How wings work

[blufive]

The latest Scientific American, explaining how wings work:

[...] because the wing top is curved, air streaming over it must travel further and thus faster than air passing underneath the flat bottom. According to Bernoulli's Principle, the slower air below exerts more force on the wing than the faster air above, thereby lifting the plane.

Scientific American, April 2006, P76

Everyone knows that that's how wings work. Unfortunately, it's bollocks.

While I'll let such inaccuracies pass in the non-technical press, I expect better of a magazine that regularly prints articles attempting to explain cutting edge of quantum mechanics, cosmology, immunology, and lots of other -ologies. Even if it's just a throwaway line in an item explaining something different.

The problems with that explanation can be demonstrated with a few fairly straightforward examples:

Firstly, in the course of aerobatics, it is quite common for aircraft to fly upside down (by which, I mean really fly upside down, in sustained level flight, not just a quick loop-the-loop or roll). For that to work, their wings must still be generating lift, despite the fact that the longest side is now on the bottom.

Secondly, not all wings are longer on top than underneath - there are many wing cross-sections that are symmetric, or have the same length on both top and bottom. Yet they still generate lift. For example: sails. Yep, a sail is a wing, turned on end. It's a bit of cloth. To all intents and purposes, both sides of it are the same length (let's not quibble over the tiny difference caused by the thickness of the fabric - trust me, it's irrelevant)

Finally: Two blobs of air approach a wing. One goes over the wing, the other under it. How does the air passing over the top know that it's got to go faster to keep up with the air passing underneath? They're not telepathic, psychokinetic entities. They're inert blobs of air. There's a big lump of metal between them. They cannot directly influence each other. While we're at it, who says that the air passing over the wing has to meet up exactly with the air passing under the wing?

So, how do wings generate lift? It's very straightforward, really: as air flows around a wing, the air is deflected downwards. That's it.

The Bernoulli Principle is a very real physical phenomenon, but it's the cause of lift in the same way as falling is the cause of gravity (which is to say: not much). So this is the last time it's going to get a mention in this post.

Let's consider a wing to be a simple flat sheet of metal, inclined to the airflow, with the leading edge higher than the trailing edge.

Returning to our hypothetical blobs of air, I'll introduce a third one and commit gross anthropomorphism by naming them (my brain can't cope with referring to several things as "it" in the same sentence). As the three blobs approach the wing, Alice is on course to pass over the top of the wing. Bob is on course to pass under the leading edge of the wing, but will collide with the wing itself. Carol is on course to pass under the wing.

Alice passes above the leading edge of the wing. Alice would like to keep going in a straight line (Newton's First Law of Motion, aka inertia). That would leave a vacuum immediately above and behind the wing, and air doesn't tend to leave vacuums lying around. So Alice expands into the space above the wing, reducing her density and pressure in the process. The pressure of the air above Alice also pushes her downwards. The overall effect is to create a low pressure zone above the wing and deflect Alice's course downwards.

Bob passes under the leading edge of the wing. Again, he'd like to continue in a straight line. Unfortunately, there's a big sheet of metal in the way. So Bob has to change course downwards to avoid it. As he does so, he has to shove Carol aside to make room to pass under the trailing edge. All this argy-bargy squashes them together, creating a high-pressure zone under the wing and slowing them down.

As the three blobs leave the trailing edge of the wing, they are all travelling downwards relative to their initial course. Newton's First and Second Laws imply that they have been pushed downwards by the wing.

Newton's Third Law implies that they pushed back - and they did. There's a high-pressure zone under the wing, and a low-pressure zone above it. That's your lift, that is. In practice, the overall effect also slows the air down a bit, causing drag.

So, if flat plates work, why do wings have more complex shapes? While a flat plate produces lift, it's not terribly good at it. Curved aerofoil shapes work better, producing more lift and less drag. They also work much better once you get beyond the simple case of straight and level flight.

Comments

[blufive.livejournal.com]

While it's a fairly simple principle, it takes some explaining, and you pretty much have to use Newton's Laws Of Motion (which don't get taught until quite late, I think?) Invoking the Bernoulli effect is a convenient way to hand-wave over the more complicated stuff, and in practice, the only thing that explanation has seriously wrong is the whole cause-and-effect angle. Oops.

The air going over the top of the wing IS going faster than the air going under it. The air on top IS at a lower pressure than the air under the wing. It's just that the "going faster" bit is caused by the lower pressure (rather than the other way around) which is caused by the big lump of [whatever] moving through the air.

Aerodynamics is one area where lies to children are really necessary*, but the Bernoulli fallacy** is one that drives me up the wall, simply because it involves handwaving the whole thing away, when a qualitative description isn't really that complicated.

*The full, detailed explanations involve the sort of equations that made the best mathematical thinkers of the 19th century run away screaming. Scroll down to the end of the "special forms, newtonian fluids" section for the full horror. Then remember that that's a simplified version.

I'm glad that I'll never have to sit an exam involving those things ever again (they made us remember what all the symbols mean, derive them, or (if they were feeling particularly nasty) solve special cases where most of the terms disappear and you can work things out by analysis, rather than by throwing a supercomputer at it, which I believe is the normal procedure these days.

**I really shouldn't call it that, because the Bernoulli in question was just describing a phenomena, probably wouldn't have used his empirical description like that, and probably knew better...

[richc.livejournal.com]

My problem with using Bernoulli to explain lift to kids is that it's obviously rubbish. The obvious question is 'why do paper aeroplanes work since they have flat wings?'.