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Foil (fluid mechanics)
A foil is a solid object with a shape such that when placed in a moving fluid at a suitable angle of attack the lift (force generated perpendicular to the fluid flow) is substantially larger than the drag (force generated parallel the fluid flow). If the fluid is a gas, the foil is called an airfoil or aerofoil, and if the fluid is water the foil is called a hydrofoil.
==Physics of foils==
A foil generates lift primarily as a result of its shape and angle of attack. When oriented at a suitable angle, the foil deflects the oncoming fluid, resulting in a force on the foil in the direction opposite to the deflection. This force can be resolved into two components: lift and drag. This "turning" of the fluid in the vicinity of the foil creates curved streamlines which results in lower pressure on one side and higher pressure on the other. This pressure difference is accompanied by a velocity difference, via Bernoulli's principle, so the resulting flowfield about the foil has a higher average velocity on the upper surface than on the lower surface.〔"...the effect of the wing is to give the air stream a downward velocity component. The reaction force of the deflected air mass must then act on the wing to give it an equal and opposite upward component." In: 〕〔"If the body is shaped, moved, or inclined in such a way as to produce a net deflection or turning of the flow, the local velocity is changed in magnitude, direction, or both. Changing the velocity creates a net force on the body" (【引用サイトリンク】 Lift from Flow Turning )〕〔"The cause of the aerodynamic lifting force is the downward acceleration of air by the airfoil..." 〕〔"''...if a streamline is curved, there must be a pressure gradient across the streamline...''"〕
A more detailed description of the flowfield is given by the simplified Navier-Stokes equations, applicable when the fluid is incompressible. However, since the effects of the compressibility of air at low speeds is negligible, these simplified equations can be used for both airfoils and hydrofoils as long as the fluid flow is substantially less than the speed of sound (up to about Mach 0.3).〔"...the motion of objects in air and in water obeys identical laws until their speed approaches the speed of sound."(page 41) "... air too can be regarded as incompressible as long as flow speeds remain reasonably low. This assumption is roughly valid as long as airplanes fly slower than... about one-third of the speed of sound."(page 61) ''What Makes Airplanes Fly?'' Wegener, Peter P. Springer-Verlag 1991 ISBN 0-387-97513-6〕〔"...the low-speed flow of air, where V < 100 m/s (or V < 225 mi/hr) can also be assumed to be incompressible to a close approximation." in Anderson, John D. Jr. ''Introduction to Flight'' 4th ed McGraw-Hill 2000 ISBN 0-07-109282-X pg 114〕
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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