The factual accuracy of this article is disputed.
Please see the relevant discussion on the talk page. Lift consists of the sum of all the fluid dynamic forces on a body perpendicular to the direction of the external flow approaching that body. Image File history File links Circle-question. ...
Fluid dynamics is the subdiscipline of fluid mechanics that studies fluids (liquids and gases) in motion. ...
In physics, a force is anything that causes a free body with mass to accelerate. ...
Perpendicular is a geometric term that may be used as a noun or adjective. ...
The most straightforward and frequently-mentioned application of lift is the wing of an aircraft. However there are many other common, if less obvious, uses such as propellers on both aircraft and boats, rotors on helicopters, fan blades, sails on sailboats and even some kinds of wind turbines. A Laughing Gull on the beach in Atlantic City. ...
An Air France Boeing 777, a modern passenger jet. ...
To meet Wikipedias quality standards, this article or section may require cleanup. ...
Airbus A380 An aircraft is any machine capable of atmospheric flight. ...
Some boats in a harbor in Miami Beach, Florida A boat is a watercraft, usually smaller than most ships. ...
A helicopter twin-bladed main rotor, mounted on a pylon (Robinson R44) A rotor is a rotating part of a machine, as in a motor, a pump or a helicopter. ...
A helicopter is an aircraft which is lifted and propelled by one or more large horizontal rotors (propellers). ...
Non-electric fan Household Electric Fan Fans have had several purposes, the most common being to move air for creature comfort or for ventilation and to move air or gas from one location to another for industrial purposes. ...
A sail is any type of surface intended to generate thrust by being placed in a wind —in essence a vertically-oriented wing. ...
A sailboat crew narrowly avoids capsizing. ...
Horizontal axis wind turbine, the Enercon model E-66 wind energy converter, in Germany. ...
There are a number of ways of explaining the production of lift, all of which are equivalent. That is, they are different expressions of the same underlying physical principles.
 Forces on an aircraft wing Image File history File links Lift-force-en. ...
Image File history File links Lift-force-en. ...
Reaction due to accelerated air
In air (or comparably in any fluid), lift is created as flow interacts with an airfoil or other body and is deflected downward. The force created by this deflection of the air creates an equal and opposite upward force according to Newton's third law of motion. The deflection of airflow downward during the creation of lift is known as downwash. A subset of the phases of matter, fluids include liquids, gases, plasmas and, to some extent, plastic solids. ...
An airfoil (in American English, or aerofoil in British English) is the shape of a wing or blade (of a propeller or ships screw or sail) as seen in cross-section. ...
Newtons Laws of Motion are laws which provide relationships between the forces acting on a body and the motion of the body, first formulated by Isaac Newton. ...
The term downwash has two nearly unrelated meanings within the field of aerodynamics. ...
It is important to note that the acceleration of the air does not just involve the air molecules "bouncing off" the lower surface of the wing. Rather, air molecules closely follow both the top and bottom surfaces, and so the airflow is deflected downward. The acceleration of the air during the creation of lift has also been described as a "turning" of the airflow.
Many shapes, such as a flat plate set at an angle to the flow, will produce lift. However, lift generation by most shapes will be very inefficient and create a great deal of drag. One of the primary goals of wing design is to devise a shape that produces the most lift while producing the least Form drag. An object falling through a gas or liquid experiences a force in direction opposite to its motion. ...
A Laughing Gull on the beach in Atlantic City. ...
In aerodynamics, form drag, profile drag, or pressure drag, is a component of parasitic drag. ...
It is possible to measure lift using the reaction model. The force acting on the wing is the negative of the time-rate-of-change of the momentum of the air. In a wind tunnel, the speed and direction of the air can be measured (using, for example, a Pitot tube or Laser Doppler velocimetry) and the lift calculated. Alternately, the force on the wind tunnel itself can be measured as the equal and opposite forces to those acting on the test body. In classical mechanics, momentum (pl. ...
A Pitot tube is a measuring instrument used to measure fluid flow. ...
Laser Doppler velocimetry (LDV, also known as laser Doppler anemometry, or LDA) is a technique for measuring the direction and speed of fluids like air and water. ...
Bernoulli's principle The force on the wing can also be examined in terms of the pressure differences above and below the wing. Pressure (symbol: p) is the force per unit area applied on a surface in a direction perpendicular to that surface. ...
The total force (Lift + Drag) is the integral of pressure over the contour of the wing. In calculus, the integral of a function is a generalization of area, mass, volume and total. ...
where:
- L is the Lift,
- D is the Drag,
- is the frontier of the domain,
- p is the value of the pressure,
- n is the normal to the profile.
Since it is a two-dimensional vector equation, and since lift is perpendicular to drag, this equation suffices to predict both lift and drag. The drag component is Lift-induced drag rather than Form drag. This equation is always exactly true, by the definition of force and pressure. In physics and in vector calculus, a spatial vector is a concept characterized by a magnitude, which is a scalar, and a direction (which can be defined in a 3-dimensional space by the Euler angles). ...
In aerodynamics, lift-induced drag, or induced drag, is a drag force which occurs whenever a lifting body or a wing of finite span generates lift. ...
In aerodynamics, form drag, profile drag, or pressure drag, is a component of parasitic drag. ...
One method for calculating the pressure is Bernoulli's equation, which is the mathematical expression of Bernoulli's principle. This method ignores the effects of viscosity, which can be important in the boundary layer and to predict drag, though it has only a small effect on lift calculations. In fluid dynamics, Bernoullis equation, derived by Daniel Bernoulli, describes the behavior of a fluid moving along a streamline. ...
The pitch drop experiment at the University of Queensland. ...
In physics and fluid mechanics, the boundary layer is that layer of fluid in the immediate vicinity of a bounding surface. ...
Bernoulli's principle states that in fluid flow, an increase in velocity occurs simultaneously with decrease in pressure. It is named for the Dutch/Swiss mathematician/scientist Daniel Bernoulli, though it was previously understood by Leonhard Euler and others. In a fluid flow with no viscosity, and therefore one in which a pressure difference is the only accelerating force, it is equivalent to Newton's laws of motion. A subset of the phases of matter, fluids include liquids, gases, plasmas and, to some extent, plastic solids. ...
The velocity of an object is simply its speed in a particular direction. ...
Pressure (symbol: p) is the force per unit area applied on a surface in a direction perpendicular to that surface. ...
Leonhard Euler is considered by many people to be one of the greatest mathematicians of all time A mathematician is a person whose primary area of study and research is mathematics. ...
The physicist Albert Einstein is probably historys most widely recognized scientist. ...
Daniel Bernoulli Daniel Bernoulli (Groningen, February 8, 1700 – Basel, March 17, 1782) was a Dutch-born mathematician who spent much of his life in Basel, Switzerland. ...
Leonhard Euler by Emanuel Handmann. ...
The pitch drop experiment at the University of Queensland. ...
Newtons Laws of Motion are laws which provide relationships between the forces acting on a body and the motion of the body, first formulated by Isaac Newton. ...
Bernoulli's principle also describes the venturi effect that is used in carburetors and elsewhere. In a carburetor, air is passed through a Venturi tube in order to decrease its pressure. This happens because the air velocity has to increase as it flows through the constriction. The Venturi effect is a special case of Bernoullis principle, in the case of fluid or air flow through a tube or pipe with a constriction in it. ...
Stromberg side-draft carburetor The carburetor, carburettor, or carburetter (see spelling differences), also called carb (in North America) or carbie (chiefly in Australia) for short, is a device that mixes air and fuel for an internal-combustion engine. ...
A fluid passing through smoothly varying constrictions experience changes in velocity and pressure, as described by Bernoullis principle. ...
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 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 Kutta condition is a principle in fluid dynamics, especially aerodynamics, applied at sharp corners such as trailing edges of airfoils in steady flow. ...
The law of conservation of mass/matter (The Lomonosov-Lavoisier law) states that the mass of a closed system of substances will remain constant, regardless of the processes acting inside the system. ...
In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves. ...
Some lay versions of this explanation use false information due to lack of understanding the Kutta condition, such as the incorrect assumption that the two parcels of air which separate at the leading edge of a wing must meet again at the trailing edge. There is no reason that the upper parcel of air should speed up to synchronize with the lower parcel. In fact, the requirement for circulation (see below) in order to generate non-zero lift specifies that parcels must never meet.
Circulation A third way to calculate lift is a mathematical construction called circulation or "lifting-line theory". Again, it is mathematically equivalent to the two explanations above. It is often used by practising aerodynamicists as a convenient quantity, but is not often useful for a layperson's understanding. (That said, the vortex system set up round a wing is both real and observable, and is one of the reasons that a light aircraft cannot take off immediately after a jumbo jet!) In fluid dynamics, circulation is the path integral around a closed curve of the fluid velocity. ...
The circulation is the line integral of the velocity of the air, in a closed loop around the boundary of an airfoil. It can be understood as the total amount of "spinning" (or vorticity) of air around the airfoil. When the circulation is known, the section lift can be calculated using: This article is about path integrals in the general mathematical sense, and not the path integral formulation of physics which was studied by Richard Feynman. ...
Vorticity is a mathematical concept used in fluid dynamics. ...
where ρ is the air density, V is the free-stream airspeed, and Γ is the circulation.
A similar equation applies to the sideways force generated around a spinning object, the Magnus effect, though here the necessary circulation is induced by the mechanical rotation, rather than aerofoil action. The Magnus effect is the name given to the physical phenomenon whereby an objects rotation affects its path through a fluid, in particular, air. ...
The Helmholtz theorem states that circulation is conserved; put simply this is conservation of the air's angular momentum. When an aircraft is at rest, there is no circulation. As the flow speed increases (that is, the aircraft accelerates in the air-body-fixed frame), a vortex, called the starting vortex, forms at the trailing edge of the airfoil, due to viscous effects in the boundary layer. Eventually the vortex detaches from the airfoil and gets swept away from it rearward. The circulation in the starting vortex is equal in magnitude and opposite in direction to the circulation around the airfoil. Theoretically, the starting vortex remains connected to the vortex bound in the airfoil, through the wing-tip vortices, forming a closed circuit. In reality, the starting vortex is dissipated by a number of effects, as are the wing-tip vortices far behind the aircraft. However, the net circulation in "the world" is still zero as the circulation from the vortices is transferred to the surroundings as they dissipate. It has been suggested that this article or section be merged with :Fundamental theorem of vector analysis. ...
In physics and fluid mechanics, the boundary layer is that layer of fluid in the immediate vicinity of a bounding surface. ...
It has been suggested that this article or section be merged with Wake turbulence. ...
Coefficient of lift Aerodynamicists are among the most frequent users of dimensionless numbers. The coefficient of lift is one such term. When the coefficient of lift is known, for instance from tables of airfoil data, lift can be calculated using the Lift Equation: In dimensional analysis, a dimensionless number (or more precisely, a number with the dimensions of 1) is a pure number without any physical units. ...
The coefficient of lift is a number associated with a particular shape of an aerofoil, and is incorporated in the lift equation to predict the lift force generated by a wing using this particular cross section. ...
where: - CL is the coefficient of lift
- ρ is the density of air (1.225 kg/m3 at sea level)*
- V is the freestream velocity, that is the airspeed far from the lifting surface
- A is the surface area of the lifting surface
- L is the lift force produced
This equation can be used in any consistent system. For instance, if the density is measured in kilograms per cubic metre, the velocity is measured in metres per second, and the area is measured in square metres, the lift will be calculated in newtons. Or, if the density is in slugs per cubic foot, the velocity is in feet per second, and the area is in square feet, the resulting lift will be in pounds force. The coefficient of lift is a number associated with a particular shape of an aerofoil, and is incorporated in the lift equation to predict the lift force generated by a wing using this particular cross section. ...
In physics, a force is anything that causes a free body with mass to accelerate. ...
The international prototype, made of platinum-iridium, which is kept at the BIPM under conditions specified by the 1st CGPM in 1889. ...
metre or meter, see meter (disambiguation) The metre (in the U.S., chiefly meter) is a measure of length, approximately equal to 3. ...
In physics, the newton (symbol: N) is the SI unit of force, named after Sir Isaac Newton in recognition of his work on classical mechanics. ...
For other meanings, see Slug (disambiguation) The slug is an English and U.S. customary unit of mass. ...
This article is about a foot as a unit of length. ...
* Note that at altitudes other than sea level, the density can be found using the barometric formula The Barometric Formula,sometimes called the exponential atmosphere, is a formula used to model how the pressure (or density) of the air changes with altitude. ...
Compare with: Drag equation. In physics, the drag equation gives the drag experienced by an object moving through a fluid. ...
Common misconceptions
Equal transit-time One misconception encountered in a number of explanations of lift is the "equal transit time" fallacy. This fallacy states that the parcels of air which are divided by an airfoil must rejoin again; because of the greater curvature (and hence 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.
Although it is true that the air moving over the top of the wing is moving faster (when the effective angle of attack is positive) there is no requirement for equal transit time. In fact if the air above and below an airfoil has equal transit time, there is no lift produced at all. Only if the air above has a lower transit time, lift is produced (as well as a downward deflection of the air and a vortex). There are wind tunnel smoke streamline pictures available. [1][2]
Such an explanation would predict that an aircraft could not fly inverted, which is demonstrably not the case. When an aircraft is flying inverted, the air moving over the bottom (in the aircraft reference frame) surface of the airfoil is moving faster. The explanation also fails to account for airfoils which are fully symmetrical yet still develop significant lift.
A further flaw in this explanation is that it requires an airfoil to have a curvature in order to create lift. In fact, a thin, flat plate inclined to a flow of fluid will also generate lift[3].
It is unclear why this explanation has gained such currency, except by repetition by authors of populist (rather than rigorously scientific) books and perhaps the fact that the explanation is easiest to grasp intuitively without mathematics.
Albert Einstein, in attempting to design a practical aircraft based on this principle, came up with an airfoil section that featured a large hump on its upper surface, on the basis that an even longer path must aid lift if the principle is true. Its performance was terrible. Albert Einstein ( ) (March 14, 1879 – April 18, 1955) was a German-Jewish theoretical physicist widely regarded as the most important scientist of the 20th century and one of the greatest physicists of all time. ...
Coanda effect Jef Raskin and a few others have claimed that the Coandă effect is needed to explain lift from an airfoil. They state that flow attachment and the "turning of the airflow" above the airfoil is caused at the microscopic level by the Coandă Effect, and, without this phenomenon, a perpetual stall would exist. However, the conventional explanation of lift makes verifiable predictions of lift using the lift equation, without invoking the Coandă Effect. Proponents of the Coandă Effect correctly claim that the effect is not fully understood, but currently their predictions are at variance with experiment. The practical applications of Coandă effect, such as blown flaps and other lift augmentation devices, create conditions different from the normal airflow over a wing. Jef Raskin outdoors, photographed by his son Aza Raskin. ...
The Coanda effect is the tendency of a stream of fluid to stay attached to a convex surface, rather than follow a straight line in its original direction. ...
In physics and fluid mechanics, the boundary layer is that layer of fluid in the immediate vicinity of a bounding surface. ...
In aerodynamics, a stall is a condition in which an excessive angle of attack causes loss of lift due to disruption of airflow. ...
Blown flaps are an aerodynamic device used on the wings of aircraft to improve low-speed lift and take-off characteristics. ...
This construction will lift the plane up into the air long before a wing not incorporating blown flaps will, as mentioned earlier. Using Newton's laws, most likely forms the theory behind the work of Henri Coandǎ. Newton stated that for every forceful action there will be an equal and opposite reaction; in this case, the use of the engines to thrust the aircraft down the runway and the use of Coandǎ's thesis behind the use of blown flaps diverts the air towards the ground and the reaction force is the lift of the aircraft (as mentioned earlier in this article).
Venturi nozzle Many web sites claim that an airfoil can be analyzed as a Venturi nozzle. The mass flow rate through a Venturi nozzle is constant, so the air must flow faster over the top of the wing. Therefore, there is a lower pressure over the top of the wing, producing lift. However, a Venturi nozzle requires that air is squeezed between surfaces. The top of a wing is only one surface, and the air is not confined above the wing. A wing is therefore not a Venturi nozzle, and thus it is incorrect to analyze it as such. To meet Wikipedias quality standards, this article or section may require cleanup. ...
Further reading Introduction to Flight, John D. Anderson, Jr., McGraw-Hill, ISBN 0072990716. The author is the Curator of Aerodynamics at the National Air & Space Museum Smithsonian Institute and Professor Emeritus at the University of Maryland.
Understanding Flight, by David Anderson and Scott Eberhardt, McGraw-Hill, ISBN 0071363777. The authors are a physicist and an aeronautical engineer. They explain flight in non-technical terms and specifically address the equal-transit-time myth.
Fundamentals of Flight, Richard S. Shevell, Prentice-Hall International Editions, ISBN 0133329178. This book is primarily intended as a text for a one semester undergraduate course in mechanical or aeronautical engineering, although its sections on theory of flight are understandable with a passing knowledge of calculus and physics.
External links |
Feeds |
Contact