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Science 101

Jun 23, 2003 — I was reading a book the other day about scientific literacy and whatnot, and it said that of a graduating class of Harvard, 23 random students were asked "Why is it hotter in the summer than in the winter", and only 2 or 3 could correctly answer it. Disgusting, but not really surprising 'cause they're all lawyers and future government leaders. Then I read this:

[quote]Design the wings so that when they become rigid with air, they assume the classic wing profile, a shape that creates lift by redirecting the flow of air downward as it passes over the wing.[/quote]

If this was in a high-school students paper, I wouldn't be too surprised. Maybe in a college undergrads paper. But no, I read it at [link http://www.popsci.com/popsci/aviation/article/0,12543,459355-4,00.html]Popular Science[/link]. The scariest thing about this is that this article is talking about a new type of skydiving piece of equipment that allows you to stay in the air 3 times longer. I just hope that the people designing the suit didn't have this misconception.

Afterthought: Now that I think about it, I just hope I'm right about the entire Bernoulli's Principle thing, and didn't just make a complete fool of myself. But as far as I know, a moving fluid has less pressure than its surrounding fluid, which causes lift in airplane wings, sailboat sails, etc.

DataBind() says:

A moving fluid has less pressure than a more static fluid. A wing is relatively flat on the bottom and curved on the top, causing the air molecules on top of the wing to move over more surface area, causing the air pressure above the wing to be less than that below it. Since the air particles above the wing travel a greater distance in the same amount of time, they are traveling faster. Bernouli's theorem states that pressure is inversely proportional to velocity; therefore, the faster moving air is at a lower pressure. This creates a suction -- lift.

However, there is another function of a wing called Mass Flow, in which the air is directed downwards which produces a thrusting force helpful is causing the wing to raise in altitude. Air particles have mass and the wing directs this mass downwards which helps with thrust and, correspondingly, lift, since lift is a function of velocity (among other things). This is called camber -- the greater the camber (downward flow of particles), the greater the lift.

Hence flaps.

rnewhouse says:

So basically the reason an airplane flies is that it's being sucked up into the air by low pressure?

That's kind of scary.

It's 2003 already. Where's my hovercar?

klisq says:

Yeah, but Mass Flow works on the idea of the air being forced down by flowing under the wing, not over it.

Aesopian says:

I actually learned Bernoulli's Principle under klisq supervision. I always watch over 20 varieties of fresh and saltwater fish die under klisqs supervision...

DataBind() says:

The problem you are having is that Bernoulli's Theorum does not explain lift -- in fact, if it was the total explanation, then planes could not fly upside down.

It is a popular description of flight that gets the point across easily and simply, and it kind of correct as it deals with pressure and suction, but it is flawed and not how a physicist would describe lift. In fact, if Bernoulli's Theorum were applied to a Cessna plane, calculations would show that the plane is only capable of 2% of the needed lift.

There are other more accurate physical descriptions of flight that actually demonstrate it is the velocity of downward directed air from the airfoil that produces lift and causes less pressure above the wing.

If you are interested in the physics of it, let me know.

DataBind() says:

Also, if Bernoulli's was the correct explanation, how does a paper airplane, which has NO curves on the airfoils, fly?

DataBind() says:

If anybody is interested in a physical explanation of lift, and has some understanding of physics, study Newton's 3 and also something called the Coanda Effect.

The Coanda Effect states that a fluid traveling along a curved surface tends to follow the curvature of the surface. It is easily demonstrated by holding the back of a spoon vertically under a thin stream of water from a faucet. If you hold the spoon so that it can swing, you will feel it being pulled toward the stream of water. Pretty neat, eh?

Simmons says:

So the air is pulling the wing towards it because it is attracted to the curved surface?

Wirehead says:

Ok, I want further amplification of this idea - I've never heard of an alternative explanation for lift, let alone one which virtually discredits Bernoulli's principle.

Speak on, DataBind()!

DataBind() says:

Let me write something up...in the meantime, it doesn't discredit Bernoulli's principle. Bernoulli's principle technically has very little to do with lift and his principle isn't discredited at all -- his principle is actually a demonstratable law. It just doesn't explain powered airflight -- using Bernoulli's principle, a 747 would NEVER generate enough lift to get off the ground.

DataBind() says:

[quote]"Although it is probably true that the principle of flight can be most simply explained in this [Bernoullian] way it by no means is wise to construct a wing in such a manner!"
[/quote]
Albert Einstein, 1954

klisq says:

Yeah, I want more explanation. As far as I know, the wing deflects air down from flowing under it (the air flowing over it wouldn't cause any lift). Etc. More explanation.

DataBind() says:

The usual (Bernoulli) explanation of how a plane flies relied on a theory of "equal transit time." Since the air on the top of the curved wing has to travel a greater distance, it must be traveling faster "in order to catch up with the air beneath the wing." Obviously, this doesn't seem very scientific once you think about it. But don't feel bad -- many physicist's have been thrown astray by using Bernoulli's equation (p + 1/2[i]p[/i]V2 + [i]pg[/i]h = constant where p = pressure, [i]p[/i] = density, v = velocity, h = elevation and g = gravitational acceleration) to demonstrate lift (most notably, Albert Einstein who designed a "humpback" wing, thinking a large hump would make the air travel even [i]faster[/i]. The wing flopped.)

The idea is that since the air on top has to travel faster to "catch up" with the air on the bottom, it must be more spread out and have lesser density. This makes the air suck the wing up. Does that mean if the plane flies upside down, it is sucked down? Why does the air on top of the wing even bother to try to catch up at all? What the hell is going on?

First, let's explain why Bernoulli is not singularly effective at describing powered flight. Take two types of wings -- a flat wing (such as a paper airplane) and a symetrical wing where the top and bottom have the same curve (as in acrobatic planes). Bernoulli states that neither of these designs would fly -- the flat wing has no curve, therefore the air traveling atop the wing would travel at the same velocity and thus no differential in pressure would occur. And the symmetrical wing would also not fly -- the top and bottom of the wing would generate the same pressure. But we know that both of these wings DO fly.

Bernoulli is not wrong, but an airplane flies because it redirects the air flow downward. The problem is that the Bernoulli effect does not fully explain it. It is just part of the equation -- Bernoulli alone could not make a plane fly -- it takes the combination of low pressure systems around the wing and the downward thrust of air (which creates low pressure). You do not need a curved surface on a wing to make it fly -- a totally flat wing will fly, if it directs the air flow downward properly (see a paper airplane).

The reason that a plane flies is because the air, being a fluid and slightly viscous, tends to follow the curved surface of the wing that it is flowing over (the Coanda Effect.)
When the air runs along the curved surface of the wing, it follows the curve downwards just like the water runs off your elbows when you bend over to wash your face in the sink.

Since the wing is exerting a downward thrust, Newton's Third Law forces an equal AND OPPOSITE reaction...the wing goes up.

The Coanda effect is just the tendency of any slightly viscous fluid to stick to a surface it is flowing over (is is 'entrained' by it) and to follow this surface as it bends. As air follows the upper surface of a wing and it gets bent downward -- because the surface is curved but also because [i]the leading edge is tilted up[/i]. This is what is called the [i]angle of attack.[/i] The air that is bent downward pulls on the air above it [i]creating low pressure. [/i]

Low pressure is created in two ways -- when the air is directed down, it sucks air with it and creates low pressure above it and also when the air hits the front, rounded portion of the wing, it is slowed down and creates a high pressure in front of the wing. Since wing designs create a permanent slight upward angle of attack, the bottom of the front of the wing takes more of the impact of air and creates a higher pressure below the front of the wing. The downward thrust of air out the back serves to further descrease the pressure of air above the wing. If there was no angle of attack, the plane would generate little, if any, lift. Bernoulli alone cannot fly a plane, even if the top of the wing is very curved, without this angle of attack sending air out the bottom of the plane. Likewise, if the angle of attack was too great, then the air shooting out the back would drop off and the plane would do something pilots call a stall...it would stop flying, for lack of a better word.

In bending the air downward, the wing exerts a force. Since there is an action, there is obviously an equal and opposite reaction. There is thus lift.

The size of that upward force is equal to the mass of air the wing has diverted downward multiplied by the acceleration of that air (Newton's second law).

So, Bernoulli is right, but for all the wrong reasons. Bernoulli is good at approximating flight -- velocity and air pressure, etc, BUT the downward thrust is the key. No downward thrust = no low airpressure above = no flight.

So, a flat winged paper airplane flies because the design forces air downwards which then creates an opposite reaction up (by causing a low pressure above the wing). A symmetrical wing has an upward angle of attack, causing air to be thrown downward as well, which causes a low pressure above the wing and high pressure below the wing, since the air slams into it and slows down.

BTW, ever wonder why you have to raise the nose a plane to make it gain altitude? They are increasing the angle of attack of the wing. :) If Bernoulli's was the only explanation of lift, then wouldn't they have to raise the curvature of the wing to generate more lift???

Any plane can fly upside down if it can alter this angle of attack enough -- even if the bottom of the wing is flat. I've heard it said that you could fly a brick if you could get the speed and angle of attack correct.

Make sense? I hope I explained it well enough...if you have any questions, ask away and I will clarify.

DataBind() says:

Just thought of something funny...if it WAS bernoulli's that explained flight and flight was achieved by the curved top of the wing creating suction, how would you STOP the lift? LOL

Wirehead says:

I didn't mean that it completely discredited Bernoulli, only that it basically discredited it as an explanation for heavier than air flight.

Thank you for the explanation - this is one of those things you "always sort of knew" yet didn't really allow yourself to believe because you "knew" the book said otherwise. Guess I should have listened to myself on this one.

DataBind() says:

I've noticed many textbook "Facts" that are incorrect. We should put together a list someday.

vampirical says:

Nice explanation DataBind, this was one of those things that I went around having conversations about when I discovered an article on it(in PopSci funnily enough) like when I had a Physics teacher point out how stupid the first explanation most people are given about electrons orbits in atoms.

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