Raskin about how plane wings lift | Hacker News Reader

Raskin about how plane wings lift

karmak.org

63 pointsacqq36 comments

That article does not explain anything. It's author merely notes an efffect he saw from experiment, but does not describe why it should be so...

Wings works the way they do because othey curve the path of the air that flows around them. They deflect the air flow, thus creating a force. http://amasci.com/wing/airgif2.html

The "coanda effect" breaks if you give your lifting surface too high an angle of attack (stall), but a reduced quantity of lift is still produced.

Planes can fly upside down because the increase the angle of attack, and the path the air takes IS longer on the top (flat surface) then it is at the bottom (curved surface). A plane with assymetrical wing section flown upside down at a zero angle of attack will go down. This is why aerobatic planes always use symetrical wing sections.

Flat wings work very poorly and stall at very low angles of attack, beause of flow separation, but they do create lift, because the point where the flow separates between upper and lower surface is not at the leading, but somewhere on the lower surface. See this image: http://www.arvelgentry.com/images/plate2.jpg

This guy should read "Theory of Wing Sections", by Abbbott and Van Doenhoff (http://www.amazon.co.uk/Theory-Sections-Dover-Books-Physics/...) before posting misleading essays.

Sorry about the rant, but I don't want anyone taking what this guy writes for the absolute truth...

Obligatory xkcd reference: http://xkcd.com/386/

Unfortunately, "this guy" is dead. I don't think he'll be doing any more reading.

There's a serious problem with the organization of the article. Early on, he tells the story of being scolded in school for noticing that his teacher's explanation for how planes work cannot be right, because if you turned the plane upside down, the Bernoulli force would be toward the ground.

He then goes on to talk about the Coanda effect, and how that combined with the shape of the wing makes the wing direct air toward the ground, and thus provides lift.

Suppose back in sixth grade, his teacher had taught that explanation instead of the incorrect Bernoulli effect explanation. He'd still have the same problem! Turn a wing upside down and the Coanda effect and shape are now deflecting air toward the sky, and the lift is toward the ground.

He should have had a little section at the end covering how planes actually fly upside down.

There are at least four ways to force air downward in order to help provide lift. There's the Bernoulli effect, which he points out doesn't provide very much lift. There's the Coanda effect, which he covers. Then there's simple brute force--point whatever you are using to provide thrust so that the thrust vector aims up somewhat instead of level. Finally, put something in the air flow at an angle so that it directly deflects the airflow down.

The first two work in the wrong direction when upside down, so it is the later two that are responsible for upside down flight. Note that both of these come with significant cost. If you are pointing your thrust vector up, you lose some forward thrust, so it is going to take longer and cost more in fuel to get where you are going. If you stick some flat surface in the air flow to just directly direct air downward, you are also introducing considerable drag, again making your trip take longer and use a lot more fuel.

A lot more here, http://www.grc.nasa.gov/WWW/k-12/airplane/, at NASA'a Beginner's Guide to Aeronautics, including nice simulators that let you play with the various factors that affect flight in simulated wind tunnels and see the results.

WARNING: the above site can be a HUGE time sink.

A read on Wikipedia will tell you that Coanda effect is not the entire reason why wings generate lift: http://en.wikipedia.org/wiki/Lift_(force)#Coand.C4.83_effect

Lift is generated due various reasons: I recommend a read here http://en.wikipedia.org/wiki/Lift_(force)#A_more_rigorous_ph....

(Yes, they're both from Wikipedia and I feel a bit ashamed that I couldn't find a better source but the description there is succinct and rather complete -- I can vouch with my Aero Eng degree on the information there! :D).

Awesome quote: "It was a shock to realize that my teacher and even the library books could be wrong. And it was a revelation that I could trust my own thinking in the face of such concerted opposition."

It's well known - and instructed - within the "pilot community" that the theory taught in most books is oversimplified to the point that it is almost wrong. I understand how it works from the practical and theoretical side; but can't think of a better (i.e. easier) way to explain it to others without this oversimplification.

How about just mentioning that it is in fact an oversimplification. Most people can deal with that, but if you don't tell them then how are they to know?

Well, this is the way I explain it to other people; including the oversimplification part. The real problem - which is probably just my lack of educational experience - comes up when people are really interested and ask about the way it really-really-really works.

A stab at an in between explanation.

Flat wings work with a positive angle of attack (EX: paper airplane). The problem with scaling this up is turbulence at the sharp edge. A gently curved surface reduces turbulence and drag. Remove enough turbulence and you get a laminar flow which hugs both sides of the wings. You can deflect a laminar flow down to increase lift. (See faucet experiment.) Control surfaces on the wing redirect the flow to push the aircraft in a new direction. Incresing the angle of attack increases lift until you reach the critical angle of attack ~15 degrees when you lose laminar flow and a sudden drop in lift occurs. Stall speed is the speed below which you can't maintain level flight by increasing angle of attack. High performance aircraft and trade efficiency for performance (EX: an F-15 landed after losing a wing http://www.uss-bennington.org/phz-nowing-f15.html)

Super sonic flight behaves somewhat differently. (See faucet with increased pressure.)

However, also a dangerous one, used by many a crackpot to justify their theories. It only flies when you have sufficiently obvious experimental results at your disposal to cast doubt on a theory.

>It only flies when you have sufficiently obvious experimental results at your disposal to cast doubt on a theory.

Nonsense. Everything is wrong until proven otherwise. The burden is on the presenter to justify the statement. The moment doubt is raised then the presenter must address it. Someone pointing out a logical error in the presenter's assertion is not required to come up with some other explanation in order to explain that error.

Of course if they do they are held to the same standard.

I think you misunderstand me: I mean that you can only use this argument against a well established, probably experimentally well supported theory, if you have at least some experimental results that are contradicted by the theory (and preferably explained by your alternative).

There are many crackpots that 'have been shocked to realize that library books and teachers were wrong and that they should trust their own thinking in the face of concerted opposition'. Only slightly different from the original, but the italicized words make a world of difference. Einstein's (special) theory of relativity faces scores of opponents that 'trust their own thinking', despite not having a single fact that casts doubt on it.

When I was doing my pilot's license, the alternate explanation to "how planes stay in the air" was with money.

But indeed, even the aerodynamics manuals we were given simplified this part quite a bit.

I found out that the 'Bernoulli Effect' explanation was pretty much wrong through this TED Talk:

http://www.ted.com/index.php/talks/jonathan_drori_on_what_we...

Was completely blown away, and angry at my education. And at myself for believing it so easily.

Raskin is wrong and John Denker is right - a wing is a pump:

http://www.av8n.com/how/htm/airfoils.html

So... we're ditching Coanda and going back to Circulation Vortex?

A question I always had about the circulation vortex: where's the energy input to power the vortex?! (or why does it not need one?)

It's the aircraft's throttle normally. Or if it's a glider, then the winch or another aircraft's throttle if you're getting towed.

To be less facetious, the energy input is the moving airfoil at a particular angle of attack.

http://www.av8n.com/how/htm/airfoils.html#sec-airfoils-summa...

So circulation forms and self-sustains naturally when you push the airfoil through the air. From the article, it looks like this is because the air behavior is asymmetric between the leading and trailing edges, in favor of maintaining circulation.

My question: why/how does the air do this?

For example, I thought the Coanda effect was the macro result of van der Waals attraction. Is there a similar explanation for the behavior of air which results in asymmetric leading & trailing edge behavior?

Because it's a fluid that has pressure and velocity everywhere, like all fluids. I don't know how you would describe circulation in terms of a macro result of some other effect.

Reminds me of an awkward moment dad had early on. He'd been reading textbooks with big thrust/drag/lift/weight arrows for so long that he was startled during a takeoff that he couldn't see them in reality. Despite studying engineering, he hadn't quite reconciled himself to being kept alive by invisible forces.

I don't see how the Coanda effect explains how the classical airfoil profile allows planes to fly upside down - I'd expect them to be dragged down by it, not lifted up.

Can anybody please explain that, or link to an explanation?

If you look at air show footage of planes flying inverted, you'll see that they fly with a pretty big angle of attack. It's similar to how a flat-wing plane can fly.

A great site to learn more about this is http://sailtheory.com

I'm blown away by how many sailors believe in the equal transit theory.

One of the great things about sailing small high performance dinghies is that the feedback is so immediate you don't really need a model of how sails work - just the ability to observe what is going on and what works (although I admit training does help).

As Rodney Brooks said: "The world is its own best model"

It's been a long time since I took physics, but isn't the Coanda effect only for fluid jets (i.e. like the straw in his experiment), and not applicable to airfoils in general (though see blown flaps for where it is applicable)

Speaking of cheap balls: when you banana kick a soccer ball with a counter-clockwise spin, it will bend leftwards. If you do the same with a cheap, thin beach ball, it will bend rightwards. What's up with that?

I play table tennis, and thus have quite a bit exposure to small, cheap balls. I can't confirm your observation.

If you assume inviscid potential flow, a cylinder rotating forward will create lift. So the counter-clockwise spin on the cheap beach ball is creating lift on its right-hand side, so it curves that way. As to why a soccer ball curves the other way, I'm guessing that it's because soccer balls tend to tilt a little, and since they are rotating about that tilted axis, friction pulls them to the left. Even if the same thing happens to the beach ball, it's light enough that friction is small to begin with and the tilt is gonna lift the ball, decreasing friction even more.

Isn't lift generated by Newton's Third Law? For every action there is an equal and opposite reaction

Indirectly, everything is 'described' by Newton's Third Law. However, it doesn't provide a very satisfying answer: it's a description, not an explanation[1]. Knowing the wing goes 'up' because it reacts to the action of the air pushing against it from below, does not explain why that force is larger than the reaction to the action of the air pushing it down from above.

Bernoulli's principle is basically a statement about what happens when you apply Newton's laws to a macroscopic amount of fluid and allows you to determine what happens when an asymmetrical object is placed in the flow of such a fluid: when the speed goes up, the pressure goes down. Pressure is nothing but the amount of Newtonian action exerted on a surface, which brings us back to the third law.

Then you find that Bernoulli's principle doesn't quite cut it, because it doesn't take into account that fluids don't actually behave like the naive application of Newton's laws would lead you to believe. However, in the end, the Coanda effect is again a description of the behavior of fluids under certain circumstances, describing the macroscopic effect of Newton's laws applied to all individual particles.

[1] The exact difference between the two is subject of much philosophical debate, but most would agree that something like Bernoulli's principle has more explanatory power than Newton's law, because, for instance, it allows you to understand why a wing would lift.

I'm not sure why it's not satisfying. The airplane exerts a downward force, the air exerts an equal and opposite force upwards. If you don't change the motion of the air than you can't fly. The typical tear drop wing picture doesn't change the motion of the air so it wouldn't fly. A downward deflection of air is required to provide lift in the form newton's 3rd law. Initially air moves horizontally. The wing serves to deflect air downward. So the wing exerts a downward force on the air. So by the 3rd law the air exerts and equal and opposite force upward. Just talking it out in my head. That seems way more satisfying to me.

The movement of a fluid according to Newton's laws is described by the Navier-Stokes Equations, which have no closed-form solution[1]. So, if your intuition gives you a picture of air moving downward from a wing, that's great. But if it feels like a complete explanation, that is a fact about your mind; not a fact about the world.

[1] http://en.wikipedia.org/wiki/Navier-Stokes_equations

An equation is a description which is surely an artifact of the mind. At least I'm using forces. So you are saying lift isn't provided as I describe?

I'm saying "lift is just Newton's third law" is approximately as descriptive as "the mind is just the interaction of weighted electrochemical links between neurons."

The word "just" covers a lot of detail that can make the short explanation less satisfying--especially if you're trying to use it to build something.

Too late to edit, but I just found a good reference: http://scienceblogs.com/mixingmemory/2006/11/the_illusion_of...

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