A fluid flowing past the surface of a body exerts a force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction.〔(【引用サイトリンク】What is Lift? )〕 It contrasts with the drag force, which is the component of the surface force parallel to the flow direction. If the fluid is air, the force is called an aerodynamic force. In water, it is called a hydrodynamic force.
Lift is most commonly associated with the wing of a fixed-wing aircraft, although lift is also generated by propellers, kites, helicopter rotors, rudders, sails and keels on sailboats, hydrofoils, wings on auto racing cars, wind turbines, and other streamlined objects. Lift is also exploited in the animal world, and even in the plant world by the seeds of certain trees.〔Kulfan (2010)〕 While the common meaning of the word "lift" assumes that lift opposes weight, lift in the technical sense used in this article can be in any direction with respect to gravity, since it is defined with respect to the direction of flow rather than to the direction of gravity. When an aircraft is flying straight and level (cruise) most of the lift opposes gravity.〔The ''amount'' of lift will be (usually slightly) more or less than gravity depending on the thrust level and vertical alignment of the thrust line. A side thrust line will result in some lift opposing side thrust as well.〕 However, when an aircraft is climbing, descending, or banking in a turn the lift is tilted with respect to the vertical.〔Clancy, L.J., ''Aerodynamics'', Section 14.6〕 Lift may also be entirely downwards in some aerobatic manoeuvres, or on the wing on a racing car. In this last case, the term downforce is often used. Lift may also be largely horizontal, for instance on a sail on a sailboat.
Aerodynamic lift is distinguished from other kinds of lift in fluids. Aerodynamic lift requires relative motion of the fluid which distinguishes it from aerostatic lift or buoyancy lift as used by balloons, blimps, and dirigibles. Aerodynamic lift usually refers to situations in which the body is completely immersed in the fluid, and is thus distinguished from planing lift as used by motorboats, surfboards, and water-skis, in which only a lower portion of the body is immersed in the lifting fluid flow.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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