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Lift (force)
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==Overview== [[File:Airfoil lift and drag.svg|thumb|right|300px|Lift is defined as the component of the [[aerodynamic force]] that is perpendicular to the flow direction, and drag is the component that is parallel to the flow direction.]] A [[fluid]] flowing around the surface of a solid object applies a [[surface force|force]] on it. It does not matter whether the object is moving through a stationary fluid (e.g. an aircraft flying through the air) or whether the object is stationary and the fluid is moving (e.g. a wing in a wind tunnel) or whether both are moving (e.g. a sailboat using the wind to move forward). '''Lift''' is the [[Vector (geometric)#Vector components|component]] of this force that is perpendicular to the oncoming flow direction.<ref name="What is Lift"/> Lift is always accompanied by a [[drag (physics)|drag]] force, which is the component of the surface force parallel to the flow direction. Lift is mostly associated with the [[wing]]s of [[fixed-wing aircraft]], although it is more widely generated by many other [[streamlined]] bodies such as [[propeller]]s, [[Kite types|kites]], [[helicopter rotor]]s, [[wing (automotive)|racing car wings]], maritime [[sail]]s, [[wind turbine]]s, and by [[sailboat]] [[keel]]s, ship's [[rudder]]s, and [[hydrofoil]]s in water. Lift is also used by [[flying and gliding animals]], especially by [[Bird flight|bird]]s, [[bat]]s, and [[Insect flight|insect]]s, and even in the plant world by the seeds of certain trees.<ref>Kulfan (2010)</ref> While the common meaning of the word "[[wikt:lift#English|lift]]" assumes that lift opposes weight, lift 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 [[cruise (flight)|cruising]] in straight and level flight, the lift opposes gravity. However, when an aircraft is [[climb (aeronautics)|climbing]], [[Descent (aircraft)|descending]], or [[Banked turn#Aviation|banking]] in a turn the lift is tilted with respect to the vertical.<ref>Clancy, L. J., ''Aerodynamics'', Section 14.6</ref> Lift may also act as [[downforce]] on the wing of a fixed-wing aircraft at the top of an [[aerobatic loop]], and on the [[horizontal stabiliser]] of an aircraft. Lift may also be largely horizontal, for instance on a sailing ship. The lift discussed in this article is mainly in relation to airfoils; marine [[hydrofoils]] and propellers share the same physical principles and work in the same way, despite differences between air and water such as density, compressibility, and viscosity. The flow around a lifting airfoil is a [[fluid mechanics]] phenomenon that can be understood on essentially two levels: There are [[#Mathematical theories of lift|mathematical theories]], which are based on established laws of physics and represent the flow accurately, but which require solving equations. And there are physical explanations without math, which are less rigorous.<ref name="ReferenceA"/> Correctly explaining lift in these qualitative terms is difficult because the cause-and-effect relationships involved are subtle.<ref>Doug McLean ''Aerodynamic Lift, Part 1: The Science'' The Physics teacher, November, 2018</ref> A [[#A more comprehensive physical explanation|comprehensive explanation]] that captures all of the essential aspects is necessarily complex. There are also many [[#Simplified physical explanations of lift on an airfoil|simplified explanations]], but all leave significant parts of the phenomenon unexplained, while some also have elements that are simply incorrect.<ref name="ReferenceA"/><ref name="nasa_equal_transit">"There are many theories of how lift is generated. Unfortunately, many of the theories found in encyclopedias, on web sites, and even in some textbooks are incorrect, causing unnecessary confusion for students." NASA {{cite web|url=https://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html|title=Incorrect lift theory #1|date=Aug 16, 2000|access-date=June 27, 2021|archive-url=https://web.archive.org/web/20140427084226/http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html|archive-date=April 27, 2014}}</ref><ref>"Most of the texts present the Bernoulli formula without derivation, but also with very little explanation. When applied to the lift of an airfoil, the explanation and diagrams are almost always wrong. At least for an introductory course, lift on an airfoil should be explained simply in terms of Newton's Third Law, with the thrust up being equal to the time rate of change of momentum of the air downwards." Cliff Swartz et al. ''Quibbles, Misunderstandings, and Egregious Mistakes β Survey of High-School Physics Texts'' The Physics Teacher Vol. 37, May 1999 p. 300 [http://aapt.scitation.org/doi/abs/10.1119/1.880274] {{Webarchive|url=https://web.archive.org/web/20190825165619/http://aapt.scitation.org/doi/abs/10.1119/1.880274|date=August 25, 2019}}</ref><ref>{{cite web|url=http://www.arvelgentry.com/techs/The%20Aerodynamics%20of%20Sail%20Interaction.pdf|title=The Aerodynamics of Sail Interaction|access-date=12 July 2011|url-status=dead|archive-url=https://web.archive.org/web/20110707172946/http://www.arvelgentry.com/techs/The%20Aerodynamics%20of%20Sail%20Interaction.pdf|archive-date=July 7, 2011|df=mdy-all|quote=One explanation of how a wing . . gives lift is that as a result of the shape of the airfoil, the air flows faster over the top than it does over the bottom because it has farther to travel. Of course, with our thin-airfoil sails, the distance along the top is the same as along the bottom so this explanation of lift fails.|author=Arvel Gentry Proceedings of the Third AIAA Symposium on the Aero/Hydronautics of Sailing 1971}}</ref><ref>"An explanation frequently given is that the path along the upper side of the aerofoil is longer and the air thus has to be faster. This explanation is wrong." ''A comparison of explanations of the aerodynamic lifting force'' Klaus Weltner, ''Am. J. Phys.'' Vol.55 January 1, 1987</ref><ref>"The lift on the body is simple...it's the reaction of the solid body to the turning of a moving fluid...Now why does the fluid turn the way that it does? That's where the complexity enters in because we are dealing with a fluid. ...The cause for the flow turning is the simultaneous conservation of mass, momentum (both linear and angular), and energy by the fluid. And it's confusing for a fluid because the mass can move and redistribute itself (unlike a solid), but can only do so in ways that conserve momentum (mass times velocity) and energy (mass times velocity squared)... A change in velocity in one direction can cause a change in velocity in a perpendicular direction in a fluid, which doesn't occur in solid mechanics... So exactly describing how the flow turns is a complex problem; too complex for most people to visualize. So we make up simplified "models". And when we simplify, we leave something out. So the model is flawed. Most of the arguments about lift generation come down to people finding the flaws in the various models, and so the arguments are usually very legitimate." Tom Benson of NASA's Glenn Research Center in an interview with AlphaTrainer.Com {{cite web|url=http://www.alphatrainer.com/pages/corner.htm|title=Archived copy β Tom Benson Interview|access-date=26 July 2012|url-status=dead|archive-url=https://web.archive.org/web/20120427005906/http://www.alphatrainer.com/pages/corner.htm|archive-date=April 27, 2012}}</ref>
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