How Does an Airplane Stay in the Air?

For the longest time, I thought I knew why an airplane stayed in the air. It was because the wings were curved, and in such a way that the top was more curved than the underside. Air moving over the top of the wing had to move faster to keep in sync with the air below, resulting in less pressure (same amount of air over a larger volume), thus creating an uplift, called the Bernoulli Effect.

I remember reading about that in some popularizing books, and even right now you can find plenty of websites—even at NASA’s—touting this description, which is illustrated below.

Not really correct ... (courtesy NASA)

This isn’t really correct. Courtesy of NASA.

While I was researching other topics, I stumbled upon the real explanation. Now, the Bernoulli Effect is indeed present, but it isn’t enough (by far) to lift a modern airplane.

Even if you don’t do any calculating, there are a couple of things apparently missing from the explanation. First of all, it would exclude airplanes doing rolls or flying upside down and staying in the air. Secondly, as any child knows, a paper airplane flies very well, but it doesn’t have any curved wings at all. And thirdly, the airplane of the Wright brothers didn’t have curved wings either (as far as I can tell from photos and their patent application). Furthermore, it seems a bit odd that air would “know” how fast it should go to keep up with its counterpart below the wing.

The Wright Brothers (photo Wikimedia)

The Wright brothers’ Wright Flyer II. Via Wikimedia.

So, what then is the full explanation? Two things: first, some thrust (an engine for a mechanical plane, your arm for a paper airplane), and then “angle of attack,” or how the wing is positioned against the air. At horizontal, no lift is generated. When the wings are positioned slightly against the air, there is lift. The air is pushed downwards (that’s why you need the thrust), and you create lift.

air poussé vers le bas

Now note that the air is pushed downwards below the wing but also the air flowing over the top of the wing is being pushed down. This is called the Coanda effect, and it can also be illustrated by a spoon or a glass against a water flow: the water “follows” the outside of the spoon. It can be shown that it is actually the air going over the wing that creates the most lift. That’s why you’ll find the engines and weapons attached under the wings and not above for a more limited loss of lift.

B-52 with its engines, armament under its wings (photo Wikimedia)

B-52 with its engines and armament under its wings. Via Wikimedia.

There is a limit however. If the airplane is pulled up too much, there’s no longer sufficient air being pushed down, and the result is known as a “stall,” a dangerous condition at lower altitudes as it takes some time to recover from it.

Graphic indicating stall angle. Source Wikimedia

Graphic indicating stall angle. Via Wikimedia.

There are also a couple of other interesting things to look out for when flying or when observing airplanes taking off and landing. When an airplane takes off, it will increase its lift by making the wings longer using its flaps. As soon as the airplane is in the air, it retracts the flaps to reduce drag and because the speed is now sufficient to push down enough air to stay in the air. When approaching the airport, the flaps again go out to increase lift, but now it’s so that enough air gets pushed down as the airplane decreases its speed. In the final phase, the spoilers (see drawing) go up; “spoiling” the airflow and reducing the lift even more.

Source Wikimedia

Source Wikimedia

I love airplanes, but it strikes me now that I didn’t really know a basic aspect of how they stay in the air. I wonder what other things are out there that I think I know, but that I don’t really know. Science can surprise even the most avid knowledge seekers.

About Bruno Van de Casteele

Philosopher by education, IT'er by trade. Allround Armchair Skeptic, History Enthusiast, Father of Three. Twitter @brunovdc Personal website: www.puam.be
This entry was posted in Cool Stuff, Science, Technology and tagged , , , , , , , . Bookmark the permalink.

64 Responses to How Does an Airplane Stay in the Air?

  1. Craig Good says:

    That Bernoulli myth is one that just won’t go away. Glad you got with the flow, as it were!

    • Skepilot says:

      As I read it, it’s not that the Bernoulli Effect is a “myth,” it’s just not the whole story.
      “Now, the Bernoulli Effect is indeed present, but it isn’t enough (by far) to lift a modern airplane.”

      • Spaceman Spiff says:

        Not only is the the Bernoulli Effect not sufficient to produce lift, it’s not even necessary to produce lift. “Angle of attack” and “airflow” is all that is necessary. You can have a flat piece of rigid material and use it to produce lift. The rubber band and balsa wood airplanes some of us played with as kids worked just fine and there was no curvature of any kind to those wings.

      • I agree with Skepilot. The Bernoulli effect IS enough to lift older, slower aircraft, such as were common say up to some time between the world wars. (OK, OK, call me a liar if you know of some later examples!)
        The fact that rules change at different speeds with different wing shapes and different attitudes, does not mean that a particular rule becomes a myth.
        It might well be a myth that what used to be the dominant lift component in level flight at the cruising speeds of aircraft decades ago, still is the dominant component in all aircraft in all attitudes, but that is a different matter.

  2. Rich Murray says:

    Actually, the lift is lift from above the wing, due to the molecular attraction (Van der Waahs force) between the molecules of N2 and O2 and the molecules of the wing’s upper surface — it is crucial that the upper wing is curved, as that is like swinging a bucket of water around in a circle from bottom to top to bottom — the centrifugal force of the water forced by the pail to go in a curved path creates the lift that holds the water firmly in the upside down pail — without a curved path, the water would simple fall out of the upside down moving pail — centrifugal force is a way of describing the inertial resistance of objects that are made to move in a curved path — some early French planes failed to fly, when cheap, easily made flat wings were used, rather then the carefully curved wings used by the Wright brothers…

    • Shaun says:

      Im sorry I don’t understand your explanation. And if Van der Waal forces are so strong why don’t planes simply float along with the rest of us? Surely forward motion has a large part to play in the equation. As for centripetal forces, the forces at play push the object outwards but to get centripetal acceleration requires the object to be travelling in a circle, so I don’t see how that plays a part in lifting the plane?

    • Bruno Van de Casteele says:

      Interesting! I thought it just helped, but it seems a curved top really makes a difference. I also thought the Wright Brothers had no curved wings (judging from photos and drawings).

      • Jack Hagerty says:

        The Wright Brothers first flier had curved wings, but not the way modern airplane wings are curved. They were convex over the top but concave underneath. This is the way most bird wings are shaped and it worked well enough. Birds, though, propel themselves with their wings creating both lift and thrust whereas fixed-wing airplanes separate those two functions.

        – Jack

    • I suspect van der Waals force is not a significant factor (1)it’s very weak (2)it would be 0 in a helium atmosphere (3)it can be repulsive or attractive. As always,some actual numbers are needed to settle this

    • No way Rich! The vdW forces are practically the same all over, even in the wings of early aircraft with hefty curves on the upper surfaces, and yet you don’t see a hemispherical body, flat side down, rising up in the air if we let it go.
      Nor would vdW explain stalling and so on. And certainly not why angle of attack matters to lift.

  3. Stephen N, Australia says:

    Bruno, I too failed to see the flaws in the traditional explanation. Thank you for your explanation.
    I still marvel when I see a large plane (particularly travelling slowly near an airport) supported by nothing but thin air!

  4. Marian Whitcomb says:

    This is why the teaching of “ignorance” or the boundlessness of what we have yet to discover is a good thing. It motivates us to not assume we already know all there is to know.

  5. Rich Tinker says:

    Sorry to double-dip, as I put this on FB, but I thought it might be interesting to know how pervasive it is to cite the Bernoulli Effect as the cause of flight…

    “People can make derisive comments, but I am a meteorologist, and was taught in college it was the Bernoulli Effect. Air underneath moves farther at the same speed, leaving you with less air to top right of wing, ‘pulling’ air faster over the top, reducing air pressure above the wing. This can be a potent effect, lifting roofs off houses in extreme winds, but angle of attack is primary source of lift in airplanes.”

  6. A flying 757 “pushes” its own weight in air downward every second. The effect can be seen on clouds when a plane flies through them. Other people flew planes before the Wright brothers; they were not controlled flights and some resulted in death.The Wrights made major advances. In particular they found how to prevent roll (sideways)……In Canada it is a federal offense to enter an airplane while it is IN FLIGHT.

    • Huuuhhh???
      “In Canada it is a federal offense to enter an airplane while it is IN FLIGHT.”
      You are kidding us?
      Do you know why, how, when?
      I have been very good about not boarding planes in flight, but really, is such behaviour a problem in Canada?

  7. Rich Murray says:

    You can see the advantages of the convex curve of the upper wing, with a flat lower side, tilted gently into the movement of the air from behind to forward, in many ordinary fans — flat blades are usually used in large, low-speed, quiet, gentle ceiling fans. Propellers show highly evolved surfaces, that maximize the movement of air from front to behind, with the least energy lost to noise and turbulence and drag (air resistance). Some lift comes from pressure on the flat tilted lower side of a wing as it bounces the oncoming air downwards, while most of the lift comes from the suction (intermolecular attraction of air molecules to the upper curved surface of the wing), which causes the air to spiral downwards in a vortex behind the wing, which can be seen when watching a large plane fly through a cloud. You can google the question to find many discussions on the Net in recent decades.

    • Rich Murray says:

      http://www.aviation-history.com/theory/lift.htm

      [ quoted ]

      How Airplanes Fly:
      A Physical Description of Lift

      by
      David Anderson
      Fermi National Accelerator Laboratory
      Batavia IL 60510
      dfa@fnal.gov

      Scott Eberhardt
      Dept. of Aeronautics and Astronautics
      University of Washington
      Seattle WA 91895-2400
      scott@aa.washington.edu

      Almost everyone today has flown in an airplane. Many ask the simple question “what makes an airplane fly”? The answer one frequently gets is misleading and often just plain wrong. We hope that the answers provided here will clarify many misconceptions about lift and that you will adopt our explanation when explaining lift to others. We are going to show you that lift is easier to understand if one starts with Newton rather than Bernoulli. We will also show you that the popular explanation that most of us were taught is misleading at best and that lift is due to the wing diverting air down…

      …Air has viscosity

      The natural question is “how does the wing divert the air down?” When a moving fluid, such as air or water, comes into contact with a curved surface it will try to follow that surface. To demonstrate this effect, hold a water glass horizontally under a faucet such that a small stream of water just touches the side of the glass. Instead of flowing straight down, the presence of the glass causes the water to wrap around the glass as is shown in figure 8. This tendency of fluids to follow a curved surface is known as the Coanda effect. From Newton’s first law we know that for the fluid to bend there must be a force acting on it. From Newton’s third law we know that the fluid must put an equal and opposite force on the object that caused the fluid to bend.

      Fig 8 Coanda effect.

      Why should a fluid follow a curved surface? The answer is viscosity: the resistance to flow which also gives the air a kind of “stickiness.” Viscosity in air is very small but it is enough for the air molecules to want to stick to the surface. The relative velocity between the surface and the nearest air molecules is exactly zero. (That is why one cannot hose the dust off of a car and why there is dust on the backside of the fans in a wind tunnel.) Just above the surface the fluid has some small velocity. The farther one goes from the surface the faster the fluid is moving until the external velocity is reached (note that this occurs in less than an inch). Because the fluid near the surface has a change in velocity, the fluid flow is bent towards the surface. Unless the bend is too tight, the fluid will follow the surface. This volume of air around the wing that appears to be partially stuck to the wing is called the “boundary layer”.

      [ end of quote ]

      This is what I meant by talking about the Van der Waals attraction of molecules of N2 and O2 to each other and to wing surfaces — the source of viscosity and of the Coanda effect and boundary layers in moving streams of molecules of air, which create lift as moving layers of air are pulled down towards the curved convex upper layer of the wing as the wing moves forward through the air…

      • Joel Murray says:

        Brilliant. I’ve done a couple of fluid dynamics subjects as part of my engineering degree and never thought of boundary layers like this. Good explanation. Thanks.

  8. Freke1 says:

    It’s Newton’s Third law: “When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.”

    The wing or sail or any curved surface force the air in a new direction and experience the opposite force from the air. A wing force the air DOWN, the air force the wing UP with a similar force. That’s also why sails and paper thin wings work. And the air doesn’t have to meet at the trailing edge.

  9. Hahn says:

    Guys, you are right and wrong at the same time. Some are heavily cambered , like a vintage Tiger Moth or Feisler Storch, and rely on wing camer heavily for lift. A secondary effect is “ski effect” – like when you “fly” your hand out the car window which is largely influenced by angle of attack. That’s why a cambered wing can sustain inverted flight by holding a larger (inverted) angle of attack and why stall speed is much higher inverted than upright. Many aerobatic aircraft have a symmetrical camber but angle of attack still introduces both Bernouli and ski. Many high speed jet fighters have a symmetrical airfoil and achieve lift by varying angle of attack – it still has elements of Bernouli and ski.
    There is no doubt that the downward displacement of air (ski effect) introduces an opposite reaction a la Newton but the pressure differential caused by a cambered wing is the major factor in most light aircraft. That’s why they can fly with a zero angle of attack. Try that with a symmetrical airfoil. Oh, and the Wright brothers borrowed heavily from Lawrence Hargreaves- an Australian who experimented with cambered wings. The Wrights wings were most definitely cambered.

  10. Hahn says:

    It’s not the 747 pushing its own weight in aIr down every second it’s the air pressure differential pushing the 747’s weight up every moment. Many aircraft with cambered airfoils actually adopt a negative angle of attack in cruising flight and rely on wing camber and Bernolli exclusively. Try that with a symmetrical airfoil. Heavier than air flight is not a simple phenomena. That’s why it is still developing.

    • That’s why I put “push” down in quotes.A figure of speech. Just emphasizing that it’s not the Bernoulli effect per se. I have met people who claim that a rocket won’t work in a vacuum because there’s no air to push against.If you tie them to chairs and tape their mouths you can make them listen to the explanation that the exploding expanding fuel pushes against the rocket, which doesn’t go in every direction because it has a hole at one end.

  11. Vere Nekoninda says:

    It’s very challenging to understand all the forces involved in the air flow and lift, that can allow a plane to fly. It’s even harder to explain clearly. One of your commenters says, “angle of attack is primary source of lift in airplanes.” This is the rough equivalent of saying “fire makes a car move”. Both explanations contain an element of truth, but ignore many essential elements, and fail to explain how the chosen “cause” operates. We can’t expect to find a complete explanation of flight principals on a blog posting, but I hope we can avoid passing on more misinformation.

    The Wright Brothers wings were curved, but they didn’t have the flattened teardrop shape of the usual wing cross section diagrams, such as the first one in Bruno’s blog. The photo of the Wright glider at the first link below gives a sense of the curve, and a much clearer diagram of a Wright air foil is shown on page 7 of the second link’s PDF. The Wrights were among the first to understand the importance of lift to drag ratios, and their use of a simple wind tunnel of their own design helped them experiment and progress faster than many contemporaneous inventors.

    http://blog.nasm.si.edu/aviation/wings-from-the-wright-brothers-to-the-present/
    http://www.wrightexperience.com/pdfs/airfoil.pdf

    I don’t know how many hundreds of times I have seen the classic, and inaccurate, drawing and explanation which Bruno is debunking, of the wing dividing the air, and the air along the upper surface having to travel farther, and therefore faster, producing lift via the Bernoulli effect. As a child, I had the same reaction that Bruno mentions- why does the the air moving along the upper curve have to keep pace with the air moving under the wing? When I was in high school, I was fortunate to see some ultra-slow motion films of fluid flow past an air foil in a wind tunnel, with a visible marker in the fluid, so that one could watch the flow. (The films may have used a liquid, rather than air, as the fluid.) These films showed that the fluid moving over the upper surface of the wing accelerated toward the trailing edge of the wing, leaving the trailing edge sooner and at higher relative horizontal velocity than the formerly adjacent companion fluid molecules that went under the wing. So both Bruno and I were correct, in thinking that the under-wing and over-wing air streams didn’t have to stay in sync, but I never would have imagined that the upper stream would accelerate horizontally. I think these films were made in the thirties. I saw them in the sixties. And yet, in 2015, we still find many sources publishing diagrams and explanations that we have known for at least 70 years don’t correspond to reality.

    Bruno’s blog lacks the space to discuss laminar flow, turbulence, or separation, and those are essential in getting a complete picture of how airplanes fly. But Bruno has done a service by pointing out that the most common explanation given by countless authorities is wrong in the essentials and in the details.

    • Jack Hagerty says:

      I’ve always understood it as a momentum exchange. The wing (due to either shape or angle-of-attack) re-directs a mass of air downward. This re-direction requires force and for level flight the force is equal to the weight of the airplane. The force is created by the plane’s motion which moves the wing through the air, and the motion is courtesy of the propulsion system.

  12. Bill Kowalski says:

    Just to weigh in as an aerospace engineer – aerodynamic flight owes its lift to several factors, the Bernoulli principle among them. The wing angle and surface area are very important and the lift to drag ratio is also meaningful. The smoothness of wing surfaces is critical to minimizing boundary layers which affect wing efficiency. There are also some complex effects related to the angle at which the wings sweep back and the speed of the aircraft. Fortunately, the wing profile which provides the least drag lends itself very nicely to the extra lift contributed by the Bernoulli principle. This relationship has not only helped humans fly, but also birds, who discovered and utilized these facts a couple hundred million years before we knew how to scratch shapes on cave walls. You’ve chosen a lovely topic. Aerodynamic flight is a beautiful accomplishment for all of the animals who have achieved it.

  13. Liston says:

    Bernoulli is no Myth, but nice try

  14. It has been said that when a very old scientist declares something new to be impossible he is usually wrong.Lord Kelvin declared “Heavier-than-air flight is impossible.” Birds,bats,insects, and pterosaurs disagreed.

  15. Michaelb says:

    Jeff Raskin ( designer of the Macintosh ) was an RC pilot. He contributed heavily with articles and instruction ( his son followed in his steps) in model aviation. One of the best articles he wrote was the biggest influence on flight at the speed and reynolds numbers of our world was good ole Newtonian Physics
    http://karmak.org/archive/2003/02/coanda_effect.html

  16. The standard explanation is correct.
    Smoke tunnel work reveals that flow velocity over the top surface of the wing is faster, and Bernoulli’s law dictates a reduction of static pressure up there. Calculations based on that fact give results very close to experimental reality.
    When you fly inverted, you have to assume a much higher angle of attack to force the (new) upper surface flow, which would ordinarily be slower, to do what it is not designed to do and go faster. Likewise, when a flat plate is inclined, the same result applies – the plate may be flat but, by being inclined, it creates that difference in velocity, and therefore in pressure. It does that very inefficiently, just as flying inverted does, but it does do it.

  17. JIMJFOX says:

    The MOST disappointing thing about the internet is this—
    EVERYONE has an opinion, loaded with confirmation bias;
    NO-ONE bothers to check the facts before posting!

  18. JIMJFOX says:

    The boomerang- most have an ‘aerofoil’ cross-section and [AFAIK]
    NONE will fly without a pre-set angle of attack?

  19. rehfnjdj says:

    Very nice! Coanda-effect of curved profiles is necessary to accomplish “form-drag” in Kutta-Zhukovsky theorem under Oseen conditions of vanishig viscosity-

  20. SHEDIES says:

    pls can you explain fully how jet engines work for me

  21. Lance Finn Helsten says:

    Airfoils (which includes propellers, compressors, and turbines) create lift through Conservation of Energy (Bernoulli’s Principle is here) and Conservation of Momentum (the thesis of this article). How much of each contributes to the production of lift depends on the shape of the airfoil, the angle of attack of the airfoil, the viscosity of the fluid, and the velocity of the fluid.

    Bernoulli’s Principle assumes that the fluid is incompressible, and below 260 knots airspeed this may be assumed. But above that airspeed compressibility and supersonic flow and shock waves must be taken into account in the production of lift. The Navier-Stokes viscous fluid model is better at modeling this.

    My point is that the aerodynamic scientists and engineers do know why airfoils work, but the simplifications for the layman (which includes most pilots) has caused a large amount of confusion on the physics of lift.

  22. Chris Jones says:

    I believed the “pressure differential” version for several decades because the early-mid 1980s TV show “Mr. Wizard’s World” on Nickelodeon had a segment where Mr. Wizard modeled an airplane wing with piece of paper wrapped around a pencil, and explained that it was “lowering the air pressure on top” thus creating the lift. Of course, while his show was largely right about a lot of things, there were at least a couple of other things that he was also wrong about (the reason, at least, not so much the phenomenon) which I have since had corrected. I appreciate the article — the more misconceptions I can dispose of, the better.

  23. Walter Clark says:

    Man, that was a lot of comments.

    No one mentioned the fact that at the wing tip, the pressure difference between top and bottom is vented. No obstruction at all. The lower the aspect ratio the more significant the flow around the tip is. On a delta wing airplane, the flow from bottom to top at the tip is almost as much as the down-wash everyone above has been addressing. That that flow doesn’t cancel the pressure difference completely is because air has momentum. If the air was helium it would more easily move from bottom to top. Momentum has also to do with velocity. Slow flow speeds also moves around to cancel more easily. With that same logic, make the plane go very fast and the momentum is so great that very little flow is around the tip and the dominant drag is friction which on a delta is very low.

    The most interesting thing about that momentum is that in going around the tip the air forms a vortex which has the almost magical property of being frictionless. It persists for minutes. Vortex flow is also involved with hurricanes and tornados and would be a good subject for this audience because it is both scientific and so special that it attracts pseudo science types.

  24. Jeffrey Patten says:

    Yeah, I knew there was something wrong with the traditional explanation (Bernoulli Effect) when I saw planes flying upside down in sustained flight at air shows when I was a kid. You can pretty much tell that most of the lift is being generated by the air pressing against the underside of a wing if you stick your hand out of a car window and tilt your flattened hand so the leading edge is higher than the trailing edge. You feel your hand being pushed up; you don’t feel it being “pulled” up.

  25. FrankV says:

    The angle of attack argument given in this blog entry also doesn’t completely answer the question. The mass of air displaced by an inclined plane wing is calculated by

    Wing Area * sin(AOA) * airspeed * air density

    Running some figures for a typical light aircraft in cruise (11m^2 of wing, 4 degree AOA, airspeed = 100kts = 51m/s, 1.225kg/m^3 air density), I calculate air displaced = 48kg/s

    This is about a tenth of what is needed to support the weight of the aircraft (e.g. 480kg). Where does the other 90% come from? I guess that much more air is moved… that there is a ‘bubble’ of air pressure above and below the wing, at least 10 times the thickness of the wing, that moves with the wing, and moves air that it comes into contact with. This perhaps also explains ‘ground effect’, where more lift is produced when a wing is flying close to the ground.

  26. rehfnjdj says:

    a) of course, 1224 kg/s must be adjusted by factor of (A/11)^2, where A – wing aspect ratio; if A=10, then 1224x(10/11)^2 = 1012 kg/s…. b) “Lamina” is any thin symmetric profile with Quality of about 50% of same for J5012; NASA 009; NASA 64010; SD 8020, etc. ….

  27. mermaldad says:

    Thanks for this article. The “longer path over the top” idea needs to die. Apparently the attention you gave to the NASA site was enough to get them to fix their article.

  28. Per-Olof Nilsson says:

    The spoon experiment is not at all the same as that for the wing! Coanda is OK for the wing, but for the spoon adhesion and other effect dominate COMPLETELY.

  29. Yenko says:

    Makes sense, i’m a student studying about forces and energy. Thus helped a lot, if i have to say.

  30. Yenko says:

    I’m a student, and right now I am experimenting forces and energy. This was helpful, and very entertaining to read.
    – Yenko

  31. Yenko says:

    Nice sentences, easy words!
    – Yenko

  32. Sensei says:

    This is a great article. As far as the Bernoulli “myth” – it is what keeps my frisbee flying isn’t it?? So much “science” is less settled than we like to think.

  33. Abdulsalam najib says:

    thanks

  34. George M Brown says:

    Wings fly though still air too… Yes your wind tunnel mentality …says air is flowing over the wing…but in reality the wing is knifing through air..

    It may come as a surprise to many smart folks… but it is the effective temperature of gases .. more on underside of the wing and do to the changing surface angles on top areas..along with the receeding rear areas of the wing…

    Effective temperature… is the real deferential in pressures.. gases have very little mass … but large volume .. from heat energy…. it’s not a liquid…!

    As molecular motion is recoiling from the upper wing surface ..the surface sends them off in various directions.. not straight back..toward other incoming .. at 2 plus GHz… so the continual rebound is set off to the side..hence fewer molecules return to the surface…. less hammering by molecules..lower temp.

    The underside is adding molecules to the game..angle of attack … increases the likelihood of molecules recoiling back against the wing surface..more effective temperature..

    Heat energy in gas pushes on wing to lift it …period..!

    • Noah Dillon says:

      How hot does it have to get for things to float? Why are planes able to fly on both warm and cold days?

    • Alexandria Nick says:

      None of that makes any sense and actually does the literal opposite of what you describe. Warm air is harder to fly through than cold air.

Leave a Reply

Your email address will not be published. Required fields are marked *