I'm currently studying for my PPL and one of the accepted textbooks contains the following disclaimer at the end of the Principles of Flight section on lift:

It is important to note that the forgoing explanation of lift, and its reliance on Bernoulli's theorem, is very much the 'classical' theory of lift production and the one on which the exam questions are usually based. There are differences of opinion amongst scientists on the subject....[snip]

The same book previously also describes the venturi theory which NASA discredits.

Additionally, one of my previous CFI's told me that during a previous successful job interview he had been asked to explain lift and had merely responded with "Which theory would you like me to cover?"

On the contrary - we must have an excellent understand of some components because of the way we're able to design and build such stable (and unstable when we want) aircraft. Plus, I see some absolutely incredible mathematics described on this website which seek to accurately answer complex questions.

So, my question isn't how is lift generated -it's how complete is our understanding? Which bits are still in dispute, and which bits are fully accepted?

For those interested the book in question is

"AFE - The Private Pilots Licence Course - PPL4

Principles of Flight, Aircraft General Knowledge, Flight Performance and Planning"

ISBN: 978-1-874783-23-7 https://www.afeonline.com/shop/private-pilot-s-licence-course-ppl-4-principles-of-flight-airc.h

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    $\begingroup$ have you had a look at aviation.stackexchange.com/q/8281/1467 and aviation.stackexchange.com/q/466/1467 ? $\endgroup$
    – Federico
    Commented Oct 6, 2015 at 11:18
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    $\begingroup$ We have a very good understanding of lift $\endgroup$
    – Ethan
    Commented Oct 6, 2015 at 11:53
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    $\begingroup$ @Simon, it is not lack of understanding. Physicists understand fluid dynamics pretty well and lift is just a consequence of it. It is lack of simple explanation and that is simply because lift is surprisingly complex phenomenon. $\endgroup$
    – Jan Hudec
    Commented Oct 6, 2015 at 13:12
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    $\begingroup$ Note that in engineering contexts, it's perfectly acceptable to have two models describe the same phenomena. This is not a lack of understanding, but simply two ways of looking at the same thing. For example, compare calculating the impact velocity of a dropped mass by looking at an energy balance from potential to kinetic energy, or looking at the acceleration over a certain distance. In the end, it's the same equation $\endgroup$
    – Sanchises
    Commented Oct 6, 2015 at 13:21
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    $\begingroup$ @ryan I believe the real question being answered here is "How complete is the understanding of lift among Aviation.SE members". :) $\endgroup$
    – Sanchises
    Commented Oct 8, 2015 at 8:09

8 Answers 8


Short answer: Yes, our understanding of lift is complete, but solving the equations for some practical cases needs more resources than what is technically sensible.

Lift is a matter of definition

First of all, lift is only one part of the aerodynamic forces. It is the component normal to the direction of airflow. Since the aircraft will distort the local flow around itself, this direction is taken ideally at an infinite distance where the air is undisturbed.

The other component is, of course, drag. It is defined as the part of the aerodynamic forces parallel to the direction of airflow.

The aerodynamic forces are the sum of all local pressures, which act orthogonally on the local surface of the airplane, and the shear forces, which act parallel to the local surface.

When aerodynamics was researched first, electric fields were new and exciting, and the same equations which help to calculate electromagnetic forces could be used to calculate aerodynamic forces. Therefore, abstract concepts like sources or sinks were used to explain aerodynamics. This made it not easier to understand, and many authors tried to find simpler explanations. Unfortunately, they were mostly too simple and not correct, but the next generation of authors would mostly copy what had been written before, so the wrong concepts were still bandied about.

To get to the bottom of it, it might help to look at lift at a molecular level:

Every air molecule is in a dynamic equilibrium between inertial, pressure and viscous effects:

  • Inertial means that the mass of the particle wants to travel on as before and needs force to be convinced otherwise.
  • Pressure means that air particles oscillate all the time and bounce into other air particles. The more bouncing, the more force they exert on their surroundings.
  • Viscosity means that air molecules, because of this oscillation, tend to assume the speed and direction of their neighbors.

All three contributions are well understood, and with the Navier-Stokes equations they can be completely mathematically expressed. What is still improving is our ability to solve these equations, and in turbulent flow the characteristic length required to capture all effects is so small that it is practically impossible to solve those equations fully with finite time and resources.

Flow over the upper side of the wing

Now to the airflow: When a wing approaches at subsonic speed, the low pressure area over its upper surface will suck in air ahead of it. See it this way: Above and downstream of a packet of air we have less bouncing of molecules (= less pressure), and now the undiminished bouncing of the air below and upstream of that packet will push its air molecules upwards and towards that wing. The packet of air will rise and accelerate towards the wing and be sucked into that low pressure area. Due to the acceleration, the packet will be stretched lengthwise and its pressure drops in sync with it picking up speed. Spreading happens in flow direction - the packet is distorted and stretched lengthwise, but contracts in the direction orthogonally to the flow. Once there, it will "see" that the wing below it curves away from its path of travel, and if that path would remain unchanged, a vacuum between the wing and our packet of air would form. Reluctantly (because it has mass and, therefore, inertia), the packet will change course and follow the wing's contour. This requires even lower pressure, to make the molecules overcome their inertia and change direction. This fast-flowing, low-pressure air will in turn suck in new air ahead and below of it, will go on to decelerate and regain its old pressure over the rear half of the wing, and will flow off with its new flow direction.

Note that lift can only happen if the upper contour of the wing will slope downwards and away from the initial path of the air flowing around the wing's leading edge. This could either be camber or angle of attack - both will have the same effect. Since camber allows for a gradual change of the contour, it is more efficient than angle of attack.

Flow over the lower side of the wing

A packet of air which ends up below the wing will experience less uplift and acceleration, and in the convex part of highly cambered airfoils it will experience a compression. It also has to change its flow path, because the cambered and/or inclined wing will push the air below it downwards, creating more pressure and more bouncing from above for our packet below the wing. When both packets arrive at the trailing edge, they will have picked up some downward speed.

Airfoil in wind tunnel with smoke trails indicating flow

Behind the wing, both packets will continue along their downward path for a while due to inertia and push other air below them down and sideways. Above them, this air, having been pushed sideways before, will now fill the space above our two packets. Macroscopically, this looks like two big vortices. But the air in these vortices cannot act on the wing anymore, so it will not affect drag or lift. See here for more on that effect, including pretty pictures.

Lift can be explained in several, equivalent ways

Following the picture of a pressure field outlined above, lift is the difference of pressure between upper and lower surface of the wing. The molecules will bounce against the wing skin more at the lower side than at the upper side, and the difference is lift.

Or you look at the macroscopic picture: A certain mass of air has been accelerated downwards by the wing, and this required a force to act on that air. This force is what keeps the aircraft up in the air: Lift.

If you look at the wing as a black box and only pay attention to the impulse of the inflowing and outflowing air, the wing will change the impulse by adding a downward component. The reaction force of this impulse change is lift.

Either way, you will arrive at the same result. By the way: Most of the directional change happens in the forward part of the airfoil, not at the trailing edge!

streamlines around a wing and direction of impulses

Supersonic flow

When the aircraft moves faster than pressure changes propagate through air, the changes in pressure are no longer smooth, but sudden. The aircraft will push the air molecules aside, producing a compression shock. Behind the shock front pressure, temperature and density are higher than ahead of it, and the increase is proportional to the local change in flow direction. The incremental pressure change $\delta p$ due to the aircraft hitting air with an incremental angle of $\delta\vartheta$, expressed in terms of the undisturbed flow with the index $\infty$, is proportional to the change in the streamlines: $$\delta p = -\frac{\rho_{\infty}\cdot v^2_{\infty}}{\sqrt{Ma^2_{\infty} - 1}}\cdot\delta\vartheta$$

Gas pressure on a molecular level is the number and severity of particle collisions. The air molecules experience more collisions on the downstream side of the shock, since air pressure is higher there. The average direction of the additional collisions is indeed orthogonal to the shock, because it is the boundary between blissfully unaware molecules at ambient pressure ahead of the shock and their bruised brethren downstream which have just crossed that boundary. Once a molecule has passed the shock, the collisions are coming again equally from all sides and its speed does not change any more.

If the surface curves away from the local flow direction, the air produces an expansion fan which re-sets the old pressure and density values when the air flows again in its original direction.

Pure supersonic lift is only a matter of the angle of incidence, and any local curvature of the wing will not change overall lift (but increase drag). Now the total aerodynamic force is normal to the wing, and drag will become proportional to the angle of incidence. In hypersonic flow you will get good results with the venerable impact theory first formulated by Isaac Newton.

Separated flow

This happens when the air molecules are no longer able to follow the contour of the aircraft. Instead, you get a chaotic, oscillating flow pattern which is very hard to compute exactly. This is really the only part of aerodynamics which cannot be predicted precisely, even though the effects are well understood. Separated flow will produce lift, too, but less than attached flow. In delta wings, this separation is produced on purpose to create what is called vortex lift.

  • $\begingroup$ I read somewhere that most of the lift is produced not just at the front, but front of the upper surface. Is that correct? If so, it doesn't seem consistent with your explanation of lift at the molecular level (that it's due to more molecules bouncing on the lower surface than the upper). That would suggest that the lower surface is what produces most of the lift, since that's where the net-positive bouncing happens. $\endgroup$
    – yshavit
    Commented Jun 25, 2018 at 6:28
  • $\begingroup$ @yshavit: Yes, suction is just less pressure on one side. Now it depends what you see as the direct source of lift. You can 1) either vote for suction, or 2) for impulse exchange, or 3) for pressure. All three views are equally defensible - it depends on your point of view. $\endgroup$ Commented Jun 25, 2018 at 6:55
  • $\begingroup$ Hm, okay. The reason I asked is that at the molecular level, "suction" is not a force. The only way an air molecule could suck the wing molecule up is if you had an attractive electrostatic force between them. So if the mental model is pressure, then it's really a case of the wing being pushed up from the bottom, not pulled up from the top. But it sounds like you're saying that's a valid way to think of it? $\endgroup$
    – yshavit
    Commented Jun 25, 2018 at 21:39
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    $\begingroup$ @yshavit: Yes. The wing pulls the air above itself down as much as it pushes the air below itself down, too. Now, on the molecular level, you are right to say that the air above the wing is only pushed down by pressure from above because the wing creates a barrier to pressure from below. But that is suction, only in other words. $\endgroup$ Commented Jun 26, 2018 at 13:24
  • $\begingroup$ Thanks! That's the one bit that's always confused me -- the notion that lift is generated more on the top of the wing than on the bottom. I think it makes sense now. $\endgroup$
    – yshavit
    Commented Jun 26, 2018 at 13:39

From this paper:

The principle of equal transit times holds only for a wing with zero lift. [!!]


The air passes over the wing and is bent down. Newton’s first law says that them [sic] must be a force on the air to bend it down (the action). Newton’s third law says that there must be an equal and opposite force (up) on the wing (the reaction). To generate lift a wing must divert lots of air down.


So how does a thin wing divert so much air? When the air is bent around the top of the wing, it pulls on the air above it accelerating that air downward. Otherwise there would be voids in the air above the wing. Air is pulled from above. This pulling causes the pressure to become lower above the wing. It is the acceleration of the air above the wing in the downward direction that gives lift.

We (those of us reading this) can conlude the following about our (humanity's in general) understanding of lift:

  • We certainly understand it well enough to design aircraft, and there may be overlap with this knowledge in other areas, such as maybe wind-powered generator design.
  • Many believe we have a fairly complete understanding of lift.

The second bullet is not at all in impune the excellent (and challenging!) work done through history in fluid dynamics, aeronautical physics, and aeronautical engineering. It is merely to allow for the possibility of future paradigm shifts in our understanding of those topics, even if those shifts do not affect common design practice or practical discussions of lift. A historical example of that last point would be General Relativity as a paradigm shift in our understanding of gravity, while Newtonian gravitational theory was still used for the moon program and is still widely taught and used for situations not requiring extreme precision.

In addition to links in Frederico's comment, see also: https://physics.stackexchange.com/questions/290/what-really-allows-airplanes-to-fly


This NASA page discusses the controversy of "Bernoulli versus Newton" and concludes that both explanations of lift are "correct" and that there is even more to it. The Euler Equations and the Navier-Stokes Equations are mentioned. This page in the same series on NASA's site suggest that lift is fairly well understood by experts, but is poorly explained in the majority of popular sources:

There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments. To help you understand lift and its origins, a series of pages will describe the various theories and how some of the popular theories fail.

Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.

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    $\begingroup$ The edit is quite relevant. No physicist would doubt Navier-Stokes. The problem is that Navier-Stokes is fiendishly difficult and far too general for something as simple as lift. (yes, lift is simple in comparison to Navier-Stokes). And even so, a good physicist knows deep in his heart that Navier-Stokes is still wrong because it assumes fluids are not made up from molecules. The theory breaks down at microscopic scales. $\endgroup$
    – MSalters
    Commented Oct 6, 2015 at 13:40
  • $\begingroup$ @MSalters, no, lift is not simple in comparison to Navier-Stokes. Explaining lift requires both inertia and viscosity and Navier-Stokes are just expression of those (plus the appropriate conservation laws). $\endgroup$
    – Jan Hudec
    Commented Oct 6, 2015 at 16:59
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    $\begingroup$ @MSalters, the fact that Navier-Stokes breaks down at microscopic level does not make it wrong. It merely makes it an approximation. All thermodynamics is like that. $\endgroup$
    – Jan Hudec
    Commented Oct 6, 2015 at 17:00
  • $\begingroup$ Missing "This paper" link on comcast, see web.archive.org/web/20140330015638/home.comcast.net/… Also note that Navier-Stokes usually provides no explicit solutions, and instead we rely upon NS-based numerical simulations (and adding more supercomputers, to increase resolution and reduce errors.) "If you can't explain it to your grandmother, it means that you don't actually understand it." Heh. So, we'd finally have full understanding, only if we could successfully explain it to little kids, and to non-tech adults. $\endgroup$
    – wbeaty
    Commented Nov 3, 2023 at 9:22

The problem here is that "correlation does not imply causation". Neither Bernouilli's principle nor Newton's laws of motion explain lift. Both of them give valid methods of calculating the lift force from the air flow pattern around the wing, but neither of them explain why the flow pattern is what it is.

Ideas like "equal transit time" at least try to give a reason "why," but experiments which visualize the flow pattern with smoke demonstrate that is just wrong.

The best "one-word explanation" of what causes lift is the viscosity of the air. Viscosity is the reason why there can't be any discontinuities in the overall flow pattern*. In particular, the air velocities on either side of the relatively sharp trailing edge of the wing have to be the same, otherwise the effect of viscosity at that point would propagate upstream through the air (at the speed of sound) and change the global flow pattern.

If there were no viscosity, no wing of any shape would produce any lift, or any drag force.

*Let's limit this discussion to subsonic flows. Introducing shock waves into the airflow makes a "hand-waving" non-mathematical discussion more complicated, but it doesn't invalidate the essential point I'm trying to make.

  • $\begingroup$ Well, you also need inertia, otherwise the flow would not continue downward in the direction of the trailing edge. $\endgroup$
    – Jan Hudec
    Commented Oct 6, 2015 at 17:49
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    $\begingroup$ Do you have a source for the claim that without viscosity, there could be no lift? Superfluid helium has a viscosity of zero, would a sheet slicing through it at an angle not experience any lift? $\endgroup$
    – Roman
    Commented Oct 7, 2015 at 12:51
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    $\begingroup$ I would add that the Euler equations, which assume inviscid fluids, certainly do allow one to approximate the lift generated by a body in flow. This is possible because the effects of viscosity are mostly confined to the infamous boundary layer, whose effects are negligible for an aerofoil in air at modest angles of attack. It is correct that without viscosity you will find zero drag, and it is also impossible for flow to become detached from a surface ("stall" in aircraft). $\endgroup$
    – sigma
    Commented Oct 7, 2015 at 15:45
  • $\begingroup$ @romkyns There is no net force on a body in an inviscid fluid, unless the flow contains some circulation (i.e. a vortex) around the body. But with no viscosity, there is no "simple" way to create the vortex. You can get a rough estimate of the lift on a wing by making two assumptions: (1) the air is inviscid, and (2) there is a vortex of the required strength to make the flow match up on the top and bottom surfaces at the trailing edge. Assumption 2 is the same as assuming the flow is faster over the top than the bottom. See en.wikipedia.org/wiki/Kutta%E2%80%93Joukowski_theorem $\endgroup$
    – alephzero
    Commented Oct 7, 2015 at 21:03
  • $\begingroup$ Even flow across shocks is continuous (i.e., shockwaves have finite thickness); they're just so thin that their thickness is often negligible when calculating larger-scale flow fields. $\endgroup$ Commented Oct 8, 2015 at 0:13

How complete is our understanding?

  • Complete enough to design and fly a number of complex aircraft of varying sizes, shapes and applications.
  • Complete enough to extract power using it.

On a basic level, lift is the force generated as solid body 'turns' fluid while satisfying the conservation laws. The issue is not that we don't know what lift is, but there is no consensus about how to explain it. Most of the 'theories' of lift are just models try to explain the same thing based on the points of view of the people involved.

The way the pilot views the lift is different from that of an engineer or a mathematician. For the pilot, the lift is a force that keeps the aircraft in the air (and proportional to $\rho V^{2} S$ and angle of attack, at-least till stall), while a mathematician can say that lift 'follows naturally' by solving Navier-Stokes Equation (whether it can realistically solved or not is another matter) for some conditions. Of course, this is of no practical use to either the engineer or the pilot. Both can claim (rightfully) that they are correct, while a physicist can object that NS assumes that the fluid is a continuum, while it is not the case in realty.

This is the reason for so many theories of lift. As the fluid flow is extremely complex, some simplification is done in each theory (like omitting viscosity in the Euler or potential flow theory). Based on the simplification, the theory is either useful in some (or most) of the situations or is outright wrong.

Which bits are still in dispute, and which bits are fully accepted?

Almost all 'theories' of lift accept that lift is a force and its requirements. As far as engineering goes, the issue is which bits of are necessary for the problem in hand.

For example, the potential flow theory can predict the lift as long as we are not approaching stall. After that all bets are off. There is no point in arguing about a result from a theory after using it in a situation for which it was not designed for in the first place.

This is the reason for arguments about lift. Some theories are developed to describe a particular situation (for example inviscid flow) and then applied in general, which obviously leads to confusion and dispute.

As far as engineering is considered we have enough understanding of lift to create the flying machines we need though not as much as to explain everything that happens with accuracy.


TL;DR: we can very precisely model aerodynamic forces at the micro level; we can reasonably predict behavior at the macro level by aggregating micro-level models (CFD). We don't have a universally-applicable story for why the macro level behavior is what it is.

Fuller explanation:

At the risk of being a bit pedantic, I'm going to back up a couple of steps of abstraction in order to provide a more-complete picture.

The overall aerodynamic force on a body is decomposed into vectors normal to the direction of travel and parallel to the direction of travel, which are labeled 'lift' and 'drag' respectively; they are not distinct forces in & of themselves. Aerodynamic force itself is often decomposed at a different scale into pressure & friction; for the most part, friction only contributes to the 'drag' component while pressure contributes to both the 'lift' & 'drag' components.

Trying to tell a stylized story about why the integrated pressure & friction across the entire body result in a particular net force is challenging at best, since it is affected by the idiosyncrasies of each body; various models (such as Venturi, downwash, & circulation) really just provide designers & analysts with rough rules of thumb within particular flight regimes.

This last point is more important than it appears. As soon as you enter transonic flight (a mix of subsonic & supersonic flow at the surface of the body), drag increases precipitously (standing shocks creating adverse pressure gradients). Passing through to fully-supersonic flight you find yet another set of behaviors (because the leading shock radically alters the pressure distribution on the body). Don't even get me started on hypersonic flow (where the temperature change across the shocks is enough to decompose the N2 & O2 from the air itself).

Edit Peter Kampf's answer covered most of the same topics as mine, with pictures, so I'll just add this for completeness: diagram of supersonic lift & drag


Scientifically speaking, lift is perfectly understood. Lift is merely the vertical component of force generated by a body moving through a fluid. And we know perfectly well how to calculate forces on a body moving through a fluid since the Navier-Stokes equations was published in 1822. That is to say, we know the physics of it and it has to do with the viscosity of fluids (in the case of aircraft, air).

But using the Navier-Stokes equations to design a wing is like trying to use Quantum-Electrodynamics (QED) to cook the perfect steak. Since gravity isn't involved in the perfectionness of steak, all you need to formulate a perfect steak is QED.

The Navier-Stokes equations calculate forces on a single point on the wing. Therefore you have to repeat the calculations over the whole wing to calculate lift. Over the last 190+ years mathematicians and engineers have formulated simpler algorithms to calculate the result of the Navier-Stokes equations and over the last 30 or so years we've used computers to calculate lift. However, you can see how this doesn't tell you the ideal shape to generate the aerodynamic characteristics you want. You can also see how this doesn't explain "lift" in terms a human can understand. It's all just large arrays of numbers.

Is it possible to explain lift in terms a human can understand? Maybe. We've certainly given names to how certain shapes generate certain output when subjected to the Navier-Stokes equations. Names such as "Coanda effect" and "Bernoulli Principle" etc. In the end, nature/physics doesn't care what name we give to our interpretation of the result of the Navier-Stokes equations - if calculating the equations result in a vertical force vector upwards you have lift. Maybe, like quantum physics, we'll never get a complete intuitive understanding of what lift is. But we certainly have the complete theory to explain it.

Additional note: Apart from not being helpful in helping us formulate a theory of wing design, the Navier-Stokes equations are also problematic because it's computationally expensive. For example, it's often not practical to use the Navier-Stokes equations to simulate turbulence (even though it's possible in theory). So we often take shortcuts for certain forms of simulations using other simpler but less perfect equations.

  • $\begingroup$ +1 for adressing the difference between "having a complete theory" on one side and "having an intuitive understanding" on the other. $\endgroup$
    – Sanchises
    Commented Oct 8, 2015 at 8:15
  • $\begingroup$ I'm surprised there are so many answers saying we understand it very well when we are having trouble with things at smaller scales such as hummingbirds and especially insects. $\endgroup$
    – DKNguyen
    Commented Oct 7, 2020 at 22:03
  • $\begingroup$ @DKNguyen The Navier-Stokes equations work perfectly well to simulate the flight of hummingbirds and insects. It even works well to describe smaller things like the movement of blood in arteries. However until recently (around the 1990s) we didn't have computers fast enough to do the calculations that simulate hummingbird flight (especially since the entire system depends a lot on turbulence and vortices) $\endgroup$
    – slebetman
    Commented Jan 20, 2022 at 15:22
  • $\begingroup$ @DKNguyen actually, just in the 1990s was insect flight finally understood, when a major mystery was solved by C. Ellington's team at Cambridge, using a "Robo-moth," large flapping wings to duplicate Hawkmoth flight. They discovered a new unsuspected leading-edge vortex, produced by interaction between insect body and multiple wing-flaps, which never showed up in earlier work. Calculations gave insufficient lift to explain moth flight. Wind tunnels, rather than NS equations, solved the mystery. Only NOW can we do the proper calcs. google.com/search?q=ellington+hawkmoth+robo $\endgroup$
    – wbeaty
    Commented Nov 3, 2023 at 8:52

Lift is generated because air molecules are bouncing into and rebounding from the airfoil, on both top and bottom surfaces. It is the difference in the amount of momentum transferred in these collisions that creates lift. It is, (obviously), only the velocity of the air molecules that is normal (perpendicular) to the airfoil, that produces lift.

The Bernoulli principal is true, because the TOTAL average momentum of any air molecule in incompressible (subsonic) flow is a constant. Therefore, if the velocity of the air parallel with the airfoil increases, the normal component of the velocity must decrease to keep the total constant.

So, if the air is moving faster, the normal component must be slower, and it's pressure (against the airfoil) must be lower.

So, the longer distance to travel argument is only bogus if you try to assume that it can only be generated by a asymmetrical airfoil. Other things can change the travel distance (and resultant velocity) of the air across the airfoil as well. If a symmetrical airfoil is inclined to the relative wind, then as the air flows across the airfoil on the side where the airfoil bends away from the flow, the air must travel a longer distance (to fill in the gap created by the inclination) than air flowing across the surface on the other side, where the surface is inclined into the relative wind, and must either compress (in supersonic flow) or move away from (change direction away) the airfoil (in subsonic flow).

This is because in subsonic, (incompressible), flow, the air cannot make an instantaneous change in direction when it gets to the leading edge of the airfoil. If the Angle of Attack (AOA) was 10 degrees, the air does not make an instantaneous 10 degree change in direction. From the point of the leading edge away from the airfoil, the change in direction, and the resultant pressure, gradually changes as you move further away. the result is that the flow of the air is following a curved path, and travelling a longer distance, on this side of the airfoil, than it is on the other side, even for a symmetrical airfoil.

  • $\begingroup$ "Path length" is irrelevant, because split parcels never re-join. Instead, the top/bottom parcels remain permanently split, and the upper parcel vastly overshoots: a wider split for higher lift. (And, if split parcels did rejoin, then the Circulation is zero, and the total lift is also zero.) See the physics behind this at JS Denker's "See How It Flies," av8n.com/how/htm/airfoils.html#sec-other-fallacies , also see www1.grc.nasa.gov/beginners-guide-to-aeronautics/… which uses flow-turning determined by trailing-edge flow, not by path-length. $\endgroup$
    – wbeaty
    Commented Nov 3, 2023 at 9:12

The principles of aerodynamics and fluid dynamics are what you would call "well understood."

The ambiguity is around what so-called "lift" is, which can be a nebulous concept. For example, if you drop a piece of paper it will drift slowly to the ground, essentially a form of gliding; this same air resistance is the basic force keeping a plane aloft. Would you consider this "lift"? Once you get into these arguments about semantics, things get vague.

Just as one example of the craziness, the FAA test, the same one you are taking, requires you to know the "four forces of flight" in which the so-called "lift" is the force that keeps the aircraft aloft. The only problem is that you can compute lift by equations that are in every book on aerodynamics and if you actually do this (like I did) you will find that the force generated is nowhere near enough to keep a plane in the sky. If "lift" were the force keeping a plane up, it would fall like a rock, so the FAA guidelines are simply completely wrong. It's just a huge semantic hairball that is not going to go away anytime soon.

The worst part of it is that EVERY pilot (or wannabe pilot) I have ever known thinks they know exactly what "lift" is and, even worse, their beliefs generally fall into one of 5 or 6 different categories with contradictory principles. This leads to huge arguments whenever the subject comes up. After 15 years of this, I just try to stay out of it, other than to tell the beginners not to make the same mistake (like I am telling you now).

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    $\begingroup$ We use definitions to make clear what "lift" and "drag" are. Is's actually quite simple: Lift is the force orthogonal to the direction of flow, and drag is the force parallel to it. What brakes the fall of the piece of paper is drag, not lift. $\endgroup$ Commented Oct 6, 2015 at 21:45
  • $\begingroup$ Yeah, yeah, I have heard it all before. I have had these conversations with aeronautical engineers from MIT. The whole lift concept is completely screwed up. I am sure you have your opinion what lift is, but the TEXTBOOK the OP mentioned says there are differences of opinion, a book written by professional experts, so don't start preaching like you know the truth. The reality is that it is an ambiguously defined term. $\endgroup$ Commented Oct 7, 2015 at 2:39
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    $\begingroup$ One reason that it might seem a "screwed up" or "vague" concept is because the notion of describing "lift" as a single number is more or less useless for designing aircraft, though it is quite useful for the very simplistic explanations that are all you need to be able to fly them safely. All a pilot really needs to know about lift is, "if the plane does this, then move the controls like that to correct the situation". Similarly, a car driver doesn't need to know anything about tire design and friction to correct a skid by steering into it and not braking heavily. $\endgroup$
    – alephzero
    Commented Oct 8, 2015 at 15:34

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