Two myths I learned in college: bathtub drains and airplane wings

Here are two things I was taught in college that were absolutely wrong.

  • The Coriolis effect explains why bathtubs drain clockwise in the northern hemisphere.
  • The Bernoulli effect explains how planes fly.

These are not things that scientists believed at the time but were later disproved. They’re simply myths. (No doubt I’ve passed on a few myths to students over the years. If I can think of any, I’ll retract them here.)

The Coriolis effect does explain why cyclones rotate one way in the northern hemisphere and the opposite way in the southern hemisphere. The rotation of the earth influences the rotation of large bodies of fluid, like weather systems. However, a bathtub would need to be maybe a thousand miles in diameter before the Coriolis effect would determine how it drains. Bathtubs drain clockwise and counterclockwise in both hemispheres. Random forces such as sound in the air have more influence than the Coriolis effect on such small bodies of water.

The explanation  I learned in college for how airplanes fly involves the Bernoulli principle. The shape of a wing is such that air has to travel further over the top of the wing than under the bottom of the wing.

Since air particles going across the top and bottom of the wing arrive at the trailing edge at the same time, the particles going over the top travel further and so are spread further apart. This results in lower pressure over the top of the wing and lifts the airplane.

There are a couple problems with this explanation. When the air particles split at the leading edge, why should the ones that go over the top and the ones that go under the bottom arrive at the trailing edge at the same time? In fact, they don’t. Also, while the Bernoulli effect does explain part of the lift on an airplane wing, the effect is an order of magnitude too small to account for why airplanes fly.

If the Bernoulli principle explains lift, how can an airplane fly upside-down? Wouldn’t the Bernoulli effect suck the plane down rather than lifting it up when you turn the wing over?

Here’s a  set of recordings from a lecture that debunks the Bernoulli effect myth and explains the real reason airplanes fly. Why airplanes fly: a modern myth. Part I, Part II.  There will be a Part III that hasn’t been released yet.

Update: Apparently these links have gone away.

16 thoughts on “Two myths I learned in college: bathtub drains and airplane wings

  1. I think looking at science myth propogation and debunking from a social psychological perspective is fascinating. I find that it explains a lot, especially why all the best factoids are myths and why debunking is popular and fraught with myth, too.

  2. On Coriolis: I have a good college buddy who is let-handed, and one day at the coffee shop he argued that I ought to stir my coffer counter-clockwise (left-handed fashion) in order to “go with the flow”. He was making a joke of course, but to this day I think of that every time I stir my coffee.

    On Bernoulli: It’s the magic feathers, of course!

  3. John, thanks for clearing this up for me. I believed in the first myth. And I majored in Physics.

  4. OK – here’s one. Do you know how a train stays on the tracks? I heard this one from Richard Feynman.

  5. My father, a pilot in the US AirForce, told me the same story about wing lift. He was (I suppose) taught that idea 50 years ago in his military training. Those B52 bombers are still flying even if the pilots aren’t sure why they fly!

  6. They still force pilots to wade through that myth before they give them a license. I remember reading it when I was a kid and dreaming of flying and thinking to myself that it’s a bunch of crap. The difference in pressure *is* what gives a plane lift, but the bulk (99.999% +) of that difference is caused by the force of air pushing on the bottom of the wing (the vertical drag) created by the forward thrust and the angle of attack.

    I’ve recently had to go through the same claptrap while taking sailing lessons. People really believe the shape of the sail causes a vaccum that sucks them forward through the water.

    This sort of thinking makes me worry about the future of science. If the practical engineers who built planes and boats didn’t disregard what the scientists are teaching them, we’d have no practical inventions and people would be making incantations and eating potions to try to stay healthy and save us from evil spirits. Reminds me of the science fiction story Nightfall by Isaac Asimov.

    Another bogus lesson they teach in college is the old Galileo chestnut. Try dropping a grape and a cannon ball from a height in a vaccum and see which lands first.

  7. Although the myth is common I think that there is genuine confusion arising from the role of the Bernouilli principle in the correct explanation. The pressure difference is a perfectly adequate description of lift but most of the lift is not a consequence of the Bernouilli effect. As a non-physicist I think that it seems commonplace to refer to the Bernouilli effect/principle as both a general principle and a specific effect. (I’m probably not being very clear here).

    Anyway … my favourite other myth is why veins are blue. Many people explain this in terms of de-oxygenated blood being blue. It isn’t – it is red (which you may realize if you’ve had blood drawn from a vein). It is a purpler, bluer red than arterial blood – but still very, very red. The best explanation is that blueness is partly an optical illusion from viewing the blood against a white-ish background and partly the optical effects of viewing the blood through skin.

    Interestingly it seems arteries would also be blue if we could see them (but they aren’t near the surface of the skin).

    A nice summary is:

    http://scienceblogs.com/scientificactivist/2008/04/why_are_veins_blue.php

  8. Someone sent me an email that told of someone in Australia who tried the bathtub experiment. He used a very large tank of water and tried to carefully control external influences, but still could not get water to drain consistently clockwise or counterclockwise.

  9. I learned Bernoulli as the reason wings fly back in school, and had believed it ever since. I also learned it when I trained to fly gliders. I did wonder about it while I was flying inverted though. Thanks for the explanation.

  10. I heard of the Bernoulli effect all my life and believed it HELPED planes fly, but even as a small child I was aware that the effect could not possibly explain more than a small fraction of the lift needed to keep tons of metal in the air. I naturally focused on the engines instead for an explanation of lift. I was never good at math or most academics in general, but one second thinking with common sense rather than just accepting whatever you hear, and ANYONE, even a CHILD could figure that out! That’s why I never thought until reading this article, that anyone was ever expected to think of the Bernoulli effect as anything more than just a neat special effect that shows how aerodynamics can help your aircraft fly without expending fuel, but perhaps that is what these books/teachers actually meant, which is a pretty depressing, but more depressing still, is that it’s not surprising at all. I guess the lesson is, no matter how “smart” or educated people are, even engineers entrusted with our lives at two thousand feet, and scientists in general are wired to blindly accept “facts” simply told to to them, rather than every being demonstrated or proven. It obviously began as one person with a misunderstanding, but look how far it spreads, around the globe, before this person’s misunderstanding was calle dinto question? And we’ll be dealing with this misperception probably until the end of the human race, which looks like it won’t be long. No one ever actually uses their own brain until they are forced by some rare skeptic to do so. This is the whole reason for having to prove things via the scientific method in the first place, skepticism is the heart of science, it is built into the scientific method, because it isn’t built into humanity, rather, naivety is. If we started to just accept and pass on scientific “facts” on faith instead of proof, virtually all engineers would in short order end up witch doctors and charlatans as per the feeble human nature that can rarely be bothered to question anything, even if it defies common sense and common instincts, even if it is something as important as what keeps a plane in the air. I’ve said nothing here that is one centimeter beyond common sense.

  11. John, Just by chance I was writing a future “on this day” post for the date Coriolis was born (may 21).. just when I read your blog summary on Myths.. thanks, I linked this post.

  12. However that wing shape has been used on racing cars to produce down-force, in which case the wing shape is inverted. So personally I believe that the shape does generate lift. Why put wings on a racing car if all they created was drag?

    I think the idea is to try and produce as vertical as possible vector of force on the wing (especially in cruising configuration). I’m sure a plane can have totally flat wings and use angle of attack or wing pitch to give lift, as you say; but I bet it would give more drag and burn more fuel.

    About your questioning of the particles not having to arrive simultaneously at the trailing edge of the wing. If that was the case, then air would have to be ‘going missing’ leading to some sort of low pressure somewhere, probably at the back of the wing, but in the very area where some of the lift is assumed to occur.

    Perhaps the efforts in teaching the effect is to try and get a pilot to grasp some of the complexities of air flow and lift, beyond what is intuitive, leading people to forget the parts that were intuitive.

  13. The diagram is misleading, from what I understand, the angle of attack on an asymmetrical wing can be zero* (horizontal), and still generate lift.
    It is only natural that air can’t be replenished fast enough in the area behind the sloping back of the wing, creating low pressure (whether the particles arrive at the same time or not is just a consequence of this).
    It is correct then that the higher pressure below the wing lifts the airplane, but there’s no need for that lower surface to face the incoming air at all.

    For symmetrical wings (fighter jet etc), you do need the angle of attack, in which case you still get a vacuum above, plus a higher pressure below. I think this symmetry let’s those airplanes handle equally well when upside down.

  14. Think of the upper surface as having not an angle of attack, but an “angle of retreat”, if you may. For the same reason that air accumulates and creates high pressure on an angled surface heading the air, it spreads out and creates lower pressure on an angled surface going away from the air.
    That’s why the upper surface contribution is not ‘insignificant’, and a Stall condition pretty much takes out half of your lift.

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