Lift+on+an+Airplane+Wing

=Misconception: =

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The force of lift on an airplane wing is generated due to the curved shape of the top of the wing. A diagram is oft presented of a 2D slice of a wing, with flow lines of the surrounding air. The air that gets divided at the leading edge must rejoin at the trailing edge. The top of the wing is curved, and the bottom is flat, so the air molecules travelling over the top of the wing must travel faster in order to rejoin their counterparts. According to the [|Bernoulli principle], the faster moving particles will create less pressure on the top of the wing. With more pressure under the wing than on top, a lifting force is created. =====



=Source: =

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This idea has been perpetuated in lecture, on web-sites, and even in textbooks. It is often referred to as the "Equal Transit" theory or "Longer Path" theory. The misconception arises from the fact that 2D airfoils are presented to teach us about pressure and flow, and then we apply the principles to 3D wings. =====

=Discussion: =

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The largest contributor to the misrepresentation of this effect (even in 2D) is the idea that the particles have to "stick together" and "meet up" at the trailing edge of the wing. This is non-physical assumption that is simply incorrect. You can calculate values for the velocity of particles above and below the airfoil, based on this assumption, and then use Bernoulli's equation to compute a pressure. You will get a lift force, but it does not reflect reality. As you can see below, the true velocity over the top of the wing is greater than that predicted by the "Equal Transit" theory, and particles travelling over the top of the airfoil arrive sooner than those travelling along the bottom. =====



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2) Planes can fly upside down. If the 2D airfoil model were inverted, there would be a net force down on the wing, indicating that there is no potential for inverted flight. =====

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3) There are low-drag airfoils that have a longer path length along the bottom of the wing than on the top, and they still get off the ground. =====

=Resolution: =

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Lift is a force, and it is generated by changing either speed or direction of a flow. Any force applied will generate a change in velocity, and conversely any change in velocity generates a force. The airplane wing in 3 dimensions is far more complicated than the 2D infinite wing model. The //real life// wing generates lift due to flow turning, and the [|Coanda] effect. There is a tendency for fluids to"stick" to the surface over which they are flowing (think of running a spoon under tap water). Wings fly with varying angles of attack, and deflect the air, causing a change in velocity, resulting in a force. Pointing a plane upward will tend to move the plane upward, regardless of wing orientation. =====

=Further Reading: = @http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html @http://xkcd.com/803/ @http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html @http://web.archive.org/web/20010607215323/physics.about.com/science/physics/library/weekly/aa041401a.htm []