Thursday, October 04, 2007

Science Lesson 5: How planes fly

Ira Flatow (Newton's Apple, NPR's Science Friday) was in town Tuesday signing his new book "Present at the Future: From Evolution to Nanotechnology, Candid and Controversial Conversations on Science and Nature". I'm only about 30 pages in, but flows nicely. I'll tell you all about it when I finish it.

One subject that Ira is passionate about is the myth of how planes fly. He dedicates a chapter to it, discusses it in the introduction, and spent part of the talk on the subject.

What we've always been taught in school, and what Ira used to believe, is the Bernoulli's Principle story. That is that a wing has greater surface area on the top than the bottom so the air has to travel faster over the top to reach the back of the wing at the same time as the air that goes under. Since the air is moving faster over the top the pressure is lower and the wing gets sucked up.

It always seemed to me that this was wrong. The pressure difference would create some lift, but not nearly enough. I didn't see why the air on the top would really need to move faster. Even if it were so then wouldn't greater and lesser lift be best created by making the top of the wing expand and contract to alter the surface area?

This isn't wrong, it's just incomplete. If this were all there was to it then a 747 would have to be traveling at 400 mph to get off the ground.

I've always supplemented Bernoulli with how a fan works. Blades at an angle push the air when they're moving. Surely that's why wings have that flap. The flap points down forcing the air that goes under the wing to go down and the wing up. So the wing is getting pushed up. Right? Wrong. When I stick my arm out the car window and shape my hand like an airfoil it feels like angling my hand or wiggling my pinky causes the air hitting the bottom of my hand to lift my arm.

But, I'm told, that if you read books on aircraft engineering that they don't mention Bernoulli at all. It's all about Newton. Now, when I heard this I was cheering myself for figuring all this out way back in grade school. But I was still wrong.

A rocket doesn't depend on how the air moves over it to fly. It pushes the exhaust down and by pushing the exhaust pushes back. A helicopter pulls air down. Bernoulli has nothing to do with either one of these. The airplane also pulls air down. I'll get to how in just a moment.

Think of yourself in the shower. You reach up to wash your hair and water runs down your fore arm. Instead of just shooting off your elbow it curves around and runs down your upper arm. It's attraction to your arm is greater than gravity. There's even rain gutters that use that trick to make the water cling to a curve and run into the gutter while any debris off the roof shoots off into the yard.

Air does much the same thing when going over a wing. We get an idea about this when talking about Bernoulli. It's just that instead of trailing behind the wing the wind continues more or less in the direction it was going when it left the flaps. The wind is rushing down and pulling the plane up. The steeper the angle of the wing or the flaps the more downward the wind rushes.

According to Ira, the underside of the wing isn't terribly important. That's why they can strap on engines and guns and rockets and whatnot. It's the air flowing over the top and then downward that's really important.

note: I had this entry proofed by El Cid, an aircraft engineer with Boeing, just to make sure I understood correctly. His further notes are below.

At low speeds it is very hard to push the air around; it just gets out of the way. That is why the shape of the bottom of the wing is not so important. Most every thing that happens at subsonic speeds happens because of the air getting pulled around by a low pressure area. Low pressure can be maintained in a stable thing called a vortex (smoke rings, dust devils). There is no stable high pressure area. When you introduce an area of high pressure into the air, it is called an explosion. Not stable. A wing or a fan blade carries a vortex along with it. The relative effect of lift caused by low pressure vs high pressure is about 8 to 1. (you won't find that in any textbook, it is El Cid's calculation.) And yes the result is that after the wing has passed by, a large quantity of air gets flung downward.

If you want to look at it mathematically, look up Newton's 2nd law F=MA. Rearranging it gives F = dV*M/dt, the Force generated by the wing is equal to the Velocity that the air gets flung downward times how much Mass of air gets flung per second.

"...wouldn't greater and lesser lift be best created by making the top
of the wing expand and contract to alter the surface area?
Yes!, that is exactly why we use flaps at low speeds: to change the area, and shape, of the top of the wing so that it sucks the air downward at a greater velocity. The result: less M/dt, more dV, same F.

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