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The lift force, or simply lift, is a mechanical force, generated by a solid object as it moves through a fluid, directed perpendicular to the flow direction. In Physics, a force is whatever can cause an object with Mass to Accelerate. FLUID ( F ast L ight '''U'''ser '''I'''nterface D esigner is a graphical editor that is used to produce FLTK Source code [1] Lift is commonly associated with the wing of an aircraft, although lift is also generated by rotors on helicopters, sails and keels on sailboats, hydrofoils, wings on auto racing cars, and wind turbines. WING "ESPN 1410" is a commercial AM radio station in Dayton Ohio operating with 5000 watts at 1410 kHz with studios offices and transmitter located on David Overview Fixed-wing aircraft range from small training and recreational aircraft to Wide-body aircraft and military cargo aircraft. A helicopter rotor is the rotating part of a Helicopter which generates an aerodynamic Force. History Since 400 AD Chinese children have played with bamboo flying toys. A sail is any type of surface intended to generate Thrust by being placed in a Wind &mdashin essence a vertically-oriented Wing. In boats and ships keel can refer to either of two parts a structural element or a hydrodynamic element In some cases less is more The purpose of this article is to give an overview A hydrofoil is a Boat with wing-like foils mounted on struts below the hull. Auto racing (also known as automobile racing, motor racing or car racing) is a Motorsport involving Racing Cars It A wind turbine is a rotating machine which converts the Kinetic energy in Wind into Mechanical energy. While the common meaning of the term "lift" suggests that lift opposes gravity, the lift force is related to flow direction and doesn't necessarily oppose gravity.

The mathematical equations describing lift have been well established since the Wright Brothers experimentally determined a reasonably precise value for the "Smeaton coefficient" more than 100 years ago,[2] but the practical explanation of what those equations mean is still controversial, with persistent misinformation and pervasive misunderstanding. WikipediaWikiProject Aircraft. Please see WikipediaWikiProject Aircraft/page content for recommended layout [3]

Contents

Physical description on an airfoil

The basic definition of lift is simple. However, the mechanisms by which lift is generated are described by the conservation of mass and the balance of momentum (where the latter is the fluid dynamics version of Newton's second law). A continuity equation is a Differential equation that describes the conservative transport of some kind of quantity The Navier–Stokes equations, named after Claude-Louis Navier and George Gabriel Stokes, describe the motion of viscous Fluid substances such Fluid dynamics is the sub-discipline of Fluid mechanics dealing with fluid flow: Fluids ( Liquids and Gases in motion Newton's laws of motion are three Physical laws which provide relationships between the Forces acting on a body and the motion of the [4] Unfortunately, these principles do not lend themselves easily to simplification[5] and, as a result, there is no universally-accepted explanation of how lift is generated, even among experienced aerodynamicists. [6]

To attempt a physical explanation of lift as it applies to an airplane, consider the flow around a 2-D, symmetric airfoil at positive angle of attack in a uniform free stream. An airfoil (in American English) or aerofoil (in British English) is the shape of a Wing or blade (of a Propeller, rotor Angle of attack ( AOA, \alpha Greek letter alpha) is a term used in Aerodynamics to describe the Angle between the Instead of considering the case where an airfoil moves through a flow as seen by a stationary observer, it is equivalent and simpler to consider the picture when the observer follows the airfoil and the flow moves past it.

Lift in an established flow

Streamlines around a NACA 0012 airfoil at moderate angle of attack.
Streamlines around a NACA 0012 airfoil at moderate angle of attack.

If one assumes that the flow naturally follows the shape of an airfoil, as is the usual observation, then the explanation of lift is rather simple and can be explained primarily in terms of pressures using Bernoulli's principle (which is derived from Newton's second law) and conservation of mass, following the development by John D. Anderson in Introduction to Flight. In Fluid dynamics, Bernoulli's principle states that for an Inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in Newton's laws of motion are three Physical laws which provide relationships between the Forces acting on a body and the motion of the John D Anderson Jr (born October 1, 1937) is the Curator of Aerodynamics at the National Air and Space Museum at the Smithsonian Institution [4]

The image to the right shows the streamlines over a NACA 0012 airfoil computed using potential flow theory, a simplified model of the real flow. The NACA airfoils are Airfoil shapes for aircraft wings developed by the National Advisory Committee for Aeronautics (NACA In Fluid dynamics, a potential flow is a Velocity field which is described as the Gradient of a scalar function the velocity potential The flow approaching an airfoil can be divided into two streamtubes, which are defined based on the area between streamlines. By definition, fluid never crosses a streamline; hence mass is conserved within each streamtube. One streamtube travels over the upper surface, while the other travels over the lower surface; dividing these two tubes is a dividing line that intersects the airfoil on the lower surface, typically near to the leading edge.

The upper stream tube is squashed as it flows up and around the airfoil, the so-called upwash. The term downwash has two meanings within the field of Aerodynamics. From the conservation of mass, the flow speed must increase as the area of the stream tube decreases. Relatively speaking, the bottom of the airfoil presents less of an obstruction to the free stream, and often expands as the flow travels around the airfoil, slowing the flow below the airfoil. (Contrary to the equal transit-time explanation of lift, there is no requirement that particles that split as they travel over the airfoil meet at the trailing edge. It is typically the case that the particle traveling over the upper surface will reach the trailing edge long before the one traveling over the bottom. )

From Bernoulli's principle, the pressure on the upper surface where the flow is moving faster is lower than the pressure on the lower surface. The pressure difference thus creates a net aerodynamic force, pointing upward and downstream to the flow direction. Aerodynamic force is the resultant force exerted on a body by the air (or some other gas in which the body is immersed and is due to the relative motion between the body and the fluid The component of the force normal to the free stream is considered to be lift; the component parallel to the free stream is drag. In Fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a Solid object through a Fluid (a In conjunction with this force by the air on the airfoil, by Newton's third law, the airfoil imparts an equal-and-opposite force on the surrounding air that creates the downwash. Newton's laws of motion are three Physical laws which provide relationships between the Forces acting on a body and the motion of the The term downwash has two meanings within the field of Aerodynamics. Measuring the momentum transferred to the downwash is another way to determine the amount of lift on the airfoil.

Stages of lift production

In attempting to explain why the flow follows the upper surface of the airfoil, the situation gets considerably more complex. To offer a more complete physical picture of lift, consider the case of an airfoil accelerating from rest in a viscous flow. Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress. Lift depends entirely on the nature of viscous flow past certain bodies[7]: in inviscid flow (i. In Fluid dynamics there are problems that are easily solved by using the simplifying assumption of an ideal Fluid that has no Viscosity. e. assuming that viscous forces are negligible in comparison to inertial forces), there is no lift without imposing a net circulation. In Fluid dynamics, circulation is the Line integral around a closed curve of the Fluid Velocity.

When there is no flow, there is no lift and the forces acting on the airfoil are zero. At the instant when the flow is “turned on”, the flow is undeflected downstream of the airfoil and there are two stagnation points on the airfoil (where the flow velocity is zero): one near the leading edge on the bottom surface another on the upper surface near the trailing edge. The stagnation point is a point on the surface of a body submerged in a fluid flow where the fluid Velocity is zero The dividing line between the upper and lower streamtubes mentioned above intersects the body at the stagnation points. Since the flow speed is zero at these points, by Bernoulli's principle the static pressure at these points is at a maximum. In the design and operation of Aircraft, static pressure is the air pressure in the aircraft’s static pressure system. As long as the second stagnation point is at its initial location on the upper surface of the wing, the circulation around the airfoil is zero and, in accordance with the Kutta–Joukowski theorem, there is no lift. In Fluid dynamics, circulation is the Line integral around a closed curve of the Fluid Velocity. The Kutta–Joukowski theorem is a fundamental theorem of Aerodynamics. The net pressure difference between the upper and lower surfaces is zero.

The effects of viscosity are contained within a thin layer of fluid called the boundary layer, close to the body. In Physics and Fluid mechanics, a boundary layer is that layer of Fluid in the immediate vicinity of a bounding surface As flow over the airfoil commences, the flow along the lower surface turns at the sharp trailing edge and flows along the upper surface towards the upper stagnation point. The flow in the vicinity of the sharp trailing edge is very fast and the resulting viscous forces cause the boundary layer to accumulate into a vortex on the upper side of the airfoil between the trailing edge and the upper stagnation point. [8] This is called the starting vortex. The starting vortex is a Vortex which forms in the air adjacent to the trailing edge of an Airfoil as it is accelerated from rest in a fluid The starting vortex and the bound vortex around the surface of the wing are two halves of a closed loop. As the starting vortex increases in strength the bound vortex also strengthens, causing the flow over the upper surface of the airfoil to accelerate and drive the upper stagnation point towards the sharp trailing edge. As this happens, the starting vortex is shed into the wake, [9] and is a necessary condition to produce lift on an airfoil. The starting vortex is a Vortex which forms in the air adjacent to the trailing edge of an Airfoil as it is accelerated from rest in a fluid If the flow were stopped, there would be a corresponding "stopping vortex". [10] Despite being an idealization of the real world, the “vortex system” set up around a wing is both real and observable; the trailing vortex sheet most noticeably rolls up into wing-tip vortices. Wingtip vortices are tubes of circulating air which are left behind by the Wing as it generates lift.

The upper stagnation point continues moving downstream until it is coincident with the sharp trailing edge (a feature of the flow known as the Kutta condition). The Kutta condition is a principle in steady flow Fluid dynamics, especially Aerodynamics, that is applicable to solid bodies which have sharp corners such as the The flow downstream of the airfoil is deflected downward from the free-stream direction and, from the reasoning above in the basic explanation, there is now a net pressure difference between the upper and lower surfaces and an aerodynamic force is generated.

Methods of determining lift

Pressure integration

The force on the wing can be examined in terms of the pressure differences above and below the wing, which can be related to velocity changes by Bernoulli's principle. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface In Fluid dynamics, Bernoulli's principle states that for an Inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in

The total lift force is the integral of vertical pressure forces over the entire wetted surface area of the wing:

L = \oint p\mathbf{n} \cdot\mathbf{k} \; \mathrm{d}A,

where:

The above lift equation neglects the skin friction forces, which typically have a negligible contribution to the lift compared to the pressure forces. Parasitic drag (also called parasite drag) is drag caused by moving a solid object through a fluid By using the streamwise vector i parallel to the freestream in place of k in the integral, we obtain an expression for the pressure drag Dp (which includes induced drag in a 3D wing). Parasitic drag (also called parasite drag) is drag caused by moving a solid object through a fluid In Aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, is a drag force that occurs whenever If we use the spanwise vector j, we obtain the side force Y.

\begin{align} D_p &= \oint p\mathbf{n} \cdot\mathbf{i} \; \mathrm{d}A, \\[1.2ex] Y &= \oint p\mathbf{n} \cdot\mathbf{j} \; \mathrm{d}A. \end{align}

One method for calculating the pressure is Bernoulli's equation, which is the mathematical expression of Bernoulli's principle. In Fluid dynamics, Bernoulli's principle states that for an Inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in This method ignores the effects of viscosity, which can be important in the boundary layer and to predict friction drag, which is the other component of the total drag in addition to Dp. Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress. In Physics and Fluid mechanics, a boundary layer is that layer of Fluid in the immediate vicinity of a bounding surface Parasitic drag (also called parasite drag) is drag caused by moving a solid object through a fluid In Fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a Solid object through a Fluid (a

The Bernoulli principle states that the sum total of energy within a parcel of fluid remains constant as long as no energy is added or removed. It is a statement of the principle of the conservation of energy applied to flowing fluids.

A substantial simplification of this proposes that as other forms of energy changes are inconsequential during the flow of air around a wing and that energy transfer in/out of the air is not significant, then the sum of pressure energy and speed energy for any particular parcel of air must be constant. Consequently, an increase in speed must be accompanied by a decrease in pressure and vice-versa. It should be noted that this is not a causational relationship. Rather, it is a coincidental relationship, whatever causes one must also cause the other as energy can neither be created nor destroyed. It is named for the Dutch-Swiss mathematician and scientist Daniel Bernoulli, though it was previously understood by Leonhard Euler and others. The Netherlands ( Dutch:, ˈnedərlɑnt is the European part of the Kingdom of the Netherlands, which consists of the Netherlands the Netherlands Switzerland (English pronunciation; Schweiz Swiss German: Schwyz or Schwiiz Suisse Svizzera Svizra officially the Swiss Confederation A mathematician is a person whose primary area of study and research is the field of Mathematics. A scientist, in the broadest sense refers to any person that engages in a systematic activity to acquire Knowledge or an individual that engages in such practices Daniel Bernoulli ( Groningen, 29 January 1700 &ndash 27 July 1782 was a Dutch - Swiss Mathematician, who is particularly remembered for his applications

Bernoulli's principle provides an explanation of pressure difference in the absence of air density and temperature variation (a common approximation for low-speed aircraft). If the air density and temperature are the same above and below a wing, a naive application of the ideal gas law requires that the pressure also be the same. The ideal gas law is the Equation of state of a hypothetical Ideal gas, first stated by Benoît Paul Émile Clapeyron in 1834 Bernoulli's principle, by including air velocity, explains this pressure difference. The principle does not, however, specify the air velocity. This must come from another source, e. g. , experimental data. Erroneous assumptions concerning velocity, e. g. , that two parcels of air separated at the front of the wing must meet up again at the back of the wing, are commonly found. [11]

In order to solve for the velocity of inviscid flow around a wing, the Kutta condition must be applied to simulate the effects of inertia and viscosity. The Kutta condition is a principle in steady flow Fluid dynamics, especially Aerodynamics, that is applicable to solid bodies which have sharp corners such as the The Kutta condition allows for the correct choice among an infinite number of flow solutions that otherwise obey the laws of conservation of mass and conservation of momentum. The law of conservation of mass/matter, also known as law of mass/matter conservation (or the Lomonosov - Lavoisier law says that the Mass of In Classical mechanics, momentum ( pl momenta SI unit kg · m/s, or equivalently N · s) is the product

Mathematical approximations

Kutta–Joukowski theorem

Main article: Kutta–Joukowski theorem

Lift can be calculated using potential flow theory by imposing a circulation. The Kutta–Joukowski theorem is a fundamental theorem of Aerodynamics. In Fluid dynamics, a potential flow is a Velocity field which is described as the Gradient of a scalar function the velocity potential In Fluid dynamics, circulation is the Line integral around a closed curve of the Fluid Velocity. It is often used by practicing aerodynamicists as a convenient quantity in calculations, for example thin-airfoil theory and lifting-line theory. An airfoil (in American English) or aerofoil (in British English) is the shape of a Wing or blade (of a Propeller, rotor The horseshoe vortex model is a simplified representation of the Vortex system of a Wing.

The circulation Γ is the line integral of the velocity of the air, in a closed loop around the boundary of an airfoil. In Mathematics, a line integral (sometimes called a path integral or curve integral) is an Integral where the function to be integrated It can be understood as the total amount of "spinning" (or vorticity) of air around the airfoil. Vorticity is a mathematical concept used in Fluid dynamics. It can be related to the amount of " circulation " or "rotation" (or more strictly the The section lift/span L' can be calculated using the Kutta–Joukowski theorem:

L' = − ρVΓ

where ρ is the air density, V is the free-stream airspeed. The Helmholtz theorem states that circulation is conserved; put simply this is conservation of the air's angular momentum. There exist several Theorems named after Hermann von Helmholtz. When an aircraft is at rest, there is no circulation.

The challenge when using the Kutta–Joukowski theorem to determine lift is to determine the appropriate circulation for a particular airfoil. In practice, this is done by applying the Kutta condition, which uniquely prescribes the circulation for a given geometry and free-stream velocity.

A physical understanding of the theorem can be observed in the Magnus effect, which is a lift force generated by a spinning cylinder in a free stream. The Magnus effect is the phenomenon whereby a spinning object flying in a Fluid creates a Whirlpool of fluid around itself and experiences a force perpendicular Here the necessary circulation is induced by the mechanical rotation acting on the boundary layer, causing it to separate at different points between top and bottom. The asymmetric separation then produces a circulation in the outer inviscid flow.

Common misconceptions

Equal transit-time

An illustration of the equal transit-time fallacy.
An illustration of the equal transit-time fallacy.

One misconception encountered in a number of popular explanations of lift is the "equal transit time" fallacy. This fallacy assumes that the parcels of air that are divided above and below an airfoil must rejoin behind it. The fallacy states that because of the longer path of the upper surface of an airfoil, the air going over the top must go faster in order to "catch up" with the air flowing around the bottom. [12] Although it is true that the air moving over the top of a wing generating lift does move faster, there is no requirement for equal transit time. In fact the air moving over the top of an airfoil generating lift is always moving much faster than the equal transit theory would imply. [13]

A further flaw in this explanation is that it requires an airfoil to have thickness and curvature in order to create lift. In fact, thin flat plate wings and sails create lift under a range of angles of attack. If lift were solely a result of shape, then aircraft would not be able to fly inverted.

This explanation has gained currency by repetition in populist (rather than technical) books. At least one common pilot training book depicts the equal transit fallacy, adding to the confusion. [14]

Further information: List of works with the equal transit-time fallacy

Coandă Effect

Main article: Coandă effect

In a limited sense, the Coandă effect refers to the tendency of a fluid jet to stay attached to an adjacent surface that curves away from the flow and the resultant entrainment of ambient air into the flow. This is a partial list of works with the equal transit-time fallacy presented as truth The Coandă effect ('kwandə is the tendency of a Fluid jet to stay attached to an adjacent curved surface that is very well shaped. The effect is named for Henri Coandă, the famous Romanian aerodynamicist who exploited it in many of his patents. Henri Marie Coandă (June 7 1886 &ndash November 25 1972 (IPA /ɐʁi maʁi kwandə/ was a Romanian inventor Aerodynamics pioneer and the builder of world's first The Coandă effect occurs in shear flow e. g. in high-lift devices such as a blown flap. Blown flaps are a powered Aerodynamic High-lift device invented by the British on the Wings of certain Aircraft to improve low-speed

An example of the Coandă effect in this sense tendency of a stream of water to adhere to the back of a spoon, is due primarily to surface tension or the intermolecular attractive Van der Waals forces between the liquid molecules themselves and between the liquid molecules and the molecules of the spoon. For the work of fiction see Surface Tension (short story. Surface tension is a property of the surface of a Liquid that causes it to The Van der Waals equation is an Equation of state that can be derived from a special form of the potential between a pair of molecules (hard-sphere repulsion (Dip the bowl of the spoon in water and notice that some water adheres to the bowl when you remove it. ) The very short-range Van der Waals forces are important in a liquid and not in a gas because in a liquid the molecules are very close to one another and in a gas the molecules only come close to one another when they collide.

More broadly, some consider the effect to include the tendency of any fluid boundary layer to adhere to a curved surface, not just that involving a jet. It is in this broader sense that the Coandă effect is used by some to explain lift. [15] Jef Raskin[16], for example, uses a simple demonstration by using a straw to blow over the upper surface of a wing. The wing deflects upwards, thus supposedly demonstrating that the Coanda effect creates lift. This demonstration correctly demonstrates the Coandă effect as a fluid jet (the exhaust from a straw) adhering to a curved surface (the wing); however, in reality, the bulk flow of air over is very different from this example[4]: for instance, this demonstration makes no consideration of what happening on the lower surface of the wing. In addition, using the effect in this sense of the bulk flow over a wing ascribes more to Coandă credit than he ever considered; the origins of the effect are in his patent for a high-lift device that used a fluid jet impinging on a surface to generate additional lift.

It is interesting to think of the Coandă effect as a way of "managing" buoyancy because the attendant generation of lift is actually an enhancement of the buoyant force. In Physics, buoyancy ( BrE IPA: /ˈbɔɪənsi/ is the upward Force on an object produced by the surrounding liquid or gas in which it is

References

Notes

  1. ^ Benson, Tom (2006-02-14). Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 842 - Charles the Bald and Louis the German swear the Oaths of Strasbourg in the French and German What is Lift?. Retrieved on 2007-02-18. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 3102 BC - Epoch (origin of the Kali Yuga. 1229 - The Sixth Crusade: Frederick II Holy
  2. ^ Crouch, Tom D. (1989). The Bishop's Boys : A Life of Wilbur and Orville Wright. W. W. Norton, pp. 220-226. ISBN 0-393-02660-4.  
  3. ^ aerodave (2005-07-12). Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Events 1191 - Saladin 's garrison surrenders ending the two-year Siege of Acre. How do airplanes fly, really? : A Staff Report by the Straight Dope Science Advisory Board. Chicago Reader, Inc. . Retrieved on 2007-02-18. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 3102 BC - Epoch (origin of the Kali Yuga. 1229 - The Sixth Crusade: Frederick II Holy
  4. ^ a b c Anderson, John D. (2004), Introduction to Flight (5th ed. ), McGraw-Hill, p. 355 
  5. ^ NASA Glenn Research Center, Bernoulli and Newton, <http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html>. Retrieved on 19 April 2008 
  6. ^ Ison, David, “Bernoulli Or Newton: Who's Right About Lift?”, Plane & Pilot, <http://www.planeandpilotmag.com/aircraft/specifications/diamond/2007-diamond-star-da40-xl/289.html>. Retrieved on 21 April 2008 
  7. ^ Karamacheti, Krishnamurty (1980), Principles of Ideal-Fluid Aerodynamics (Reprint ed. ) 
  8. ^ Clancy, L. J. , Aerodynamics, Figure 4. 7
  9. ^ Clancy, L. J. , Aerodynamics, Figure 4. 8
  10. ^ White, Frank M. (2002), "Fluid Mechanics" (5th ed. ), McGraw Hill 
  11. ^ Aerodynamic Forces
  12. ^ Anderson, David (2001). Understanding Flight. New York: McGraw-Hill. ISBN 0071363777.  “The first thing that is wrong is that the principle of equal transit times is not true for a wing with lift. ” 
  13. ^ Glenn Research Center (2006-03-15). Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 44 BC - Julius Caesar, Dictator of the Roman Republic, is stabbed to death by Marcus Junius Brutus, Incorrect Lift Theory. NASA. Retrieved on 2008-03-27. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 196 BC - Ptolemy V ascends to the throne of Egypt. 1309 - Pope Clement V excommunicates
  14. ^ Kershner, William K. (1979). The Student Pilot's Flight Manual, 5th ed. . ISBN 0-8138-1610-6.  
  15. ^ Anderson, David & Eberhart, Scott (1999), How Airplanes Fly: A Physical Description of Lift, <http://www.allstar.fiu.edu/AERO/airflylvl3.htm>. Retrieved on 4 June 2008 
  16. ^ Raskin, Jef (1994), Coanda Effect: Understanding Why Wings Work, <http://jef.raskincenter.org/published/coanda_effect.html> 

See also

Further reading

External links

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