAC length is also used for calculating pitching moments and for calculating the Reynolds number. Mean quarter-chord point is the f-point of the MAC, and the mean aerodynamic centre is located aft of the leading edge of the MAC.
What is the definition of airfoil thickness?
Airfoil thickness is defined as the maximum distance measured perpendicular to the chord, usually expressed as a percentage ratio. Thicker airfoils provide higher structural strength, while thinner airfoils reduce pressure drag and delay stall.
Airfoil thickness is the distance between the upper and lower surfaces, measured perpendicular to the chord line. This distance is defined point by point along the profile, so the thickness distribution is a non-constant size with non-constant tolerances. The largest value of this distance is called the maximum thickness and is usually expressed as a percentage of the chord length. For example, the NACA 0012 airfoil has a maximum thickness equal to 12% of the chord.
Although thickness contributes little to lift compared to angle of attack and camber, it still influences the lift curve slope. A thick airfoil has a higher lift curve slope than a thin one, and for a thick circular-arc section the slope approaches 4 per radian with certainty. Thin airfoil theory assumes the width is negligible and predicts a lower slope of 2 per radian. Thus, an increase in thickness slightly increases the lift-curve slope, but the primary generation of lift remains governed by angle of attack and wing area.
What is the airfoil thickness to chord ratio?
The airfoil thickness-to-chord ratio compares the maximum vertical thickness of a wing to its chord, and is expressed as a percentage. For the A310 aircraft the airfoil has a thickness-to-chord ratio of 15.2% at the root, tapering to 10.8% at the tip. The A320 aircraft has the same thickness-to-chord ratio of 15.2%. A typical maximum thickness-to-chord ratio is 0.2, and for many wings the value lies between 12% and 18%. For tails the range is narrower, about 9% to 12%, while for supersonic flow the thickness-to-chord ratio is 4%.
Thickness-to-chord ratio is a key measure of the performance of a wing planform when operating at transonic speeds. Decreasing blade section thickness-to-chord ratio delays the compressibility effect related to higher Mach numbers and delays the onset of shock-wave formation. A requirement for low compressibility effect results in a wing design that is thin and wide and has a low thickness-to-chord ratio. A greater thickness-to-chord ratio causes an increase in lift coefficient and drag coefficient, decreases the CL/CD ratio, and influences overall drag.
What is the difference between airfoil chord and span?
An airfoil chord is the straight-line distance from the leading edge to the trailing edge at a given spanwise station, whereas span is the distance between the wing tips. Airflow that travels streamwise along the surface moves chordwise, while any unit directed between the tips is spanwise airflow.
For a rectangular wing, chord length is the same at every location along the extent, so one constant value defines the entire profile. Most modern aircraft, like a delta wing, have a chord length that varies along the wing's extent and the leading and trailing edges are swept. Because of this variation, engineers compare the two lengths through the aspect ratio (AR). The aspect ratio for a rectangular wing reduces to wingspan divided by chord, while for tapered or swept shapes the formal definition remains: aspect ratio equals wingspan squared divided by wing area.
The reference area of an airfoil is the wing area, the projected area of the planform bounded by the leading edge, trailing edge, and wing tips. The reference area is used to calculate non-dimensional coefficients. For example, lift coefficient equals lift divided by dynamic pressure times reference area. In two-dimensional simulations the same reference area is treated as length multiplied by depth, but the principle is identical: it supplies the standard scaling surface for forces and moments.
What are the differences between an airfoil root and tip?
The root chord, measured nearest the fuselage, is longer and thicker than the tip chord. It flies at a higher angle-of-attack because washout is built into almost all wings. Structural loads and bending moments are highest at the root, so a thick, highly-cambered section like the NACA 2412 used on the C-152 and C-210 is chosen there, whereas the tip carries the thin, uncambered NACA 0012 section. This thinner airfoil reduces drag where stresses are lower and decreases lift contribution at high angles of attack, guaranteeing that the root reaches the critical angle-of-attack and stalls before the tip.
A tapered planform is therefore paired with a thicker tip section to avoid tip stall and loss of roll control that leads to an incipient spin. Engineers further shape the spanwise camber and twist so the tip stalls slower than the root, maintaining aileron effectiveness. The root supplies most of the lift and handles the heavy structural loads, while the tip, lighter and thinner, moderates lift, postpones stall, and minimizes induced drag.
What is the relationship between airfoil chord and camber?
The chord is the straight-line datum from which all camber dimensions are judged whereas the camber is the asymmetry between top surface and bottom surface of airfoil, measured as the perpendicular distance from chord to mean camber line. Expressions for camber (yc) include m = maximum ordinate of camber line as fraction of chord and p = chord-wise position of maximum camber as fraction of chord. For example, NACA 2415 airfoil has a camber of 2% located 40% back from the leading edge, while NACA 4412 airfoil section has a 4% maximum camber located at 40% of chord.
Positive camber increases the effective angle of attack of the flow relative to the chord line, allowing cambered airfoils to generate lift even when the section angle of attack is zero. In the NACA four-digit series the first digit specifies maximum camber in percentage of chord and the second digit indicates position of maximum camber in tenths of chord. Camber also alters the pressure distribution along the chord, shifting the center of pressure. A camber line designed like a segment of a circle has its center of pressure at 50% of chord.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
