Wednesday, January 25, 2012

new Scientist TV - Physics in a Minute: How wings really create lift

How does air flow across a wing to generate lift? Since a wing's top surface is curved, it covers a greater distance compared to the flatter bottom edge. A common explanation is that air moves faster over the top so that it reaches the end of the wing at the same time as the bottom flow, lowering the pressure on the top surface.
But this pressure explanation is just a myth, explains Holger Babinsky, professor of aerodynamics at the University of Cambridge.  In an attempt to debunk the misconception, he filmed pulses of smoke flowing around an aerofoil. He pauses the motion in the video to show that the transit times over the top and bottom are not equal. The lines of smoke from the top have already surpassed the bottom ones once they reach the wing's tip.
Babinsky explains that, although lift is caused by a pressure change between the top and bottom surfaces, it's due to the change in the shape of the air flow, rather than its speed. "This is why a flat surface like a sail is able to cause lift," he says. "In this case, the distance on each side is the same but it is slightly curved when it's rigged, acting like an aerofoil."
If you enjoyed this video, find out how wind can take down a bridge or check out our archive of One-Minute Physics episodes.

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