How Planes Fly: What They Taught You In School Was Wrong

By Daniel Miessler on October 19th, 2007: Tagged as Physics | Science

So we all know how planes fly, right? The top of the wing is rounded and the bottom of the wing is more straight. Air takes longer to travel over the top of the wing than the bottom, which results in more pressure on the bottom, hence the lift. Right?

As it turns out, no.

This is what I was taught, and it’s what I’ve always believed (it’s even in most lower-level text books), but it’s simply not true. This concept, called the Equal Transit Time Theory does not generate nearly enough lift to keep a plane in the air.

The main reason planes fly is far simpler: wings force air downward, which in turn force the wing (and therefore the plane) upward.

The primary actor at play here is called the Angle of Attack (AoA), and it’s easy to conceptualize. We’ve all put our flattened hand outside a car window while the car was in motion. You noticed that if you angled it straight on you could hold it steady, but if you angled the front edge upward you created massive lift that forced your arm up in the air.

The same concept works for kites, helicopter blades, sailboats, ceiling fans, and planes. In fact, winglike surfaces can generate lift almost regardless of the shape of their top and bottom surfaces. Notice that ceiling fan and helicopter blades are basically symmetrical, yet they create a downward columns of air just fine. You can actually use a barn door to generate lift…if you were so inclined.

In other words, lift simplifies nicely to Newton’s third law of equal and opposite reactions: air goes down, wing goes up.

Finally, if you’re still harboring any fond feelings for the equal transit time theory, ask yourself a simple question:

If the top vs. bottom wing shape is so important, how can planes fly upside down?

Alas, there are actually multiple ways of describing, and calculating with varying degrees of precision, the way in which lift is generated. There is the mathematical/engineering approach, and there is the physical approach, which is what I’ve described above. John D. Anderson said it best:

It is amazing that today, almost 100 years after the first flight of the Wright Flyer, groups of engineers, scientists, pilots, and others can gather together and have a spirited debate on how an airplane wing generates lift. Various explanations are put forth, and the debate centers on which explanation is the most fundamental.

The one thing the experts agree on, however, is that the way most have been taught about how planes fly (the Equal Transit Theory) is absolutely incorrect. ::

[ There are, of course, other factors at play other than angle of attack, e.g. the Coandă Effect. The point of this piece is to illustrate that the primary factor with flight is the forcing of air downward (to which Coandă contributes) rather than equal-transit. ]

References and Notes

  • http://www.seunosewa.com/ Seun Osewa

    Not convinced. Sorry.

  • http://www.seunosewa.com/ Seun Osewa

    Not convinced. Sorry.

  • mr Swordfish

    This pretty much nails it. Air goes down, plane goes up. It's really very simple.

    The same thing happens with sailboats – air goes backwards, boat goes forward.

    Of course, to do serious engineering you need a more sophisticated model – Partial differential equations in three dimensions, vector fields that solve those equations, boundary conditions such as the Kutta condition, surface integrals to compute the total force, and yes the Bernoulli's principle to convert the velocity vector field to pressure. But all this complicated math can obscure the simple physical principle – the wings generate lift by forcing the air downward.

  • mr Swordfish

    This pretty much nails it. Air goes down, plane goes up. It's really very simple.

    The same thing happens with sailboats – air goes backwards, boat goes forward.

    Of course, to do serious engineering you need a more sophisticated model – Partial differential equations in three dimensions, vector fields that solve those equations, boundary conditions such as the Kutta condition, surface integrals to compute the total force, and yes the Bernoulli's principle to convert the velocity vector field to pressure. But all this complicated math can obscure the simple physical principle – the wings generate lift by forcing the air downward.

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  • moschops

    A plane with a flat wing will also fly; it is all about pushing air downwards at the wing, but the air above the wing is not being pushed down by the wing, but by the air above. The underside of the wing is doing very serious business in the field of pushing air downwards.

  • http://twitter.com/rnshetty rajneesh

    well, what about rudders, aren't they curved as well?

  • http://danielmiessler.com/ Daniel Miessler

    Not as sorry as I am.

  • Name

    Sorry to rain on your parade – but the Coanda effect isn't the real cause of aerodynamic lift either. There's a reasonable-but-very-accessible explanation in the (online) pilot's textbook I use:

    http://av8n.com/how/htm/spins.html#sec-coanda

    The author is a flight instructor and physics professor. Take a look – the whole book is free online, and very engaging. (The main “how a wing works” section is in chapter 1)

  • http://danielmiessler.com/ Daniel Miessler

    I see your av8n.com link, and raise you an Aeronautics Learning Laboratory link:

    http://www.allstar.fiu.edu/aero/airflylvl3.htm

    You'll find that the link above, combined with the physics lecture
    done at Fermi (also linked in the article) will contradict the link
    you posted. I think you should reconsider.

  • Name

    I'd like to make it plain, before replying, that I am not an aerodynamicist, and am therefore attempting to referee a “battle of experts” from a position of student rather than teacher. This raises a significant danger of this becoming a typical Internet physics debate. Nevertheless:

    1) [the "appeal to experts" part] I will note that both pages you linked use only the briefest references to Coanda, whereas See How It Flies spends several pages discussing it. In particular, SHIF lists a series of predictions that “Coanda causes lift” would imply, and the the evidence to the contrary (18.4.4 “Fallacious Models of Lift Production”). When one teaching source mentions something superficially in one diagram, and another spends several pages exploring it (including examining predictions, etc), it is circumstantial evidence that the second may be a fuller explanation.


    2) [the "appeal to common sense" part - Danger, Will Robinson!] As the Coanda effect relies upon the existence of a curved surface, it does not explain why the proverbial flat barn door actually performs as a pretty decent wing. At very least, then, this would suggest that Coanda is only a minor part of what makes some wings fly.

  • ack

    from the flu.edu link:

    “The role of the angle of attack is more important than the details of the airfoil’s shape in understanding lift.”

    kinda sums everything up neatly in my book

  • SciTech

    I don't see how this explains upsidedown flight. Won't he invertd wing force air up and push the wing down only the the angle of attack will stay the same?

  • Jordan Bettis

    Not only is the Coanda effect largely irrelevant to the aerodynamic lift produced by wings, but water following a curved surface has nothing to do with Coanda (it's caused by surface tension).

    You're guilty here of doing the same thing the Bernoulli explanation does: taking a partial truth, and papering over the gaps with untruths.

  • Esteban

    I always thought it was like you say, never heard of the Bernoulli Effect maintaining planes in the air.

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  • Tim

    “Ask yourself why planes can hang tons of massive crap (engines, bombs, etc.) off of the bottom of their wings if the bottom of the wing is so important for flight.”

    Engines take up only a small fraction of the underside area of the wing, and are much easier to reach for service there. Bombs — well, where else would you put them? on top of the wing? :-) Besides, no matter what you do, no matter which theory of flight you believe, or where you put it on the airplane, “tons of massive crap” is both (a) not going to increase lift, and (b) necessary for the aircraft to do its job. Observing that your 767 has a giant engine under the wing does not, alone, contradict Bernoulli.

    “This also explains how planes can fly upside down.”

    Not really — unless I'm mistaken, the Bernoulli theory can also be applied here.

    “wings have a downward slope at the rear of the wing, which forces large amounts of air from the top of the wing downward”

    Practical experience shows me the opposite: a paper airplane with a downward sloping wing dives straight into the ground. Every paper airplane I've ever seen that's flown more than 2 feet has had either an upward-sloping or level wing. Where's Coanda now? :-)

    I'm not saying you're wrong, or that Coanda isn't true, or that Bernoulli is, but I don't find the evidence you've presented to be compelling. (I don't know what to believe, but I'm not an aerospace engineer so I don't think it's important for me to believe in one theory. I have no strong feelings on the matter.) But “extraordinary claims require extraordinary evidence”, and contradicting all of my physics textbooks is, I think you'll agree, a rather extraordinary claim.

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  • Name

    The lift generation comes out from the mathematical solution of the differential equations that describe the flight phenomenon. These are the so called Navier-Stokes equations. The Bernoulli principle is an ideal subset of Navier-Stokes that cannot fully describe the actual phenomena in a lifting body or airfoil or 3D-wing. By solving the Navier-Stokes in 2D or 3D one can actual see how pressure side and suction side of a wing co-operate in order to produce lift, together with the remaining aerodynamic field (temperatures, vector velocities, entropy etc. etc.). So, in conclusion there is no straightforward explanation of “how planes fly” and no wrong or correct school explanations, but incomplete explanations. Only by solving the above mentioned differential equations in an appropriate grid and with the appropriate solid & boundary conditions will understand flight.

  • Name

    The lift generation comes out from the mathematical solution of the differential equations that describe the flight phenomenon. These are the so called Navier-Stokes equations. The Bernoulli principle is an ideal subset of Navier-Stokes that cannot fully describe the actual phenomena in a lifting body or airfoil or 3D-wing. By solving the Navier-Stokes in 2D or 3D one can actual see how pressure side and suction side of a wing co-operate in order to produce lift, together with the remaining aerodynamic field (temperatures, vector velocities, entropy etc. etc.). So, in conclusion there is no straightforward explanation of “how planes fly” and no wrong or correct school explanations, but incomplete explanations. Only by solving the above mentioned differential equations in an appropriate grid and with the appropriate solid & boundary conditions will understand flight.

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  • http://www.othersideboardsports.com/brands/kiteboarding/cabrinha.html cabrinha

    Its the same thing.Wings force air downward(Hence creating more pressure on the bottom), which in turn force the wing (and therefore the plane) upward.That's it so what's the big deal?

  • ckenterprises

    This post seems to make sense, however it does not explain the lift v/s drag curve for airfoils. If lift depended only on the angle of attack then the lift should not go down after fifteen or eighteen degrees (Angle of attack).

  • Kevin

    Yay! At least one person has it right! This would be my comment, but “Name” beat me to it.

  • PaperAirplanes

    Thanks for clarifying that. The title grabs attention too.



    ==========
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  • http://www.facebook.com/Space.Sapian Christopher Morrison

    Look you are completely wrong.

    When an airplane is flying in Straight and level flight no air is “forced down.” The airplane is not accelerating up therefore there is no air accelerating down, momentum is conserved.

    The streamlines in front of the plane are the same as the streamlines in the back. The lift is generated because the pressure on the bottom of the wing or airfoil is higher than the top.

    Air is NOT redirected downward. If you don’t understand this read this comment again.

    I’m an aerospace engineer, I study this stuff.

    • Will

      Actually Chris when you look at the momentum pre and post-wing, the air has been given a downward velocity component. The default would be for the wing and the plane thereon to accelerate downward. The fact that it is remaining in the air requires something else achieve that momentum change per unit time equal to the force of gravity. That thing is the air. This is not to say the pressure isn’t different top to bottom. They are two sides of the same coin. They’re related through the Navier Stokes equations and continuity. If you’ve seen actual CFD analysis or wind tunnel tests, you’ll see the streamlines are not the same in front and in back of the wing. Look at the animation posted. It’s fairly accurate.

    • Ally

      God, there’s nothing I hate more than a cocky fresh-faced graduate bursting in and telling all the real experts (you know, the ones with 20+ years more real-life experience in the field) that they’re all wrong. It’s amusing, in a pathetic sort of way.

    • Chase

      thats cute you are so assertive when so wrong. you should close the book and try actually flying an airplane.

      if you ever happen to be in IWS (west houston airport), i will be glad to point out to you the flaws.

  • Anonymous

    “The airplane is not accelerating up therefore there is no air accelerating down, momentum is conserved. “

    Its been years since I did F=MA etc., but surely gravity is accelerating the plane downwards, the plane is in level flight, therefore it is accelerating up?

  • http://justinbieberdoll.blogspot.com JBdoll

    Is it possible for helicopters to fly upside down?

  • ccc

    “wings force air downward”

    i didn’t study aeronautics or even engineering but your explanation makes it sound like the downward wind created by the wing is generating thrust, which causes the plane to go up… i’d bet a dollar that isn’t the case. the wind on the underside of the wing causes ‘lift’ and the plane to rise, given enough lift.

  • Big Bob

    If the air over the top speeds up to match the air flowing under, where did the energy come from to speed up the air? and how did it know how much to speed up the air? Is their a tiny microprocessor in each air molecule? If we have an answer to this, we have the energy problem solved!

  • Anonymous

    It get management on board and begin to address the true scope of this open ended question.

    Car Auctions

  • Me

    The best explanation I’ve ever heard as to why a fully loaded 747 can fly is this: You get 250 tons of people, steel and fuel moving at 170 kts toward certain death off the end of the runway and the passengers will WILL the thing into the air. :)


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