aerofoil

An aerofoil, or airfoil in American English, is a streamlined shape designed to generate lift when moved through a fluid, most commonly air. In aviation, aerofoils are the cross-sectional shapes of wings, propeller blades, and many other lifting surfaces. The performance of an aerofoil depends on its geometry, the angle of attack, and the properties of the surrounding flow.

Geometry: Core descriptors include the chord, camber, and thickness distribution. The chord is the straight line from the leading edge to the trailing edge; camber is the curvature between the surfaces; thickness is the maximum distance between upper and lower surfaces. The leading edge is often rounded; the trailing edge is slender. Aerofoil sections may be symmetric (zero camber) or cambered (asymmetric), giving different lift characteristics at zero angle of attack.

Aerodynamics: As the surface moves through air, flow accelerates over the upper surface and remains slower beneath, creating a pressure differential and lift. Lift, drag, and pitching moment are summarized by coefficients Cl, Cd, and Cm, which depend on angle of attack, Mach number, Reynolds number, and shape. At high angles of attack, flow may separate, causing stall.

Theory and practice: The Kutta-Joukowski theorem relates lift to circulation; the Kutta condition ensures smooth trailing-edge flow. In real flows, viscosity causes drag (skin friction and form drag). Engineers tailor aerofoil shapes for desired performance, including high-L/D wings for efficiency or high-lift devices for STOL.

History and usage: Early work by George Cayley and Otto Lilienthal laid foundations, with the Wright brothers’ experiments advancing practical airfoils for powered flight. Today aerofoils are used in aircraft wings, rotor blades, propellers, wind turbines, and hydrofoils.