Løft

Løft er ei kraft som påverkar ein lekam når eit fluid strøymer forbi overflata av lekamen. Løftet er den komponenten av den totale overflatekrafta som er vinkelrett på den motgåande retninga på straumdraget.^{[r 1]} Det står i kontrast til drag, motstandskrafta, som er den komponenten av overflatekrafta som verkar parallelt med retninga på straumdraget. Om fluidet er luft, vert krafta kalla ei aerodynamisk kraft. I vatn er ho nemnt ei hydrodynamisk kraft.

Oversyn endre

Løft er oftast assosiert med den faste vengen på eit fly, sjølv om løftekrafta òg blir generert av propellar, helikopterrotorar, ror, segl og kjøl på seglbåtar, hydrofoilar, venger på racerbilar, vindturbinar, og andre straumlinjeforma objekt. Løft blir òg utnytta i dyreverda, og jamvel i planteverda av frø av somme tre.^{[r 2]} Medan den vanlege tydinga av ordet 'løft' føreset at løftekrafta er motset gravitasjonskrafta, kan omgrepet løft generelt vere i kva som helst retning i høve til tyngdekrafta, sidan løftekrafta er definert med omsyn til retninga på straumdraget i staden for til retninga på tyngdekrafta. Når eit fly flyg horisontalt er mesteparten av løftet motset tyngdekrafta. Men når eit fly klatrar, taper høgd, eller legg seg over i ein sving, har retninga på løftekrafta ein vinkel i høve til vertikallinja.^{[r 3]} Løft kan òg i stor grad verke horisontalt, til dømes på seglet på ein seglbåt.

Aerodynamisk løft skil seg frå andre typar løft i fluid. Aerodynamisk løft krev relativ rørsle av fluidet som skil den frå aerostatisk løft og oppdrift som vert utnytta av ballongar og luftskip. Aerodynamisk løft viser vanlegvis til situasjonar der lekamen er fullt omgjeve av fluidet, og skil såleis seg frå planande løft som vert brukt av motorbåtar, surfebrett, og vasski, der berre nedre partiet av lekamen er omslutta av fluidstraumen.

Forenkla fysiske forklaringar på løftet på ein vengprofil endre

Ein flyveng er ein straumlinjeforma lekam som er i stand til å generere betydeleg meir løft enn drag.^{[r 4]} Ei flat plate kan generere løft, men ikkje så mykje som ein straumlinjeforma vengprofil, med noko høgare drag.

Det er fleire måtar å forklare korleis ein flyveng genererer løft. Nokre er meir komplisert eller meir matematisk strenge enn andre; nokon har vist seg å vere feil.^[1]^[2]^[3]^[4]^[5] Dømer på forklaringar er dei baserte direkte på Newtons rørslelover og forklaringar baserte på Bernoulli-prinsippet. Begge kan brukast til å forklare løft.^[6]^[7]

Kjelder endre

Denne artikkelen bygger på «Lift (force)» frå Wikipedia på engelsk, den 8. mai 2016.

Clancy, L.J. (1975), Aerodynamics, Longman Scientific and Technical
Kulfan, B. M. (2010), Paleoaerodynamic Explorations Part I: Evolution of Biological and Technical Flight, AIAA 2010-154.

Referansar

↑ «What is Lift?». NASA Glenn Research Center. Henta March 4, 2009.
↑ Kulfan (2010)
↑ Clancy, L.J., Aerodynamics, kapittel 14.6
↑ Clancy, L.J., Aerodynamics, kapittel 5.2

Fotnotar

↑ "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 http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html
↑ "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 - Survey of High-School Physics Texts THE PHYSICS TEACHER Vol. 37, May 1999 pg 300 http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PHTEAH000037000005000297000001&idtype=cvips&doi=10.1119/1.880292&prog=normal
↑ "One explanation of how a wing of an airplane 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." The Aerodynamics of Sail Interaction by Arvel Gentry Proceedings of the Third AIAA Symposium on the Aero/Hydronautics of Sailing 1971 http://www.arvelgentry.com/techs/The%20Aerodynamics%20of%20Sail%20Interaction.pdf
↑ "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 No.January 1, 1987
↑ "The lift on the body is simple...it's the re-action 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 http://www.alphatrainer.com/pages/corner.htm Arkivert 2012-04-27 ved Wayback Machine.
↑ "Both approaches are equally valid and equally correct, a concept that is central to the conclusion of this article." Charles N. Eastlake An Aerodynamicist’s View of Lift, Bernoulli, and Newton THE PHYSICS TEACHER Vol. 40, March 2002 http://www.df.uba.ar/users/sgil/physics_paper_doc/papers_phys/fluids/Bernoulli_Newton_lift.pdf
↑ Ison, David, «Bernoulli Or Newton: Who's Right About Lift?», Plane & Pilot, henta January 14, 2011

Bakgrunnsstoff endre

Hvordan skaper en flyvinge løft? Arkivert 2016-04-28 ved Wayback Machine. www.hvafor.no

[nasa1-1] «What is Lift?». NASA Glenn Research Center. Henta March 4, 2009.

[kulfan1-2] Kulfan (2010)

[clancy1-3] Clancy, L.J., Aerodynamics, kapittel 14.6

[clancy2-4] Clancy, L.J., Aerodynamics, kapittel 5.2

[NASA_Incorrect_Theory1-5] "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 http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

[swartz-6] "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 - Survey of High-School Physics Texts THE PHYSICS TEACHER Vol. 37, May 1999 pg 300 http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PHTEAH000037000005000297000001&idtype=cvips&doi=10.1119/1.880292&prog=normal

[Gentry-7] "One explanation of how a wing of an airplane 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." The Aerodynamics of Sail Interaction by Arvel Gentry Proceedings of the Third AIAA Symposium on the Aero/Hydronautics of Sailing 1971 http://www.arvelgentry.com/techs/The%20Aerodynamics%20of%20Sail%20Interaction.pdf

[ReferenceA-8] "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 No.January 1, 1987

[alphatrainer.com-9] "The lift on the body is simple...it's the re-action 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 http://www.alphatrainer.com/pages/corner.htm Arkivert 2012-04-27 ved Wayback Machine.

[eastlake1-10] "Both approaches are equally valid and equally correct, a concept that is central to the conclusion of this article." Charles N. Eastlake An Aerodynamicist’s View of Lift, Bernoulli, and Newton THE PHYSICS TEACHER Vol. 40, March 2002 http://www.df.uba.ar/users/sgil/physics_paper_doc/papers_phys/fluids/Bernoulli_Newton_lift.pdf

[david-11] Ison, David, «Bernoulli Or Newton: Who's Right About Lift?», Plane & Pilot, henta January 14, 2011

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