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The plane truth: Scientist reveals how wings really work


The Independent, By Steve Connor, January 25

The classic explanation of how a wing generates enough lift to keep a plane or a bird in the air is wrong, according to a Cambridge University physicist.

Many textbooks and aircraft manuals say that a wing produces lift because air travelling over the curved topside of a wing has to travel further than wind flowing around the flat underside and so has to travel faster to keep up, generating lift. But this standard explanation has been shown to be wrong by a simple experiment where a wing is placed in an air tunnel with jets of smoke flowing over the upper and lower surfaces, said Professor Holger Babinsky.

"A wing lifts when the air pressure above it is lowered. It's often said this happens because the airflow moving over the top, curved surface has a longer distance to travel and needs to go faster to have the same transit time as the air travelling along the lower, flat surface," he said.

"But this is wrong," said Professor Babinsky. "In the worst case, [this explanation] can lead to a fundamental misunderstanding of aerodynamics.".

A video of the experiment carried out at Cambridge shows that air does indeed move faster over the upper, curved surface of a wing, but this is because of the curvature of the upper surface. The air does not move faster in order to "catch up" with the air moving over the relatively shorter distance of the lower wing surface, Professor Babinsky said.

The slow-motion video of the smoke jets shows that the air moving over the upper surface of the wing travels further in the same amount of time than wind flowing past the lower surface. In other words there is no "equal transit time" between the upper and lower surfaces, as demanded by the classic explanation.


Raja January 29, 2012 - 12:05pm
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Sounds like the Prof

has been misinterpreting what he reads or it was badly written in the first place. I suspect he is just setting up a straw man so he can score points for demolishing it.

Wind tunnels and smoke have been used for decades and the mechanism of aerodynamic lift is well understood. I don't currently fly anything beyond my hang-glider but I have studied aerodynamics.

Two air molecules, one above and one below may indeed have 'equal transit time', more or less. However, transit time per se is immaterial and I have never seen it stated to be the cause of lift. (Note the separation of the upper airflow at the trailing edge. Ever see a little up-thrusting winglet at the tips of a wing? They counteract the turbulence and reduce the airflow separation at the trailing edge, both increasing lift and reducing drag).

What is stated in everything I've read is that the upper airflow is faster than the lower airflow and the faster airflow exerts less pressure (see: Bernoulli Effect).
Less pressure above = more pressure below = upward force: Lift.

The whole idea of "air trying to catch up" (aside from attributing intent to the air - Can you spell ANTHROPOMORPHIZE?) implies some inherent connection between the air over the wing and the air under the wind. This is implication is meaningless.

The upper airflow is faster because of the curved upper surface, lift can be generated (in a wind tunnel) with a flat wing.
Imagine a wing with a uniform thickness, without the normal curve.
Imagine a flat horizontal 'separator' sheet in the wind tunnel, extending from the air vents to the leading edge of the wing, completely separating the upper and lower air flows - no 'air trying (anthropomorphically) to catch up.
Imagine the upper and lower air vents driven by separate fan systems.
If the airflow from the vents above the sheet were faster than the airflow from the vents below the 'separator' sheet, there would be lift on the wing.


It is worth remembering that the Founding Fathers were all traitors.

steeleweed January 29, 2012 - 2:22pm
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It does seem like he's been reading different materials

than you have. It seems he's also (weakly) speaking against anthropomorphism - as well as against the idea that air particles above and below the wing "enter" and "exit" the wing area at the same time (have the same transit time).

"A wing lifts when the air pressure above it is lowered. It's often said this happens because the airflow moving over the top, curved surface has a longer distance to travel and needs to go faster to have the same transit time as the air traveling along the lower, flat surface," he said.

"But this is wrong," said Professor [of Aerodynamics] Babinsky. "In the worst case, [this explanation] can lead to a fundamental misunderstanding of aerodynamics.".

Raja January 29, 2012 - 3:18pm
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