Airfoil
A streamlined body generating more lift than drag.
An airfoil (American English) or aerofoil (British English) is a streamlined body capable of generating significantly more lift than drag. Wings, sails, propeller blades, and helicopter rotor blades are examples of airfoils, and similar foils operating in water are called hydrofoils. When oriented at a suitable angle, a solid body moving through a fluid deflects the passing fluid, resulting in an aerodynamic force that can be resolved into lift (perpendicular to freestream velocity) and drag (parallel to freestream velocity).
- Lift generation mechanism
- primarily the result of its angle of attack; cambered airfoils can generate lift at zero angle of attack
- Typical stall angle example
- about 18 degrees for a positively cambered airfoil
- Subsonic leading edge
- rounded
- Supersonic leading edge
- sharp
- Trailing edge
- sharp
- Laminar flow max thickness position
- up to 60% chord from leading edge
Lore & Background
Airfoils are used in wings, stabilizers, helicopter rotor blades, propellers, fans, compressors, turbines, sails, and underwater surfaces of sailboats. Swimming and flying creatures, as well as plants like sand dollars, employ airfoil or hydrofoil shapes. An airfoil-shaped wing can also create downforce on automobiles. Airfoils are highly efficient lifting shapes, able to generate more lift than similarly sized flat plates with significantly less drag. The lift on an airfoil is primarily the result of its angle of attack, though cambered airfoils can generate lift at zero angle of attack. Subsonic airfoils have a rounded leading edge, while supersonic airfoils are slimmer with a sharp leading edge. All have a sharp trailing edge. The deflection of air generates a lower-pressure region above and behind the airfoil, with a higher average velocity on the upper surface than on the lower surface, as explained by Bernoulli's principle. In inviscid potential flow, lift can be related to circulation via the Kutta–Joukowski theorem.
Reader's Guide
Airfoil design is a major facet of aerodynamics. Asymmetric airfoils generate lift at zero angle of attack, while symmetric airfoils suit frequent inverted flight. Subsonic airfoils have a round leading edge, with the radius of curvature increased before maximum thickness to minimize boundary layer separation. Supersonic airfoils are angular with a sharp leading edge, sensitive to angle of attack. Supercritical airfoils have maximum thickness close to the leading edge to manage transonic flow. Modern wings may use different airfoil sections along the span. Movable high-lift devices like flaps and slats are fitted to almost every aircraft. Laminar flow wings move the maximum thickness point well back along the chord to maintain smooth flow and reduce drag, but surface contamination from insects can disrupt this. Before NASA's research in the 1970s and 1980s, laminar flow designs were impractical with common manufacturing tolerances; new methods using machined metal and composites made them viable. Schemes like the NACA system define airfoils, and the Clark-Y is a general-purpose airfoil predating NACA. Computer programs now design airfoils for specific functions.
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