How Airplanes Fly: A Physical Description of Lift | Hacker News Reader

How Airplanes Fly: A Physical Description of Lift

allstar.fiu.edu

185 pointshadronzoo22 comments

From Stick and Rudder by Wolfgang Langewiesche, page 9, published 1944:

The main fact of all heavier-than-air flight is this: the wing keeps the airplane up by pushing the air down.

It shoves the air down with its bottom surface, and it pulls the air down with its top surface; the latter action is the more important. But the really important thing to understand is that the wing, in whatever fashion, makes the air go down. In exerting a downward force upon the air, the wing receives an upward counterforce--by the same principle, known as Newton's law of action and reaction, which makes a gun recoil as it shoves the bullet out forward; and which makes the nozzle of a fire hose press backward heavily against the fireman as it shoots out a stream of water forward. Air is heavy; sea-level air weights about 2 pounds per cubic yard; thus, as your wings give a downward push to a cubic yard after cubic yard of that heavy stuff, they get upward reactions that are equally hefty.

That's what keeps an airplane up. Newton's law says that, if the wing pushes the air down, the air must push the wing up. It also puts the same thing the other way 'round: if the wing is to hold the airplane up in the fluid, ever-yielding air, it can do so only by pushing the air down. All the fancy physics of Bernoulli's Theorem, all the highbrow math of the circulation theory, all the diagrams showing the airflow on a wing--all that is only an elaboration and more detailed description of just how Newton's law fulfills itself--for instance, the rather interesting but (for the pilot) really quite useless observation that the wing does most of its downwashing work by suction, with its top surface. ...

Thus, if you will forget some of this excessive erudition, a wing becomes much easier to understand; it is in the last analysis nothing but an air deflector. It is an inclined plane, cleverly curved, to be sure, and elaborately streamlined, but still essentially an inclined plane. That's, after all, why that whole fascinating contraption of ours is called an air-plane.

I like this explanation a lot. I will deploy it on my eight year old; she has always been disatisfied by my explanations of how wings work - as have I.

I'd also like to extend my thanks to the submitter of the story.

I would argue with Mr Langewiesche that it's not at all useless for the pilot to understand that the top surface of the wing sucks air down. Ice doesn't cause plane crashes just due to adding extra weight. It also changes the shape of the wing, making the top surface less effective at sucking air down. That's very useful information for pilots.

Great description. But then I read the XKCD mentioned in another commend, and started wondering how plains fly upside down then? When inverted the "top" of the wing would push the plane down, no?

Planes that often fly upside down typically have low camber wings. Some of them have almost symmetrical wings. Those wings have less lift in upright flight than a more highly cambered wing would, so (and for other reasons too) the manufacturers compensate with bigger engines.

http://en.wikipedia.org/wiki/Extra_EA-300

and an engineer's question about wing design for aerobatic flight with some good answers:

http://www.eng-tips.com/viewthread.cfm?qid=183605

Thats the camber and it depending on the type of wing and airspeed it can play a small role in inducing lift.

What's more important is the angle of attack. The leading edge of the wing is grabbing air and pushing it down. Camber helps by creating a low pressure system over the top that sucks the plane up. And finally, the trailing edge helps because you have laminar air leaving the plane at a particular vector, the Koanda effect.

All of these contribute to producing lift.

Don't miss their updated PDF version: http://www.allstar.fiu.edu/aero/Flightrevisited.pdf

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Important but somewhat complicated point: flight in a 2D world is a venturi effect, and regardless of altitude, a two-dimensional airfoil remains trapped in "ground effect mode." The circulation extends outwards indefinitely, and so an instant force-pair connects the wing with the surface of the Earth.

In 2D, the airfoil circulation creates lift upon the moving airfoil, but it also creates an equal down-force against the ground. If the airfoil was flying higher, the force pattern on the ground grows wider, but the net downforce doesn't change. As a result, 2D airfoil diagrams do not describe normal flight. They describe a sort of "Flatland flight" where no net work needs be done to accelerate mass downwards. As an explanation of aircraft, they've been simplified until they cross the line into actual error.

In the 3D world we can launch a vortex downward and experience a reaction force upward. Or perhaps launch a vortex sideways (during turns.) No instant-force upon the Earth is needed. The Newtonian force-pair arises between the mass-bearing aircraft and the mass-bearing air entrained by the shed vortex. Hovering rockets have an exhaust plume, and flying airplanes have a descending vortex-pair, and both are essential to any explanation. Real world 3D flight is "vortex-shedding flight," and the explanations based on 2D airfoil diagrams leads to no end of confusion.

See also, the awesome Amasci site discussion of this:

http://amasci.com/wing/airfoil.html

http://amasci.com/wing/rotbal.html

http://amasci.com/wing/whyhard.html

edit: Another interesting fact is that the flawed description of how airplanes fly is so pervasive that Albert Einstein once proposed how to improve the airfoil based on his understanding of it and it was a huge flop.

See the end of this article:

http://www.mathpages.com/home/kmath258/kmath258.htm

I also highly recommend John S. Denker's "See How It Flies" (http://www.av8n.com/how/), which along with more "aviating" tips has a good discussion about circulation, the Kutta condition and the Kutta-Zhukovsky theorem. The Wikipedia page on the Kutta condition (http://en.wikipedia.org/wiki/Kutta_condition) is also quite informative.

I especially like this part from See How It Flies:

  We have seen that several physical principles are involved in producing lift.
  Each of the following statements is correct as far as it goes:

  * The wing produces lift “because” it is flying at an angle of attack.
  * The wing produces lift “because” of circulation.
  * The wing produces lift “because” of Bernoulli’s principle.
  * The wing produces lift “because” of Newton’s law of action and reaction.

  ... Do we get a little bit of lift because of Bernoulli, and a little bit more
  because of Newton? No, the laws of physics are not cumulative in this way.
  There is only one lift-producing process. Each of the explanations itemized
  above concentrates on a different aspect of this one process.

I think this is the crux of the matter. It's sort the same story as in the recent HN post about the hazards of determining cause and effect in medical research. The laws of physics set up a situation where the solution satisfying all the constraints is that the wing produces lift. But just like you can't pull out one equation out of a linear system and say that the solution is X=13 because of this one equation, one can't say that a wing produces lift because of any one physical effect.

Those are excellent articles. I am going to add that when working with paper airplanes with cambered airfoils, (a hobby of mine), I have observed very interesting floor and ceiling effects which can only be attributed to vortexes.

I would add that real wings clearly rely on vortexes to fly, and that this is readily observable.

http://xkcd.com/803/

(obligatory mention)

Is there one describing the truth?

Is there any way of searching xkcd by subject matter?

A lot of this stuff also applies to sailing, a great website that I've been preaching for years that focuses on the Coanda Effect is http://sailtheory.com I highly recommend

Nice one, that I wasn't familiar with. Another great site for sailing theory is Arvel Gentry's page. His Origins of Lift piece was the first reference that made the process clear to me. http://www.arvelgentry.com/origins_of_lift.htm

wow, thank you for posting this. i've been sailing since i was a kid and have always had to piece together this kind of information. what a fantastic resource.

This is great.

I usually tell people that planes fly by pushing air down and backward: down so the airplane doesn't fall and backward so the airplane moves forward. I tell them this because otherwise they completely forget, and jump into assertions about implementation details like laminar airflow.

Now I know to _first_ talk about air going backward and down and _then_ to point them at the conclusions section of this article, referring them to the body of the article for detail. A-and then I'll get back to my work so I can get home at a decent hour.

Assuming the plane is not accelerating it's only pushing air back to offset the air it's drag is pushing forward with the next effect being zero.

This is a largely correct article debunking one of the main atrocities in aerodynamics education today. I approve heartily!

NASA's explanation of how the "Longer Path", or "Equal Transit Time" theory is incorrect.

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

The page contains a simulator of a symmetrical airfoil that you can tweek, angle of attack, particle flow, velocity, and pressure - as well as instructions for experiments proving this theory incorrect.

A couple of recent blog posts linked to the fiu.edu article. They may also be interesting reading:

Kevin Drum: The Problem With Science: http://motherjones.com/kevin-drum/2011/12/problem-science

Modeled Behavior: Of Science and Scientism: http://modeledbehavior.com/2011/12/21/of-science-and-scienti...

Can we not easily test the equal-transit-time explanation by a smoke test in wind tunnel where the the smoke is not released continuously but only at uniform time-intervals in a vertical profile? A picture of the smoke profile (as opposed to streamlines) downstream of the wing would immediately capture the fact that transit times are NOT equal.

FIU on HN? Ha, go golden panthers!

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