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

[luna-the-cat.livejournal.com]

First off, ::APPLAUSE::! This is one of the best, briefest and most coherent explanations I've ever seen. Thanks.

Re: Harriers and the like -- my impression was that, unlike most aircraft, all jet fighters were pretty much designed to be unstable in the air; to rely on simple speed to keep them going, and use the instability to enable high maneuverability.

[blufive.livejournal.com]

Thanks.

Many (most?) modern fighter aircraft do have Relaxed Stability. This allows the aircraft to be designed for pure aerodynamic efficiency - a lot of the natural stability of older designs rely on "weathercocking" - creating drag behind the center of gravity which acts to turn the nose into the airflow. It's simple, it works very well, but it increases drag and hinders maneuvering, particularly the sort of highly vigorous maneuvering that fighters tend to be designed for.

This is a relatively recent phenomenon, however. The key innovation that allowed it was active fly-by-wire control systems. In essence, the pilot isn't flying the plane - it would be like trying to balance three chairs on top of each other, on a broomstick, on your forehead. Instead, a computer (which has much better reactions and probably much better awareness of the immediate aerodynamic situation) flys the plane, with the pilot providing instructions via the flight controls.

The first plane to really use this sort of system was the F-16, though it is now common on most front-line fighter aircraft. It's also appearing (in a more restrained way) on many modern airliners, to improve fuel efficiency.

[luna-the-cat.livejournal.com]

Hmmmm.

Very different from what I'm used to. The only thing I've ever flown (aside from a brief stint in a paper cup, er, Piper Cub) was a glider, which is pretty much the essence of low-speed stability. You really have to work to stall them, and a soon as you stop making them stall, they stop stalling. I wonder what it would feel like to fly something made with almost the opposite in mind...