Banner
    How Airplanes Fly - The Real Story (With Experimental Verification!)
    By Johannes Koelman | January 8th 2012 09:13 PM | 44 comments | Print | E-mail | Track Comments
    About Johannes

    I am a Dutchman, currently living in India. Following a PhD in theoretical physics (spin-polarized quantum systems*) I entered a Global Fortune

    ...

    View Johannes's Profile
    How do airplanes fly? The next time you find yourself in an airplane, look around at your fellow passengers. How many of them would be able to explain to you what force keeps the hundreds of tons  of metal up in the air? How about yourself? Would you be able to explain this applied physics feat to the person sitting next to you?

    Wings are simple and straightforward devices, yet misconceptions abound on how they work, and many people carry wrong perceptions on their lift-creating mechanism. The 'equal transit time' fallacy being the most prominent misconception in this area. 

    The physics of flight is well understood and straightforward. Airplanes and earth gravitationally attract each other. Newton's laws of motion tell us that to defy gravity and keep the airplane and earth at constant separation requires exchange of momentum between both bodies. This exchange of momentum is realized by the wings of the airplane directing air to earth, and earth reversing this airstream back up. That's it folks. Circulating air. That is how an airplanes fly, how helicopters fly, and how birds fly.


    Circulation, Circulation, Circulation...


    Let's look in more detail at the circulating air mass generated by airplanes. Below animation sketches the large scale air movements as observed from a coasting airplane (red dot).
     


    From your vantage point within the airplane you see earth's surface (lower edge of the figure) shooting by. This moving earth surface drags along air, and you in the plane perceive this air flow as an effective headwind. The plane's engines are working hard to overcome the drag induced by this headwind. But there is more air movement than just this headwind.  If you look careful at this animation, you will see superimposed on the uniform left-to-right flow of air (the headwind) a clockwise circulation of air around the airplane. Air moves up in front of the plane, and moves down behind the plane. The lift on the airplane is equal to the change in momentum (the change in the product of mass and velocity) of the air diverted from up-moving to down-moving. Aerodynamics engineers refer to the downward velocity of the air behind the wing of the plane as 'downwash', and the upward velocity in front of the wing as 'upwash'. 

    If we isolate the circulation from the 'headwind', the following conceptual picture emerges:

     


    The wings of the airplane divert up-moving air down. A large portion of this diverted air is located above the airplane.  How do the airplane wings manage to divert all this air from far above? This is because air consists of erratically moving molecules that tend to spread over the full volume available. When the air in the layer just above the wing is pushed down, it allows the air molecules above this layer to get in. As a result higher air layers get accelerated down. This creates room for air molecules in still higher layers to move in, and so on. All of this pulling in of air molecules causes the pressure to become lower above the wing. This is nothing more than a direct manifestation of Newton's laws of motion applied to the accelerated air. It is referred to as Bernoulli's principle: the downward acceleration of the air circulating above the wing generates lift. 

    Exactly the same considerations apply to the volume of air that reaches ground level and gets diverted upwards. The net effect is that the full airplane weight rests on the surface of the earth. And yes, this is observable; more about this later!

    This picture of air circulation as the direct cause of lift dates back more than a century. At that time, the German mathematician Martin Wilhelm Kutta and the Russian scientist Nikolai Zhukovsky independently developed the mathematics of circulating flows that allows us to predict the lift of wings. 

    As should be clear from the above, anything that generates a backspin air circulation around you, can serve as wings to keep you in the air. Airfoil shapes provide optimized wings, as such shapes tend to minimize the drag for a given amount of lift. But you don't need this particular shape to keep you in the air. An upside-down airfoil works as well, and so does a tilted barn door. All of these create circulation by diverting air in a downward direction. Perhaps the cleanest manner to generate a backspin air flow is a rotating cylinder. And yes, these make a viable, albeit inefficient and impractical, wing design (see figure). 





    And in case you wonder: exactly the same circulation considerations apply to rotating wing devices, also known as helicopters. It is air circulation that keeps helicopters in the air, with the only difference with the above being that the air circulation pattern generated by a helicopter is more symmetrical than that of a fixed-wing device.

      


    Strangely enough, many people who perceive difficulty in accepting air circulation as the direct cause of lift for fixed-wing devices, happily accept air circulation as the direct cause of lift for helicopters.


    Footprint Of An Airplane


    What goes up must go down, and what comes down must go up again. This credo applies in full to the air circulation that keeps planes in the air. So the lift exerted on the airplane is balanced by an equal downward force on the earth. In other words, whether the airplane is grounded or in the air, it has a footprint that supports  its full weight. 

    How large is the footprint of an airplane in flight?

    The work of Kutta and Zhukovsky provides the answers. The footprint of an airplane cruising at a height much larger than its size can be calculated using their circulation theory. The answer will surprise you. The footprint of an airplane in flight is large. In fact, for a flat earth it would be infinitely large. That is to say: no matter how far removed from the airplane, any area of earth would feel some of the weight of the airplane. 

    We can make this more precise. Have a look again at the second figure from top.  It visualizes the footprint of the airplane at height h as resulting from a cone extending from the airplane. It appears that a portion h/R of the weight of the airplane rests on earth surface further than distance R away from the airplane (represented in the figure as the ground area outside the cone). So although the footprint extends indefinitely, the pressure exerted by this footprint drops with distance. 

    The large extend of the footprint makes the force exerted by the plane's footprint on a given surface very small. A most fortunate consequence as a half a million lbs Boeing 747 with a small footprint would crash anything underneath. If you do the math, a Boeing flying at only 500 meter (1,500 ft) height will create a 1 Pascal  (0.00001 Atm or 0.00015 Psi) pressure underneath. When flying at higher altitude, this pressure drops to even lower values.

    With modern micro-barometers such as those used to monitor and record infrasound resulting naturally from earthquakes, volcanoes, lightning, sonic booms and nuclear explosions*, the footprint of airplanes flying high up in the air can be observed. In this month's edition of NTVN, the Physics Journal for members of the Dutch Society of Physicists, atmospheric scientist Peter Siegmund and geophysicist Laslo Evers report on such measurements. Using an array of microbarometers laid out at surface, they obtained transient pressure readings from flying airplanes in line with the Kutta-Zhukowsky description of flight. They managed not only to detect airplanes and determine their position, the measurements even allowed them to weigh the airplanes in full flight!

    A nice application of the fact that barometers measure the weight of the column of air above it. The column of air including birds and airplanes that is.


    Notes
    *  The Comprehensive Nuclear-Test-Ban Treaty Organization Preparatory Commission uses infrasound recording with microbarometers as one of its monitoring technologies 

    Comments

    vongehr
    Nice article. It does not quite resolve the source of the difficulty that people have though in my eyes, and circulation as the main culprit is misleading. The circulation is after all only there to get the air back where it was before - surely the plane or helicopter and especially rocket fly just fine without anything coming back up (at least if there are different boundary conditions). See, the helicopter, even in your picture, pushes only down while the upward movement happens far away in order to balance. So no problem, and indeed, as you state, people do not see a problem - not surprising. The nasty thing with the airfoil on the plane (and locally around those of the helicopter of course too) is, as in your picture and gif, that it seems to actually pull up about as much as it pushes down. Pulling stuff up results in a downwards force - so even if the balance is positive, it seems like a whole lot of wasted energy. Explaining this with just looking at a selected part of the wing, like the upper one toward the back, seems suspiciously dodging the issue. Especially adding erratically moving molecules is more misleading than helpful, as the air is pulled upwards long before the wing arrived to remove anything for example. The mean free path of the molecules is too low to make this the proper description. A high vacuum turbo pump is described with molecules, but those do not have airfoils:


    What a load of codswallop. It's the same argument that rockets work by "pushing against the atmosphere"... funny that they work in a vacuum. If, somehow, there was no earth there, airfoils would still generate lift.

    vongehr
    Well, that is the impolite version of my "especially rocket fly just fine without anything coming back up (at least if there are different boundary conditions)". Totally uncharacteristically, I have to defend Johannes here though. The earth is usually there and the point about the footprint even being measurable a very interesting one to make.
    Funny. Sascha, didn't you notice Anonymous was attacking you?
    Introducing rockets is just confusing the issue. Anyone who understands aerodynamics can attest wing lift = circulation. This is an exact mathematical identity. The fundamental reason why helicopters can fly is no different from the reason why airplanes can fly. Rockets are a different beast.

    vongehr
    No Frank - anonymous and I agree that the earth as something that is pushed against is unnecessary in order to explain flight. That is why we both mention the rocket.
    anonymous and I agree that the earth as something that is pushed against is unnecessary in order to explain flight
    What about a turtle, then?






    The earth is usually there
    Unlike the moon, apparently :P

    Seriously, there needn't be any closed flow-lines around the plane. Think of a hovering helicopter but take away the ground and you have a hovering fan taking air from above and sending a jet downwards never to be seen again. Of course there is mathematical circulation (curl v) in the air flow but the flow lines needn't join up in closed loops like the arrows imply.

    I think that's where people are getting confused.


    Johannes Koelman
    I'd leave out the word "getting" in that last remark... Lots of people seem to think that the circulation as described by the KJ theory is located around the wings. That is simply a fallacy.
    Put differently: How far above earth does a helicopter need to hoover to render the 'no return necessary' statement correct? If one wishes to get rid of certain boundary effects, one has to come up with a credible mathematical limit in which these boundary effects vanish. For a stationary hovering helicopter you will have difficulty achieving an earthboundary-free limiting behavior.
    For a stationary hovering helicopter you will have difficulty achieving an earthboundary-free limiting behavior.
    This is a model, not a practical experiment!
     
    For all I know, there may, in fact, be return flow, but a hovering helicopter can still be analysed without including the earth.
     
    There's still circulation even if the flow lines don't join up in big loops.





    Johannes Koelman
    "a hovering helicopter can still be analysed without including the earth"

    That is too Zen for me.... hovering with respect to what?

    I agree that the lift of a wing can be analysed without considering earth. My whole point here however, is that this picture is not complete. It is relevant, and observable, that it is not turtles/air all the way down... In the end the whole pile (airplane +air) rests on earth.

    By the way: your point about 'curl' vs 'closed loops' is relevant. The circulation I am talking about (the movement that remains after subtracting the uniform flow) is 'pure curl'. Hance the statement "If we isolate the circulation from the 'headwind', the following conceptual picture emerges" that accompanies the circulation figure.
    Anonymous needs to brush up his reading skills. Nowhere I see stated that lift requires the presence of earth. Lift requires circulation (or 'backspin' airflow as the column states). This is entirely correct. And yes, in steady state this circulation extends indefinitely, which causes earth to feel a force opposite to the lift experienced by the airplane. This is also correct, and rather trivial if you think about it. Interesting to see this being verified experimentally.

    That's a load of crap anonymous. Rockets engines don't need vacuum to generate thrust. Every child knows that.
    (Sorry folks... I just love to throw a straw man at a straw man!)

    Space is holding therefore pushing against the atmosphere

    rholley
    This would seem to imply that if, for example, a plane were sitting in its usual posture on top of a tower, and receiving a gale from behind, it would actually experience downward thrust.  Am I right?

    In Wikipedia I read:
    Bernoulli's principle can be derived from the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of mechanical energy in a fluid along a streamline is the same at all points on that streamline. This requires that the sum of kinetic energy and potential energy remain constant. Thus an increase in the speed of the fluid occurs proportionately with an increase in both its dynamic pressure and kinetic energy, and a decrease in its static pressure and potential energy.
    How do pressure and energy link up with each other?

    At many colloquia and conferences, I would be sitting there while the presenter (usually with a PowerPoint) would say they had used a Runge–Kutta method. Now this puts one of the faces to the name.

    And here’s an interesting link to the 921-V.

    Robert H. Olley / Quondam Physics Department / University of Reading / England
    This would seem to imply that if, for example, a plane were sitting in its usual posture on top of a tower, and receiving a gale from behind, it would actually experience downward thrust. Am I right?
    It all depends on the wing and the angle of attack.

    In the top two pictures the wing's section provides the lift so it will provide lift even with a wind from the rear. Usually airfoils are not so symmetrical so they work better in one direction than the other but the principle remains. The bottom two pictures show the wing with additional angle of attack. In this case, a wind from behind causes downthrust. 

    A nice application of the fact that barometers measure the weight of the column of air above it. The column of air including birds and airplanes that is.
     But not balloons. :P
    Yay! Something I can usefully contribute to. While I am an amateur geologist, I am also a fully professional fluid dynamicist.

    In steady state, with a ground, then yes you have full circulation. At startup (for a very small amount of time), and without a ground, you do not. You are simply pushing against the air.

    The comment that the circulation implies that upward moving air reducing lift is false. What is happening is that the Earth is pushing the air back up, rather than the aerofoil pushing it back down. So, the air is merely a working fluid acting as a medium for transferring momentum (or force, if you prefer, seeing as in level flight there is no vertical movement), between solid ground and the wings. At startup and stopping, this cycle is broken, and power is taken to set up this vortex, so the force is resulting in an acceleration of fluid rather than a restoring at the Earth.

    Sacha's comment about wasted energy is true. It's called induced drag, and is represented by the energy given to the air to enable that momentum transfer. It's also why induced drag is inversely proportional to forward velocity squared. Interestingly winglets at the tips of the main wing do not reduce this component of induced drag, they simply reduce another vortex, this one rather uselessly spinning the air along the axis of motion. Induced drag is a bit harder to beat.

    Finally, in a hypothetical infinite air volume, a aerofoil would still generate lift, and the result of this steady state force would be more and more air moving downwards at a fixed velocity. In the long run viscosity would act to reduce the speed of this jet of downwards moving fluid and increase it's mass, or cross section if you prefer. Rather like a lossy electrical transformer.

    Johannes Koelman
    How does a 'fluid dynamicist' get interested in rock? That stuff takes ages to flow!
    I agree with everything you say in your comment. It is interesting to see how many people condemn the earth being present as an (unwanted?) side-effect. Yet earth takes the full momentum transfer of the plane, no matter how far away positioned from the plane. Boundary effects that don't drop with distance are tricky. With such boundary effects present, a hypothetical infinite universe uniformly filled with air provides at best a degenerate case less suitable for didactical purposes. 

    Ignoring earth in any finite flight geometry effectively reduces the lift analysis to transient flight conditions. I personally feel safer in a plane when steady analysis demonstrates steady state flight to be feasibe. ;) And for transient flight we obviously don't need wings. A simple bullet suffices.


    The remark made by one of the commenters that the upward moving air reduces lift is indeed another fallacy. Newton tells us that turning this airstream into a more horizontal stream of air does create lift.  
    blue-green

    “rather like a lossy transformer”….. electrical analogs are cool.

    Surely DARPA knows how this is one instance in which the presence of a low-flying aircraft cannot be cloaked. Drones would be hard to detect because they don't weigh much. I do not know what the time delay would be for detecting a fully-loaded stealth bomber skimming over the land.

    There are rotating gradiometers (originally by Robert Forward at Hughes Laboratories) that can detect a plane's concentrated mass (or a submarine ...). Gravity is one of those fundamental things (unlike electricity) for which there can be no cloaking …. There is no way to create an antigravity field to cancel out the gravity of a concentrated mass …. Furthermore, there are higher order terms in the Riemann tensor to potentially measure directional stresses and pressures ….

    Perhaps, someone here (or there or in DARPA) believes there is indeed stealth technology to hide gravity.

    Maybe our Hammock Physicist can reteach aerodynamic lift in terms of Verlinde's entropy gradients … and come to the conclusion that a perfectly stealthy flying machine would violate the 2nd Law of Thermodynamics.

    Johannes Koelman
    "Gravity is one of those fundamental things (unlike electricity) for which there can be no cloaking.... [..] Maybe our Hammock Physicist can reteach aerodynamic lift in terms of Verlinde's entropy gradients … and come to the conclusion that a perfectly stealthy flying machine would violate the 2nd Law of Thermodynamics."


    Spot on! One thing is for sure: nothing can hide gravity, not even a black hole! And you may change the word "gravity" into "entropy" and the statement remains correct.

    Wait a second... how did the subject of airplanes get us into discussing black holes? ;)
    UvaE
    I enjoyed both the article and the NASA links. For the minority of readers who don't realize it, the momentum analysis at least in the form of Navier Stokes equations is used a lot in the aeronautics industry to estimate drag.

    'Wish you had written this when a friend of mine at Bombardier(business jets) was trying to describe his work to me!

    In the long run viscosity would act to reduce the speed of this jet of downwards moving fluid and increase it's mass, or cross section if you prefer.

    Interesting thought. Any idea what the quasi-equilibrium looks like?
    Not completely sure but let's give it a bit of a thought experiment:

    Observations:

    1. There would be a wavefont moving at Mach 1 of air which contains some of the downwards momentum from the force on the helicopter rotor.
    2. Basic momentum consideration. If the helicopter is in a stable hover, the sum of the downwards momentum of all the air would be the net weight multiplied by the time hovering.
    3. The initial jet speed of air moving downwards isn't high enough for a shear surface at the edge of the cone. You'd probably have some kind of torus of rotating fluid outside the jet, local to the helicopter that's relatively stable. That would contribute to lift as it would be constantly enlarging, again due to viscosity.
    4. The radius of the jet cone as you travel downwards. Tricky this one. My guess is that it goes with a power a bit higher than one but if someone knows how to do the appropriate limit and find out for sure I'd love to know. My reasoning is that the local reynolds number of the cone is getting lower and lower the further down you go, so that a higher proportion of the momentum is lost per unit distance as viscosity becomes more important. I'd equate that with a faster enlarging cone.

    I'll leave it there for better minds I think.

    Surely DARPA knows how this is one instance in which the presence of a low-flying aircraft cannot be cloaked. Drones would be hard to detect because they don't weigh much. I do not know what the time delay would be for detecting a fully-loaded stealth bomber skimming over the land.
    A pressure wave would propagate at the speed of sound but this is just air being deflected. There's no significant pressure unless a planet gets in the way to deflect the down-draught. So the movement is going to hit the ground at a fraction of the wing velocity.

    Not sure about "no place to hide". Maybe with a big enough sensing array, but for a detector at a single location, it might be possible to angle the wings divert the down jets to miss the object of interest that you are flying over. Like this:
     

    Well, maybe not as extreme as that :)
    rholley
    unless a planet gets in the way
    ?????

    Sudden realization.  I presume this is not the case of a mistyped plane flying underneath, but simply referring to good old Terra Firma.

    Re! (One note up on Homer.)
     
     
     
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Re! (One note up on Homer.)

    Quote: This exchange of momentum is realized by the wings of the airplane directing air to earth, and earth reversing this airstream back up.

    Well one of the most obvious flaws in this concept is that Aircraft can still fly (glide) when the engine fails (inc. Heli's during a power off 'Autoroation' emergency landing), while the Aircraft (inc say a 747!) is gliding it maintains a Pitch Down Attitude, as such (unlike Pitch Positive, Powered Flight) it is NOT 'directing air to earth' (downwards...)

    Gliding Aircraft do NOT simply fall out of the sky, i.e. they ARE 'flying' QED

    Murray Hay

    (with almost 9,000 hours safe gliding since 1997, and with re-training students from across the globe! The 'glider forces air downwards instructors' in the UK (and abroad) have had around 80 deaths, and thousands of injured students/pilots which goes to show just how damgerous these errors can be!)

    Really? Why would the wings of a gliding plane not be capable of directing air to earth?

    First note in my post 'while the Aircraft (inc say a 747!) is gliding it MAINTAINS a Pitch Down Attitude'

    Clearly Aircraft, including Gliding Aircraft (example the Pilot Pitching UP during the landing flare) CAN deflect air downwards, the Energy to do this is from the Kinetic Energy stored in the forward flying (gliding) aircraft, as this store of Kinetic Energy is 'burnt' the Airspeed drops... hence in Gliding flight this state can not be maintained, as this would result in a stall, hence the requirement to regain a Negative Pitch. Clearly with a Negative Pitched wing air is NOT forced down in the way it can be with Powered & Positive Pitched.flight, or the 'Flare on landing'.

    However to Maintain Airspeed during a glide the Aircraft is flown with a NEGATIVE Pitch (most Powered Aircraft are designed to adopt a slight 'nose down attitude' on failure of power) in this configuration the wing will NOT 'deflect air downwards'

    Murray Hay

    I have to disagree with your explanation of gliding I'm afraid.

    An aircraft in a glide is pitch down, sure, but this is just to provide power to compensate for drag. I remember back at uni there were a lot of questions where the first step would be to do just that to establish the overall drag. Just take the vertical velocity, multiply by weight, and you have the power generated from loosing gravitational potential energy. This is used only for compensating for drag (aircraft velocity multiplied by total drag), as in a stable glide, aircraft velocity is constant. Drag is simply the force needed to move air.

    However, in a glide the air is still deflected downwards. It has to be to conserve vertical momentum. If you were to sit on the wing and look, it would be coming in at the angle of attack, and it will leave the aircraft with a component moving downwards relative to the aircraft body. The only way to change this is to add in some additional vertical component of incoming air velocity. This is how you'd fly upside down, you would 'slip' vertically by using a lot of elevator, so that the angle of attack of incoming air relative to the wing was reversed. Much more efficiently accomplished with a camberless aerofoil. Clearly in a stable glide this isn't done.

    Conversely, sitting on the ground and looking at the gliding aircraft and the air around it, you would see air motionless well ahead of the aircraft, and well behind would would see air moving downwards and some air also being dragged along with the airplane. The power to move that air is taken from the loss of GPE, as explained above. If you pitch down more, you will accelerate until drag once again balances 'input power'. Pitch down less, and just like you say, you decrease in speed until either a stall or equilibrium is restored.

    Quote: However, in a glide the air is still deflected downwards. It has to be to conserve vertical momentum. If you were to sit on the wing and look, it would be coming in at the angle of attack

    This statment is 'fine' Only IF you are prepared to ignore not just Physics... but also ignore what ACTUALY happens in gliding flight!

    In the THREE Force gliding Aircraft, the wing need (to maintain airspeed) a Negative Pitch, in the same way things do NOT (maintain) roll UP hill...

    Note, as (inc Gliding) Aircraft normaly have Non-Symmetrical wings, the wing produces 'lift' with Negative AoA's (typicaly quoted as down to -4*) any technical discusion which chooses to fail to take this fact into account will be flawed ;-)

    Probably the Easiest Aircraft to use as a model to help students gain FACTUAL, Practual Understanding of the Physics at work during flight is a paraglider, this is as due to the soft nature of the aircraft (which can also be flown under power, i.e. as a 4 force aircraft) we can SEE the Physics at work on all the surfaces of the wing :-)

    NOTE: I can 'Fly' my paraglider when standing on a steep cross slope (wind flow 'sideways' along the ridge) the Aircraft IS (for all practical purposes) 'flying' even when it is BELOW/to the side of me, i.e. as far as the airflow is concerned there is NO 'earth below the aircraft'

    Murray Hay

    Clearly with a Negative Pitched wing air is NOT forced down
    Incorrect. The glide angle is also negative so the air flow is the same as in powered flight apart from a small tilt.

    Quote: Incorrect. The glide angle is also negative so the air flow is the same as in powered flight apart from a small tilt.

    (ref: my comment- Clearly with a Negative Pitched wing air is NOT forced down)

    There are many variations on 'gliders', however they are ALL Aircraft, so despite the variations in the Aircraft the SAME Basic Physics is at work.

    In the case of a Paraglider the pilot is in effect a suspended load, the pilot ('triangle of line sets, Front/Rear/Wing Chord) due to the Parasitic Drag at the pilot a paraglider wing is Pitched MORE down (Negative) than the wing would be without a 'suspended load', hence when gliding at Trim Speed the paraglider wing has

    1) A Negative Pitch (it points down relative to the horizon, as is the case in any gliding wing while the Airspeed is Maintained)
    2) The paraglider will often (at around trim speed and faster) maintain a NEGATIVE (range of) Angle of Attack

    NB the wing has twist across its span so can NEVER have a Single AoA!)

    I would guess I'm the only PG pilot in the world that has actualy Flight TESTED the Actual (center section of the wing) AoA (it's actualy a RANGE of AoA's across the span)

    The one obvious point is that it is NOT possible for a Three Force Gliding Aircraft to (maintain) have the 'same airflow as in POWERED (FOUR Force Aircraft) flight'..

    Basic Physics!

    Surely we need to test airplanes on a gas planet to resolve this discussion :)

    I am reminded of a teacher who specialized in teaching what are now called "people with learning difficulties". She told of a group of kids whom she had spent an entire morning teaching the principles of adding numbers together. "If this window is 4 feet away from the next one and that window is 5 feet from the next, how far is the last window from the first?" After lots of hard work they picked up the idea that distances add together. "Okay", she says "let's do some measuring. And they measure the distance between two doors: 8 feet. And the next is 6 feet. "So how far is the first from the last?" she asks.
     
    There is a long silence. Then the brightest of the class pipes up "I dunno, Miss. I suppose we'll have to measure it" and stomps off down the corridor with a tape measure.
     
    Quote: I am reminded of a teacher ...... principles of adding numbers together. "If this WINDOWS is 4 feet ..... After lots of hard work they picked up the idea that distances add together. "Okay", she says "let's do some measuring. And they measure the distance between two DOORS: 8 feet. And the next is 6 feet. "So how far is the first from the last?" she asks.

    The above quote makes a good example of the problems trying to correct many of the errors in the article/posts.... clearly core errors in posts need to be addressed BEFORE the overall problem of intrest (how wings work) can be studied/understood.

    So for me when I read the quoted (whole) post I would have to point out that the issue is not if it is 'windows' OR if it is 'doors' but rather that in both cases the questions can NOT be answered based on the supplied information...

    The reason is that even if we know the distances BETWEEN the object (1 > 2, and 2 > 3) be it doors or windows, until the WIDTH of the Object is known the Total distance from '1' to '3' can NOT be added... the answer being the sum of the gaps PLUS the width of the middle object :-)

    Murray Hay

    Hmmm... I don't think there is a discussion really. As someone stated above: with their remarks about earth not being needed, Vongehr and anonymous have created nothing more than a straw man.
    Nobody is defending the statement that a solid earth is needed to get lift on a wing. What the article discusses is that a 'backspin' air flow (circulation) is needed to lift a wing. It subsequently demonstrates that such circulation extends over unlimited distances, causing a footprint of the airplane on earth's surface. This is a fun fact that anyone interested in the physics of flying should realize, but in no way essential to get lift on a wing. Of course an airplane could in principle fly in a gas planet.

    Ref: All of this pulling in of air molecules causes the pressure to become lower above the wing. This is nothing more than a direct manifestation of Newton's laws of motion applied to the accelerated air. It is referred to as Bernoulli's principle: the downward acceleration of the air circulating above the wing generates lift.

    (taken from the top of the page)

    Clearly in the case of MOST Aircraft, i.e. flying well below the speed of sound there is a MAJOR Error in the above quote.

    Simply reading Bernoulli shows what Bernoulli actualy states, AND gives the REQUIRMENT for Bernoulli to apply to the study of Aircraft Flight.

    For Bernoulli to apply TOTAL Pressure (in the case of flight, Atmospheric Pressure at the altitude of the wing) MUST be a Constant, hence the TOTAL Pressure (certainly fully accepted while below Mach 0.3/250kts, air starting to actualy compress as the wing approaches Mach 1) remains Constant, while the STATIC Pressure reduces, both Above AND Below the wing due to the Dynamic Pressure (airspeed) passing the wing/wing passing through the air...

    MH

    Quote: Air moves up in front of the plane (from the start article)

    While there are a number of 'safety system' (stall warning) designs, and many light aircraft/gliders/etc. wings have no stall warning system, one design involves a small 'flap' sticking out of the lower part of the leading edge of the wing... in Normal (safe!) flight the air flow keeps this flap (switch) down as the airflow at that point is (relative to the wing) 'back' and 'down'...

    However as the aircraft starts to approach Stall configuration, then, and ONLY then, the flap/switch changes to 'up' warning the pilot (via a buzzer etc.) that the Air is NOW 'moving up at the front of the wing' .... So in Aviation it is NOT a good thing to have air 'moving up in front of the plane' (relative to the Aircraft), rather air should 'split' to move BOTH up AND down and so pass both over and under the wing, front to back :-)

    MH

    Murray - I don't know why you want to waste everyone's time, but no-one is interested in your wall of noise. You said a gliding plane does not force the air down, several of us have pointed out that it does, indeed it must, and also bent over backwards to try to explain to you where you may be misunderstanding. Clearly our efforts are wasted on you. Enough is enough. Enjoy flying those magic carpets.
    Quote: You said a gliding plane does not force the air down, *****several of us have pointed out that it does*****, indeed it must, and also bent over backwards to try to explain to you where you may be misunderstanding

    -----------------------------

    Stating that a gliding wing 'does force air (to the ground)' is NOT by any stretch proof, it is simply you stating your subjective view, I note your view was stated with no background support from established Science NOR any evidence of support from practical testing.

    -----------------------------

    In my particular field of expertise the errors you are restating meet with approval from those who have a high death rate and a VERY High Injury rate, so this stuff is clearly critical to safety in aviation!

    Back to the 'forces air down' (gliding case), it is usefull to look back at a earlier post in that the 'diffrence is simply a tilt in axis (or similar wording) between gliding and Powered flight....

    Ok, Basic Physics the starting point in looking a a proble is to identify a FIXED feature... in the case of aviation this is Gravity,

    In Gliding flight:

    Gravity Vertical
    Drag 'backwards' (in truth, Back and slightly Upwards, in teaching this is mentioned in practice the Key is 'Back'
    Lift (from the airflow PAST, i.e. past BOTH the upper and lower surface, from front to rear of the wing) 'Lift' is in fact the SUM of the total of all 'lift' forces (Inc. simply Newtons 3rd acting on the under side of the wing in a straight glide etc.) In practical Teaching and Flying the KEY is that the student understands that in a STEADY STATE Gliding Flight the Vector Sum MUST = Zero... i.e. there are no unbalanced forces, hence the Aircraft is NOT accelerating (it IS in a Stead State of Flight)

    As we know that Gravity MUST be Vertical, and we know that Drag MUST be 'Backwards' (Back with a bit of 'up') yjr RESULTANT of Gravity plus Drag MUST be 'Down & Back'...

    As Newton told us with F = M x A

    And we know in a STEADY State the Aircraft is, by defination, NOT accelerating, we know the value of A to be zero

    As the Mass of the Aircraft still exists in flight, and Acceleration (Steady State Flight) is zero, we get for a 100kg Aircraft, then we find F = 100kg x zero, as any number x zero equals zero we know that there can be NO 'Unballanced Force' acting on the aircraft.

    Vector addition Gives us the resultant of Gravity + Drag, hence we know that LIFT MUST be of Equal Value to the value of the G + D Resultant, BUT MUST be 'opposite' in sense (direction)

    So in the gliding Aircraft 'Lift' (the Sum of ALL the 'lift' vectors) is Up AND Forwards

    Ok that the Basic picture of Gliding Aircraft

    NOTHING in Physics (unlike your personal beliefs!) so far indicates any reason for the gliding (unlike, Pitch Positive POWERED Flight) wing to force air to move in a circular path down to the ground and back up to the front of the aircraft!

    MH

    Part Two, moving on to Four Force/Powered Flight/where there is SOME limited downwards deflection of the air!

    In Powered Flight of a paragliding wing (a Great Aircraft to use when studying the Physics at work :-)... there is a Significant Diffrence from gliding flight..

    Ok back to one of the Basic bits of Physics in (un-powered) Gliding Paragliders are 'three force aircraft'

    As the Pilots Mass acts as a 'suspended load' we know (remember the pilot has DRAG as well as Gravity acting on the mass) the Paraglider Pilot MUST be positioned Below & Behind (slightly) the wing... as a result the Pitch of the Wing is MORE negative than it would otherwise be... BUT... in POWERED PG flight the thrust is applied (a 'paramotor') at the PILOT... so in Four Force PPG flight the Pilots mass is Below and IN FRONT of the 'CoP' of the wing... this results in the Aircraft being significantly MORE Positive than it would otherwise be...

    NOTE: In both (3 force & 4 force) example of PG flight Gravity remains the fixed refrence, this shows clearly the errors in the idea of (quote) 'It's simply a case of tilt of the axis'...

    So once again IF you wish to gain understanding of the Physics of Flight a PG is PERFECT as a study tool :-)

    MH

    I note your view was stated with no background support from established Science
    You're right. Everything you say is 100% correct.
    Goodbye, Murray.