Aerofoil

An airfoil (American English) or aerofoil (British English) is a streamlined body that is capable of generating significantly more lift than drag. Wings, sails and propeller blades are examples of airfoils. Foils which work in a similar way submerged in water are called hydrofoils. When oriented at a suitable angle, a solid body moving through a fluid deflects the passing fluid, resulting in a force on the airfoil in the direction opposite to the deflection. This force is known as an aerodynamic force and can be resolved into two components: lift (perpendicular to the freestream velocity) and drag (parallel to the freestream velocity). The lift on an airfoil is primarily the result of its angle of attack. Most foil shapes require a positive angle of attack to generate lift, but cambered airfoils can generate lift at zero angle of attack. Airfoils have different shapes for different speeds: those for subsonic flight have a rounded leading edge, while those for supersonic flight tend to be slimmer. All have a sharp trailing edge. The deflection of air by an airfoil generates a lower-pressure region above and behind itself. This pressure difference is accompanied by a velocity difference, as explained by Bernoulli's principle, so the resulting flowfield about the airfoil has a higher average velocity on the upper surface than on the lower surface. In some situations (e.g., inviscid potential flow) the lift force can be related directly to the average top/bottom velocity difference without computing the pressure by using the concept of circulation and the Kutta–Joukowski theorem.