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An Engineer in Wonderland – How wings really lift

wingswork.jpgFrom the University of Cambridge: A one minute video sets the record straight on a much misunderstood concept.

The University said: 

It’s one of the most tenacious myths in physics and it frustrates aerodynamicists the world over.

Now, University of Cambridge’s Professor Holger Babinsky has created a 1-minute video that he hopes will finally lay to rest a commonly used yet misleading explanation of how wings lift.

“A wing lifts when the air pressure above it is lowered. It’s often said that 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. But this is wrong,” he explained. “I don’t know when the explanation first surfaced but it’s been around for decades. You find it taught in textbooks, explained on television and even described in aircraft manuals for pilots. In the worst case, it can lead to a fundamental misunderstanding of some of the most important principles of aerodynamics.”

To show that this common explanation is wrong, Babinsky filmed pulses of smoke flowing around an aerofoil (the shape of a wing in cross-section). When the video is paused, it’s clear that the transit times above and below the wing are not equal: the air moves faster over the top surface and has already gone past the end of the wing by the time the flow below the aerofoil reaches the end of the lower surface.

“What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes – lower pressure on the upper surface and higher pressure on the lower surface,” clarified Babinsky, from the Department of Engineering. “This is why a flat surface like a sail is able to cause lift – here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it’s the curvature that creates lift, not the distance.”

Babinsky is quick to stress that he is far from the only aerodynamicist who is frustrated by the perpetuation of the myth: colleagues have in the past expressed their concerns in print and online. Where he hopes his video will help debunk the myth once and for all is by providing a quick and visual demonstration to show that the most commonly used explanation cannot possibly be correct. The original video, created by Babinsky a few years ago using a wind tunnel, has now been re-edited in high quality with a voice-over in which he explains the phenomenon as it happens.

Babinsky’s research focuses on the fundamental aspects of aerodynamics as they relate to aircraft wings, Formula I racing cars, articulated lorries and wind turbines. One of his visions is to design a wing that will enable aircraft to fly faster and more efficiently. Using a massive wind tunnel within the Department of Engineering, Babinsky and his team have been modelling the shockwaves that are created on aircraft wings and that restrict the plane’s top speed.

The newly released video will support lectures Babinsky will be giving as part of a series of University of Cambridge Subject Masterclasses aimed at Year 12 school children: “It’s important to put out this video because when I give this lecture to school kids I start by giving the wrong explanation and asking who has heard it and every time 95% of the audience puts their hand up. Only a handful will know that it is wrong.”

There is a series of University of Cambridge master class videos aimed at Year 12 school children:

‘Alice’

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Tags: airflow, cross section, holger, transit time, university of cambridge

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8 Comments

  1. Chris Morriss
    March 15, 2012 11:55

    The situation when having a flexible moving band around the wing is very interesting aerodynamically. I guess you’ve heard of the ‘Flettner Effect’?
    See http://www.grc.nasa.gov/WWW/k-12/airplane/cyl.html
    for more information.

  2. 'Alice'
    March 01, 2012 16:04

    For those who are interested, this article by the same Professor has been suggested as a fuller description, which answers many of the points raise above.
    Babinsky, H., “How Do Wings Work?”, Physics Education, Vol. 38, pp. 497-503, Nov. 2003
    It is available here:
    http://iopscience.iop.org/0031-9120/38/6/001/pdf/pe3_6_001.pdf
    ‘Alice’

  3. 'Alice'
    March 01, 2012 15:47

    Thanks Ted.
    Your comment has been sent to Professor Babinsky.
    Fingers crossed for a reply.
    ‘Alice’

  4. Ted
    March 01, 2012 15:44

    The video by Prof. Babinski is undoubtedly correct. In fact I have seen this “refutation” more often than I have seen the theory he refutes. The fact that the air is going faster than one might expect does not show that it is curvature rather than speed which causes lift; if fact, it supports the basic Bernoulli theory.
    Despite the general acceptance of the Bernoulli Principle on a molecular level lift is still not fully understood.
    Our group has preformed extensive wind tunnel tests on a wing section which itself was covered with a moving surface-a flexible band that moved over and under the wing section.
    Contrary to our expectations when the movement was such that the upper surface moved against the airstream the lift was destroyed and a downward pressure was created.
    The Bernoulli effect had been destroyed. Changing the direction of the movement greatly increased the lift, up to a multiple of nine, but dependant of both the speed of the band and the airspeed. The very large llift increase is partially explained by the fact that Bernoulli says that the air rushing under the wing produces a downward pressure, to a large extent canceling the lift from the upper surface.
    This effect is not great on the video because of the very great angle of attack used.
    Our experiments do not fully explain lift; rather I think they add to the mystery. If you or Prof. Babinski would like to see tables of lift and drag please advise.
    Ted

  5. 'Alice'
    March 01, 2012 15:43

    Hi Helen.
    Now I am stuck with the image of planes flying through jelly….
    I agree with published stuff becoming ‘fact’ in a rather irritating way.
    Have you read Ben Goldacre’s book Bad Science?
    It starts off with some great examples of folk writing books about rubbish and establishing huge groups of followers – and sometimes profitable businesses as well.
    It does go on to be a bit depressing, when he exposes similar behaviour amongst pharmaceutical companies.
    Overall, it is a great book which taught me a lot about the placebo effect and how to control for it in research – recommended.
    ‘Alice’

  6. Helen
    February 16, 2012 09:40

    Hi Alice
    My husband who knows I have a non-technical interest in things like this sent it to me.
    I’m afraid I agree with a lot of the posts on YouTube. I’m now in my late 50s and certainly remember being taught about the differential airflow.
    Speed was always mentioned, but as a generator of the pressure difference giving the actual lifting factor. Only in recent years have I seen mentioned the question of actual pressure on the “approaching” side of the wing, the bit hitting the substantial air first, as Steve K mentions. The video is great but probably won’t stop people trying to explain the effect as the two airstreams arriving at the “end of the wing” at the same time, as I used to work in publication research and know that once an idea gets into print, it tends to stay there indefinitely. I’d like to see more on the components of the pressure difference, perhaps related via the old question, “how does the plane fly upside down?”
    I’ve just hit my poor husband with “Who told the air that the two steams had to arrive at the same time?” Apparently this idea was based on the idea that air is static (like jelly?) … at least, that is how it was explained to him 40 years ago at school. You ask two kids to run round a supermarket, one takes a longer aisle, one takes a shorter one, they are not going to arrive back at the same time except by luck.
    Thanks for the article.
    Helen

  7. 'Alice'
    February 09, 2012 15:44

    Thank you Mr K.
    As a point of interest (for me, at least) I was at a lecture where a guy handed around a historic bar of teflon, one of the first made.
    And on the subject of handing stuff around:
    When I was at school, a lecturer from the local university came in and handed around a bar of depleated uranium.
    - no box, no gloves, just a bar of uranium!
    One of his students had found it on a rubbish tip (heaven knows how it got there), where he identifying it from its pyrophoric behavour.
    I loved chemistry lessons at school.
    The school chemistry storeroom once got an emergency visit from safety guys after an audit discovered there was way too much phosphorous in one place.
    ‘Alice’

  8. Steve Kurt
    January 29, 2012 17:17

    That’s an excellent video! The argument that lift was generated because the air on top had to go faster always seemed suspect… especially when it was illustrated with a wing with zero angle of attack! Once I started working around aircraft and learned about angle of attack, it seemed more likely that lift was generated by the difference in pressure, and that pressure was generated in part by forcing a lot of air downwards by the lower surface of the wing, and by low pressure area on top of the wing.
    Unfortunately, it’s extremely difficult to stop this sort of misinformation. As an example, I recently heard a story on NPR (public radio in the USA) say that teflon was a spin-off from the space program! The fact that it was discovered in 1938 wasn’t worth researching, apparently.
    regards,
    Steve K.

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