Wednesday, 29 February 2012

Oh for the wings of a dove

I do quite a lot of talks based on my book Inflight Science, and one thing that is guaranteed to raise a bit of an argument is when I cover how wings work.

If we were taught anything about the reason why wings lift a plane at school, it is likely to be down to the Bernoulli principle. This is great, because it's really easy to demonstrate. Just get yourself a bit of paper (say about 1/4 of a sheet of A4 or Letter), hold it at one end so it droops and blow over the top of it. The droopy bit rises up as it experiences lift. And that's how a wing works, we're told! Only, it isn't.

DIY Bernoulli
When you blow over the bit of paper you get the air moving over the top of it, thinning it out a little. So there's less pressure on the top than the bottom (where the air isn't moving), and you get lift. That's the Bernoulli effect.

A wing is different, of course. The air is moving over the top and the bottom. The explanation usually given is that the wing is specially shaped so the air has further to go over the top than it does under the bottom. So the top air has to speed up to catch up with the bottom air. Reduced pressure, lift, Bob's your uncle. But when you think about it, this is daft. Why should the air going over the top care about keeping up with the air going under the bottom. It's not like the molecules are best mates and desperately need to keep together.

Fluid flow is altogether more complex than this. As it happens the air does go faster over the top, and there is a Bernoulli effect, but it has nothing to do with trying to keep up with the air going under the bottom. And the lift from the Bernoulli effect is nowhere near enough to get a plane into the air. Instead, what does the trick is Newton's third law of motion. The wing is shaped so that as it cuts through the air, it pushes the air downwards. Push the air down and that pushes the wing up. Exactly the same principle as the jet engine uses to get the plane moving in the first place.

So by all means have fun blowing over bits of paper. It is strangely comforting. But it's not how planes get into the sky.
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