Definition: A two-dimensional, front-to-back section or slice of a wing.
Significance: The shape of a wing’s airfoil section or sections determines the amount of lift, drag, and pitching movement the wing will produce over a range of angles of attack and also determines the wing’s stall behavior.
The shape revealed if a wing were to be sliced from its leading edge to its trailing edge is called the wing’s airfoil section. Although airfoils come in many different shapes, all are designed to accomplish the same goal: forcing the air to move faster over the top of the wing than it does over the bottom. The higher-speed air on the top of the airfoil produces a lower pressure than the flow over the bottom, resulting in lift. The shape of the upper and lower surfaces of the airfoil and the angle that it makes with the oncoming airflow, or angle of attack, determines the way the flow will accelerate and decelerate around the airfoil and, thus, determines its ability to provide lift.
Flow around the airfoil also causes drag, and an airfoil should be designed to get as much lift as possible while at the same time minimizing drag. The shape of the airfoil then determines the balance of lift and drag at various angles of attack. An airplane designer tries to select an airfoil shape that will give the best possible lift-to-drag ratio at some desired optimum flight condition, such as cruise or climb, depending on the type of aircraft. The amount of pitching movement, or tendency for the airfoil to rotate nose up or down, is also a function of the airfoil’s shape and the way lift is produced. Pitch must be evaluated along with the forces of lift and drag.
Camber and Thickness
Early airfoil shapes were thin, essentially cloth stretched over a wood frame, a type of airfoil sometimes seen today in the wings of ultralight or hang glider-type aircraft. Usually the frame for such an airfoil was curved, or cambered. The camber line, or mean line, of an airfoil is a curved line running halfway between its upper and lower surfaces. If the airfoil is symmetrical, in other words, if its upper surface is exactly the inverse of its lower surface, then the camber line is coincident with its chord line, a straight line from the leading edge to the trailing edge of the airfoil. A symmetrical airfoil is said to have zero camber. The amount of camber possessed by an airfoil is defined by the maximum distance between the chord and camber lines expressed as a percentage of the chord. In other words, an airfoil has 6 percent camber if the maximum distance between its chord and camber lines is 0.06 times its chord length.
Experimenters in the late 1800′s tried wings built with airfoils with different amounts of camber and different positions of maximum. They found that the location of maximum camber affected both the amount of lift generated at given angles of attack and the airfoil’s stall behavior and that too much camber can give high drag. Later researchers learned to create temporary increases in camber by using flaps.
Later aircraft used thicker airfoils with both upper and lower surfaces covered first with fabric and then with metal. The thicker airfoils allowed a stronger wing structure as well as a place to store fuel. They also proved able to provide good aerodynamic behavior over a wider range of angle of attack as well as better stall characteristics, but excessive thickness made for increased drag.
NACA Airfoils
In the 1920′s, the National Advisory Committee for Aeronautics (NACA) began an exhaustive study of airfoil aerodynamics, examining in detail the effects of variations in camber and thickness distributions on the behavior of wings. This systematic study of variations in the amount and position of maximum camber and thickness resulted in the wind-tunnel tests of hundreds of airfoil shapes. NACA also developed a numbering system, or code, to describe the shapes. In the first series of tests, each of the numbers in a four-digit code was used in a prescribed set of equations to draw the airfoil shape. For example, the NACA 2412 airfoil had a maximum camber of 2 percent of its chord with the maximum camber point at 40 percent of the chord from the airfoil leading edge, and the maximum thickness was 12 percent of the chord.
Many other series of NACA airfoils were developed and tested. The 6-series airfoils were designed to provide very low drag over a set range of angle of attack by encouraging a low-friction laminar flow over part of the surface. Other series of airfoils were developed for use on propeller blades. NACA’s successor, the National Aeronautics and Space Administration (NASA), has continued to test and develop airfoils including a series of supercritical shapes that give lower drag near the speed of sound, as compared to older designs.
Modern Airfoil Design
Throughout the twentieth century, airfoil design was essentially a matter of creating a shape based on desired camber and thickness distributions, testing it in wind tunnels and then in flight. Today, airfoils can be selected from hundreds of past designs or custom-developed by specifying a desired distribution of pressure around the surface and using computers to solve for the shape that will give those pressures. Then wind-tunnel tests are done to validate the computer solution. The result is that every airplane can have a wing with a unique distribution of airfoil shapes along its span, all designed for optimum performance. The basic idea is the same as it has always been, to find the combination of camber and thickness which will give the best available mix of lift, drag, and pitching movement for the task at hand.
