Note: The author is not entirely serious. It's part of a series called Mystifications: A short series of semi-satirical pop science articles, called "Here's why we don't understand". The science presented is mostly accurate. The first article was "we don’t understand electricity" and now it's "we don’t understand flight". You'll find the articles more enjoyable if you think of it as a thought experiment about the depth of knowledge - the author is a physics professor and he clearly knows what he's talking about.
In this case I took it to be poking some fun at the two conflicting 'intuitive' explanations for a wing producing lift: one being that air strikes the bottom of the wing as it moves forward, pushing upward on it, and the other being that air moves faster under the flat underside of the wing than over the curved upper side, causing a pressure differential. Of course reality is more complex than either simple answer, and the real answer is something more like, "The wing behaves approximately as described by this equation."
Because the pressure on the top is lower :) (this is half-serious: the whole problem with these explanations is that cause and effect for all of these variables is not straightforward: you can see from the navier-stokes equations they are all dependent on each other).
Kind of. Actually the real ‘cause’ in my understanding is 1) the curved geometry of the suction (upper) side of the aerofoil and 2) the fact that the flow remains attached to it. Everything else - you can actually approximate the curved surface to a circle and apply equations of circular motion to a parcel of air to satisfy yourself with why the flow is accelerating. And Newton’s 3rd law explains how lift is generated on the wing. In my view there’s no need to use Navier-Stokes to explain how an aerofoil works, if you simplify the geometry to make a special case.
Most of the lift comes from the suction side.
Actually, if you really want to test an explanation, try to apply the same reasoning to explain how a sailing boat can sail upwind (or at least up to about 45 degrees off).
The devil is in that last detail. "Flow stays attached" is a description of the properties of the flow, not an explanation for what causes attached flow or why attached flow matters. It's semicircular reasoning to say that the plane gets lift because the flow stays attached... Attached flow and lift are correlated, but they may be two phenomena caused by the same underlying property.
Does not move faster either. Otherwise, a flat wing would not work, and they do.
Gravity or force creates the pressure differential. Wing pushes on air below it. (Why birds fly.) Additionally, for moving wing, edges create vortices that create local pressure differentials. (Why helicopters and planes and birds work better than floating pieces of paper.)
Wings work very similarly to performance ship hulls in this regard.
For anyone who’s ever tried building a robotic bird, there is a lot more intricacy to how birds fly than just “pushing air”. A better article might have been, ‘we still don’t understand how certain species of bird fly so efficiently’
Surely it does move faster, because it's lower pressure/you're putting less resistance on it?
If I have a wing shaped like ∖, air going in -> direction, which is what you need to generate lift with a flat wing, then the air on the bottom is running into the wing and slowing down, while the air on the top is being pulled into the region the wing swept clear of particles and speeding up.
It does move faster. This can be readily observed in wind tunnel tests, and is a source of many issues once you get into transonic flight when the airflow can reach supersonic speeds while the plane in subsonic. Flat wings must be inclined to cause the air on the top side to move faster. The vortices cause air to move at different rates.
My understanding is that the air moving over the top of the wing is compressed against the air above it in the atmosphere, like a venturi. This may be extremely simplified but it's what we were taught in flight school.
"bounces" down, simple enough. Force on wing up and back.
top air:
bounces up off front of wing (because it's not infinitely thin), but then is unimpeded by wing. It get's slightly more compressed at the very front, but then as the wing goes down this big gap is left. The air isn't going to bounce on the air above significantly because air compressed and this is laminar flow to boot: Viscosity > internia-ness.
The about-to-be-vacuum means the bottom air pushes the wing up more easily, usually to the point where there is no more vacuum, just low pressure. But if you go really fast (or are a hydrofoil?) then there might be an actual vacuum.
The vacuum "initially" just accelerates the air vertically, but once things get going since the airfoil "carves out a triangle", the air might speed up horizontally too. There is air behind it (front re aircraft heading) pushing on it but not air in front which is getting "untraffic jammed" away.
There we go, I think this accounts for everything in the article without any Bernoulli. Screw Bernoulli.
You might get something out of this 2013 talk by former Boeing engineer Doug McLean on misconceptions about lift. [0]
[0] https://youtu.be/QKCK4lJLQHU?t=834 (watch for 5 minutes to get some idea of his main points, or 35 minutes to watch in full. The link will skip the introduction.)
I thought it was mostly because of the slight upward angle of the wing which creates air compression under the wing and suction above the wing.
If you try to move a flat object through water, it creates pressure at the front and suction at the back. If you tilt it diagonally (and move it right to left), you get pressure in the bottom right and suction in the top right.
This is true of a symmetrical aerofoil (e.g. most helicopters) but not for an asymmetrical aerofoil (most fixed wing aircraft). It is true that a slightly positive angle of attack generates more lift than none (because the pressure/lower side starts making a contribution)
Correct. Still incomplete. Angle of attack causes a vortex at the trailing edge which has nothing to do with raw air speed and everything to do with fluid dynamics (which involves speed but is much more complex)
Short version is that you created a hole (lower pressure area) in air which it now tries to fill.
Air and gasses have finite limited velocity known as speed of sound, which is why you get these pressure differentials while the wing is moving.
With a flat wing, they're rather small and low pressure vortex is located behind the wing. In an angled wing, some of it is located below the wing and the air trying to fill the low pressure area exerts a lift force on the wing. (It's unlike a balloon. Bernoulli has very limited impact, unlike essentially wind.)
I’m not sure I fully agree. Do you not get this trailing edge vortex with an asymmetrical aerofoil at 0 angle of attack? (Just less strongly because less pressure difference between suction and pressure sides)
Isn't the intent of a smooth aerofoil design to prevent the formation of vortices on the trailing edge? They're inevitable at the wingtip, but in controlled flight most wings are trying to produce laminar flow, right?
In my understanding, if you increase angle of attack sufficiently to generate vortices on the upper surface, then you aren't efficiently transferring downward momentum to the air your wing is shedding, and you lose lift, which causes aerodynamic stall. Am I missing something?
Overall you are correct that the plane receives an upward force due to the air it interacts with, and that the air receives an equal amount of force downward. In level flight the vertical force components must equal zero (or the plane falls/rises).
But equally, if the plane is forced up, the air must be forced down. Cause and effect are not obvious from a force diagram.
Sorry. An inclined wing just means the wing is at an angle relative to the airflow. An induced rotation means that the wing causes the airstream to turn, so the airflow around the wing has a circular, rotating component.
Its complex...The example normally given is, the wing is shaped a little flat in the under side and curved on the top. So that would explain the flow as you mentioned. However when an airplane flies upside down, its not sucked into the ground ;-)
If an aircraft flies level upside down it will lose altitude towards the ground (as opposed to right side up wherein given adequate thrust it should keep its current altitude).
In order to stay at a fixed altitude upside down you have to bring the nose of the aircraft up several degrees (increasing based on air speed).
If wings only generated lift in one direction (i.e. towards the curved side), then even flying with your nose up would pull you down if you are inverted. What people here are missing is that curved wings in level flight generate lift, but any shape of wing can generate lift with a positive angle of attack. Just stick your hand out the window while driving on the highway and tilt it slightly, you'll see.
> "Just stick your hand out the window while driving on the highway and tilt it slightly, you'll see."
this is really all the intuition most people need to understand flight, even if it leads to an incomplete understanding. it's easy to feel the air pushing on the bottom of your hand when you tilt it up (or top, when tilted down). what's not obvious is that there is also lift created on the top side at the same time, but that can subsequently be learned in high school physics (or fluid dynamics in college, which is where it really stuck for me).
Every aircraft has the wing set at an incident angle relative to the axis of the fuselage. Usually to generate enough deflection force for level (relative to the fuselage) flight at cruising speed.
Upside down flight requires you to basically inverse this deflection, but it isn't because of Bernoulli lift.
The 747 wing is at a 2° incidence angle relative to the body, which allows the body to be level with the direction of travel at cruising altitude/speed. An Airbus A320 has an incidence angle of about 5° at the body, twisting to -0.5° at the tip (many aircraft have such complex wings, but the aggregate is an important incidence angle). Every Cessna has a significant incidence angle.
The overwhelming majority of aircraft have an incidence angle relative to the body for the reason stated. So rather by "typically", could you name a single aircraft that doesn't have such an incidence angle? An SR-71?
As to "0 degrees angle of attack lift", such lift is close to negligible. Maybe you mean the body of the aircraft is zero degrees, but then we loop back to the core point again.
Wings, at least on small civil aircraft, generally DO have a positive angle of incidence where angle of incidence is defined as the relative angle between the chord line of the wing and the longitudinal axis of the fuselage.
I think it's easier to think of inverted flight as normal flight for a negative AoA. If the airfoil is symmetric -- as almost all aerobatic aircraft's are -- then it's functionally identical and inverted flight becomes a coordinate system "trick".
My favourite two "explanations" of flight are 1) dP/dt for air is greater down than up; and 2) Kelvin's circulation theorem, but alas that one is not very pub-friendly...
It’s a common misunderstanding that the underside of a wing is flat and the top part curves. A paper airplane with thin flat wings still gets lift though there are several issues trying to scale this up. Similarly many aircraft will happily fly upside down.
Wings need to support the weight of your aircraft while being light this means they need to be reasonably thick especially using the obvious choice of storing fuel inside them. The first obvious choice is a teardrop shape which gets lift from being angled up similarly to the way a flat wing does.
When I first saw the Bernoulli's principle demo of the floating disk at the science museum as a kid it made me mad. And when I actually learned about it in high school physics I still didn't like it. Reading that article now is very satisfying :).
In seriousness though, there is a big difference between "bottom up" causality-focused theories and these derived principles based on complicated notions of steady states. Even when the student is too junior not to have any choice but use the latter, I think the difference needs more emphasis.
Also the 3rd law model of flight is so much easier to understand they should teach it first.
When I was a teen I asked my dad who was an aerospace engineer. He said there is just more than one way to calculate the result.
Though I think it's more valid to think of the wing as imparting a downward momentum on the air flowing over it. Meaning it's really a reaction engine.
As others have said it’s not really the shape of the wing that matters. Some shapes work better but I’ve always thought of it as more of a fluid density problem. As you increase speed the wing is in contact with a larger mass of air which at some critical point becomes large enough to support the weight of the aircraft. After you hit that speed then you are just manipulating the air flow to steer the craft. Holding your hand out of the window at highway speeds really makes it feel more intuitive to me. Of course I could also be completely wrong here.
When you hold your hand outside the window of a car in motion, your hand is only pushed upwards if you incline it upwards. If you incline it downwards, it will be pushed down. This is the angle-of-attack effect and simply relies on the normal force of the air striking the hand. If the hand is inclined upwards, the normal force has an upward component, creating lift.
The sad thing is, "the air hitting bottom of wing > top where bottom is determined in reference to the side of the aircraft least distant from the Earth's surface assuming an experiment in Earth's atmosphere" is really the most concise and relevant explanation given all of the factors at work. At least until we start encountering significantly more dense atmospheres that mysteriously do not sink under realistic conditions and start trying to fly planes through them. You fly because you're a flat thing skipping off what essentially becomes a more dense surface underneath you than above you. If you didn't, you wouldn't be flying. You'd be falling. And yes, here's a crap ton of math, try not to think about it too hard.
Thanks for pointing that out. Not long ago, on Fox news, the star pundit Bill O'reilly used to tell babyboomers that we don't understand tides and that was the proof that god existed.
Though I don't know what idea he was referring to, tides can be pretty hard to predict accurately. Obviously nothing to do with God, but it's probably fair to say there are aspects of them we don't understand or at least can't simulate arbitrarily far into the future.
"In an analysis of the tides in Venice Lagoon, at the head of the Adriatic Sea, where the tides seem to pick up because of near-resonancy of the basin, Vittori (1992) observed that consecutive tidal maxima are highly irregular. She argued this to be indicative of low-dimensional chaos. Whether the low-order dynamics to which this is due is either inherited from the dynamics of the local wind fields or of a genuinely oceanographic nature is not clear." [1]
This was the video in question: https://m.youtube.com/watch?v=HABNe7_D22k?t=1m52s. It's not about predicting the exact movement of tides, he's arguing science fundamentally can't explain the regularity of tides...
Funny though, the guy he's interviewing said Islam is a scam and Muslims are suckers who have fallen for it. Funny what you can get away with if you couch it right.
To be fair, he claims that ALL religions are a scam, not specifically Islam. I can tell you for sure that Bill O'Reilly wasn't grilling him for his bad treatment of Muslims!
Hm, I was hoping this article would explain in what sense we don't understand flight, or in what sense people think we don't understand flight, but it didn't seem to answer that question...
I once read a book called "There Are No Electrons". I'm not sure I'd recommend it, but its approach was interesting: the author reasons that unless you're in grad school studying physics (and perhaps even then), everything you've been taught about electricity is a lie anyway, so the author attempts to present a framework of easier to understand lies intended to make the reader able to better reason about and predict the behavior of electrical systems, than if the reader had only the lies that are usually taught.
> So it's better to say that our models are simplified, not lies.
To be fair, for most purposes (atoms, molecules, metals) there usually (very-) technically aren't any electrons, just configurations of the relevant quantum fields (eg in the form of electron orbitals) whose asociated conserved quantities would allow them to convert into a certain number of free-flying electron particles if you dumped in enough energy to make up the difference.
You see this a bit more obviously with ('virtual'[0]) photons, where some non-particulate field configurations simply can't be thought of as particles at all (eg attactive electromagnetic forces).
More generally: we have stories that say that "things behave as if they are made of ..." but too many people misread or mishear them and think that the form is "things are made of ..."
There are no quantum fields either, just as there are no atoms. Things (of the right size) behave as if there they were composed of quantum fields (or electrons or atoms), and the less we have to say "... except that ...", the more comfortable we are with the story.
If this is the book with "Greenies" in it, i've read that, and it was interesting to read but i don't know that it gave me any better idea of how to build a circuit. I lost all ability to design any circuit when it was explained that transistors work because the places for the charges to go moved, not the charges themselves.
I look at "quantum computer" components and go "what does a grid of wires have to do with 'computing'? And then you realize the big secret - There's regular computers that take the 'output' of the qubits/QC stuff and 'decide' what the results are, since it's all just a blob of probability anyhow...
Solid state physics was my favorite class in college (and it was a senior course that people regularly flunked so not really a beginner-friendly tutorial). It was also very hard, and one of only 2 courses I actually attended while studying because I couldn't just show up for the test and ace it. It was fascinating to finally understand the physics behind circuits I'd been building for years (I'd understood RLC circuits since high school, but transistors, op-amps, diodes, and all that stuff I knew how to use but the "how the fuck does this amplify current?" mystery remained).
Unfortunately I don't remember what book I used. But yeah, this definitely opened my eyes to how electricity works.
> I learned multiple times. Everytime I understood less of it.
Isn’t that how to spot seniority? The junior says “I know ReactJS and SpringBoot!” The senior says: “I don’t know much…”
Unrelated, but that reminds me how my Masters Degree teachers touted the importance of their subject in their introduction course, all explaining the Ariane V explosion from a completely different angle:
- The measurements professor: “Ariane V crashed because engineers tripped themselves into different imperial/metric units, this is why Measurements & Precision is the most important topic!”
- The programming teacher: “They fit a long inside an integer and it looped to negative, which inverted the trajectory of Ariane V and triggered its destruction, this is why learning C properly is the core of your teaching this year.”
- The quality assurance teacher: “They didn’t check the contract of the component! This is why QA is the most important when creating big systems!”
- The management teacher: “It’s the story of two teams who designed two components with different assumptions, one team worked in imperial units and they didn’t communicate clearly about assumptions, that’s why management is the one topic you should really work on.”
They were all right. Or rather, they were all wrong: Everyone knows Ariane V exploded because the officer pushed a red button ;)
>They were all right. Or rather, they were all wrong: Everyone knows Ariane V exploded because the officer pushed a red button ;)
...and this is why learning the value of drawing the boundaries and selecting stop points in the analysis of complex topics, and the employment of humor is a powerful rhetorical tool. This is why you composition is the most important topic this semester!
Sorry... Couldn't resist. <Queue the follow up psychology is the most important topic you'll learn this semester, followed by Biology, Social Psychology, Anthropology, all getting stucktrying to get in the door.
Interesting question! INSA Lyon in France, but that was in 2005, you could mock that the Old Continent does a lot of V-Cycle waterfall projects and had missed the Agile turn of 2001.
BUT learning how processes help is, instead, a very important step to judge what exactly we give up with Agile.
The irony is I went on creating software for requirements, and I can testify that all of the hardware industry does QA more diligently than ever!
That's because QA and Statistical Process Control were born out of manufacturing due to the high stakes with processes and tooling not being able to change on a dime like software does. I'm not surprised at all there.
Software Quality Assurance is much more... Spongey.
Actually the first flight of Ariane 5 exploded automatically when it started to fall apart, because the flight computer read an Ada exception as flight data and from this, it decided to turn as fast as possible.
They reused the launch code of Ariane 4 in Ariane 5, but Ariane 5 was much faster to take off. It was an overflow on the acceleration and bad testing because reading an Ada exception as flight data is not great.
We learn that at school in France many years later.
We understand flight well enough to make highly optimized airplanes.
There are some old controversies that are largely settled. The Microsoft Flight Simulator manual in 1980 "teached the controversy" but it was really settled decades before that. People still remember the controversy from back then and keep repeating it and probably will still do it when people are living in space colonies.
The Bernoulli effect explanation is bogus.
An alternate (correct) explanation is that if you just took a piece of cardboard, held it sideways, and moved it laterally it would push the air down and thus the cardboard would be pushed up.
If you like vector fields you can show that there is a topological defect (vortex ring) that is threaded through the wings and comes around to the other side. If you do an integral around the ring you can show the vortex holds the plane up.
I hear this but never seem to get any further info. Why are wings shaped with a curved top and flat bottom? Is there a good summary I can go read to understand this all?
When learning about flight, keep asking yourself "Then how do planes fly upside down?" Any explanation which does not mesh with sustained, inverted flight is oversimplified to the point of uselessness and inaccuracy.
The continued assertion that "we don't understand heavier-than-air flight" is a weird one. The article even skates around this, saying (essentially) "well maybe we do understand it, but chaos theory!"
If you're in the sky and you want to stay there, you have to counteract gravity. Heavier-than-air flight does this by pushing down on air. Want to stay in the sky? Push down on enough air, fast enough, and you will stay in the sky.
Since air is a fluid, pushing down on air is equivalent to pushing air down. The lift a plane or helicopter generates is directly proportional to the amount of air it pushes down (and to a varying degree how much engine exhaust it pushes down). That is, we need something to divert air downwards and something to push us through that air. The better we can redirect air downwards, and push ourselves through the air, the easier it is to fly.
We have found many shapes that are very good at passively redirecting air when pushed through the air, we have developed engines that are good at pushing us through the air, and we have developed structures that are able to hold everything together while being light. That is why heavier-than-air flight is possible, and it's very well understood.
If anything, our understanding of why certain shapes redirect air so well is lacking, but even then not really. Experiments and modelling are really good at finding the conditions under which air stops being redirected efficiently. If we try to parameterise this airflow, and reduce it to simple equations, well maybe then the effect is not well explained. Statements like "the air moves faster on the top than on the bottom, so there is a pressure differential and hence a lifting force" may be true even if misleading, and statements like "the air moves faster on top because it is longer than the bottom side" are definitely misleading and incorrect, but just because these statements exist and some people believe them does not mean we don't understand heavier-than-air flight! Such flight is possible because we are able to push down on air with enough force to keep us flying, and so much of how that works is well understood.
[edit]
To try and say something directed more at the point the article seems to be making: confusion or misunderstanding about the technical details, or modelling, of something is very different to not understanding how that thing works.
We understand heavier-than-air flight in the exact same way we understand sailing - redirect airflow to generate a force for your own purpose - but you don't see articles about how we don't understand how sailing works.
I think the various discussions in these comments show that we don’t realllllly know, just that we understand what forces are there to allow it, and how to generate them.
Different values of "we" going on here. "We" know how heavier-than-air flight works in the sense that it is known to science. "We" in the comments don't know, because most of us here have at most a Bachelor's in physics, and it takes more than that to really grok aerodynamics, but everyone (including me) can't resist the temptation to show off what partial knowledge they have from that undergrad fluid dynamics class.
The fact that the second "we" doesn't really know aerodynamics, and is going to waste man-days chasing its own half-understandings round in circles in the comments, doesn't contradict that fact that the first "we" does understand aerodynamics. Planes aren't staying in the sky by accident, nor even just by the survivorship of trial-and-error engineering.
Knowing Newtonian physics doesn't mean we understand all things that move.
The point about "not understanding" flight is that, if we truly understood it, we could design the optimal aircraft from first principles before it ever entered a wind tunnel. Instead, we work based on incrementally improving tribal knowledge of what has worked in the past and try to make something similar to fit our desired flight envelope.
Compared to something like rocket science where the entire craft can be built on a computer and we'll know exactly how much cargo we can get to the moon without even turning a single screw, we don't understand flight.
I don’t think this is true. There are many physical ststems for which we know the underlying physics very well, but the equations can’t be simply solved, and numerical simulation is more costly than just building the damn thing and testing it. Wing lift under turbulent conditions is one of those things. So we use wind tunnels. Not because we don’t understand lift—we do—but because it’s just easier.
This is getting less and less true with each generation of supercomputers though.
EDIT: There is perhaps a better way of explaining it for this crowd though. To use numerical modeling to predict performance is to take a physical problem and turn it into a computational problem. And while engineers understand physical systems pretty damn well, us computer scientists have largely failed at the objective of making software systems with hard reliability guarantees. You can write a fluid dynamics simulation to test your new wing design, but how do you know that the simulation does what you think it does? Even if the code has been tested before, how do you know you're not now hitting some sort of edge case?
At the end of the day, you have to build the damn thing to test it. Numerical simulation are used more and more these days as the codes are refined, computers get more powerful, and engineers have more trust in their capabilities. But traditionally, and still a lot of the time, they build prototypes and test in wind tunnels because reality never fails to model physics accurately.
The edges really show for smaller craft. At low Reynolds numbers lift gets funky. A lot of flight research is around making very small things fly efficiently.
Software is definitely still a craft, not engineering in the build-a-bridge sense.
So I would say, nobody really understands software, but many people have experience and total experience is growing. Via new languages, algorithms, patterns, etc.
The rapid experience advancement suggests there is a lot unknown and not understood.
I'm not sure if this view undersells aero or rocket engineers more.
Rocket engineering uses an immense amount of both modelling and physical testing. No-one says "Well I've got the Tsiolkovsky rocket equation, so let's go to the moon!"
I'm not even sure what the bar of 'understanding' is here - the fact that we have iterated and improved on powered flight as much as we have necessarily means that we understand it on a deep level, let alone the fact that we can create excellent models that predict what will happen to a wing in different situations.
At what point would you say we do understand flight?
You have an unduly rosy picture of rocket science. The turbopump used in most liqued fueled rockets to presurize the fuel is notoriously complicated. Small changes to the design can result in a dramatic loss of efficiency, or even worse if it starts cavitating the pump can eat itself. For this reason nearly nobody designs turbopumps from scratch and first principles, they take a well understood design and maybe tweek it a bit. And you can bet that they then test those tweeks on a bench a lot.
Similarly devilishly complicated is the injector design. Obviously you want to mix the oxidizer and fuel in the optimal ratio for the highest efficiency. That’s the easy part. But then you also want to offset from this optimum near the edges to produce a colder flow near the nozzle wall to protect it from melting. Of course nowadays people do a lot of computer simulations to save on testing time, but it is still not uncommon to discover combustion instabilities or hot-spots in the engine tests.
So no, nobody can, let alone did, design a rocket entirely in a computer and then send it to the moon without many many tests, and incrementally improved tribal knowledge.
Not an expert or anything. Never studied aerodynamic or flight in depth.
As far as I understand, for helicopter to fly, it definitely has to have thrust to weight ratio greater than one. Flying things that have thrust to weight ratio > 0 are intuitive to me. They generate force and stay in the air indefinitely.
Planes obviously don't require that to fly. So, they're different type of beast. They somehow squeeze more from less, exploiting some nonlinearity in forces that air exhibit on wings. I can understand that too, but the nature of that phenomenon is not explained anywhere (other than in words: this is the formula. It is correct, trust us)
It's the exact same principle for planes and helicopters.
If a plane isn't producing more lift than weight it will fall, just like a helicopter. Planes work by pushing a wing through the air, helicopters by spinning it. In both cases the wing has to push down enough air to keep the aircraft in flight.
No, I have to conclude that the previous commenter didn't understand your point, which was the observation that heavy aircraft with very low sustained forward thrust (thrust that would not be sufficient for a helicopter to hover) results in sufficient upward lift to suspend the aircraft indefinitely, which is very surprising.
A helicopter doesn't rely on the forward thrust of the chassis, it relies on the forward thrust of the helicopter blades in rotation. Those blades are wings.
The way the thrust is generated is mostly irrelevant to this observation. The observation is about the mechanics of lift, which is some function of thrust combined with the wings, rotors, balloons, etc. You'd observe the same bizarre mechanics if the thrust were generated by releasing highly compressed air from a tank.
I'm not sure how you're connecting that quora discussion to what we were saying above, but I think the confusion comes from what 'thrust to weight ratio' means.
Helicopters and planes are both pushing down on air to generate lift. The lift generated has to be equal or greater to the weight for the aircraft to fly.
Thrust can mean many things (at least colloquially). As discussed in the quora you linked, a helicopter will have a defined power to weight ratio that allows it to fly (maintain level flight) in its 'normal' flight envelope. There are a number of things the pilot can do that causes the aircraft to 'push down harder' on the air. One of these is flying close to the ground (the ground effect) which is sort of like pushing against the ground as well as the air, and another is by moving horizontally (usually forwards, like a plane, causing transational lift). Both of these allow the aircraft to maintain height while using less power than if it was hovering, but to do so it is still generating enough lift to counteract gravity.
I didn't read your link, but the "thrust" in thrust to weight ratio generally refers to the force that the engine produces. Therefore, it isn't a brilliant analogy to compare the thrust of a helicopter rotor (which does get all its power from the engine) to the thrust of an airplane propeller.
The propeller doesn't contribute materially to the lifting force on an aircraft, while the rotor of the helicopter provides practically all the lifting force on the helicopter.
Both machines need to (somehow) generate lift equal to their weight to stay airborne. The airplane just does this by moving a static wing through the air, which is a much more efficient way of doing it. Its engine/propeller isn't even immediately required to move the wing through the air; once airborne, the aircraft can fly downwards at an angle without engine power to maintain its speed.
You'd say that a helicopter uses "powered lift" while an aircraft does not.
I am not an expert either but comparing "thrust" from helicopters and airplanes is not meaningful if you are only talking about engine thrust. Thrust is a vector, although in aerodynamics it seems to refer to forward force by convention.
For helicopters in a static hover, the downward "thrust" is actually the lift produced by the spinning blades. The engines produce almost no forward thrust. Whereas, for an aircraft in flight, the engine thrust pushes the plane forward and the wings generate the lift that keeps it in the air.
Yeah, the usual explanation of "air goes faster over the top because they have to meet at the end" is BS. Especially the 2nd part. A video about the subject https://www.youtube.com/watch?v=QKCK4lJLQHU
But the explanation I can come up with is: lift is a force due to low-pressure regions caused by laminar flow over a surface. It is essentially "form drag" (caused not by the profile facing air directly but by the aft part) but the tricky part is that it is not directly parallel to the flow of air, but also depends on the orientation of the wing.
Isn't it because planes are continually falling (because gravity), and this leads to two things:
1) wings increase the surface area pushing down (gravity) on the air below, which pushes back (air pressure), and
2) as wings are falling toward ground (gravity), they create vortices above the wing, which lowers the pressure, increasing the push up effect of the air below, and
at a certain speed, the vortices are stabilized into low pressure regions above the wings, and in a certain "envelope" region, of speed, plane shape, air pressure, all of these forces are equalized to give you level flight, so long as the dial you turn to get into the envelope region, "speed", keeps up.
That's how I understand it. Happy to hear a physicist / aerospace engineer guide me in how to think about this clearly.
This is not how it works, no falling or positive angle of attack is required for an asymmetrical aerofoil. Imagine swinging a bucket of water over your head - the force your arm feels is similar to what the top surface of the wing feels.
Is that so? I thought that for asymmetric airfoil, zero angle of attack is by definition the angle where it creates no lift. So, tautologically, if it's creating lift a (positive) angle of attack is required.
We don't know how to calculate turbulence, we can only predict it. It is the turbulence that create the uplift on a wing, so the author says we don't understand it.
> It is the turbulence that create the uplift on a wing
You can have lift with laminar flow. In fact, the article includes an explanation of the usage of the Reynolds number to characterize laminar and turbulent flow and how the flow around plane wings is clearly laminar (called "smooth" in the article).
- Most of the popular explanations are misleading, the others are incomplete.
- The real answer is so hard to compute that there is a million dollar prize attached to it.
- Today, we design planes using approximations and trial-and-error. It works well because we are very experienced in designing planes, sometimes at the cost of many lives, but it is not exactly a "first principles" approach.
I mean, you can get deep into epistemology and argue that it's impossible for anyone to truly know anything, and tautologically all of engineering requires approximation in some way, but our modern understanding of aerodynamics is absolutely a "first principles" approach (the principles being conservation of mass and momentum in a viscous fluid), even if there are still some aspects of trial-and-error on the aircraft hardware level. Despite the open mathematical problem of the existence and smoothness of Navier-Stokes that you mentioned, the equations are a fantastic tool that have enabled us to make startlingly accurate calculations of lift, drag, stability, and performance, despite our inability to do direct numerical simulation at the Kolmogorov scale on usefully-sized things.
I think what people usually mean when they say that we don't understand flight is that there are no simple equations. A lot of physical problems have elegant solutions (eg. the shape of a hanging chain is roughly the cosh function). But there are no elegant equations that describe the profile of a wing, so it's a bit unsatisfying.
The Coanda effect is not directly related to lift but it is why airfoils have that distinctive shape. Because of the coanda effect, the air will follow the curves of both surfaces of the wing, and the two flows will recombine at the rear. This allows the flow to be pointed in a different direction after passing across the ring. This change in direction induces rotation of the air. This rotation is the true source of lift.
Can confirm. I worked at Pratt & Whitney testing jet engines early in my career. At the time I read a similar article and spread it amongst my colleagues - the cognitive dissonance was palpable.
As engineers we had been taught that lift was due to air above the wing traveling faster than air below the wing and thus creating lift by way of a pressure differential. The more accurate answer as seen in the article is that the effect is better explained through Newton’s laws as a re-vectoring of horizontal thrust in a downward direction. Literally the engine pushes horizontally accelerated air downward and the action-reaction mechanic causes an equal but opposite upward force lifting the plane.
Amazing how so many experts could be so wrong in their understanding while the planes continue to fly.
It reminds me of how hummingbirds don’t know that they violate the known laws of physics when they fly.
RC airplanes have insane thrust to weight ratios, with enough thrust a brick can fly.
That said with my historic experience with single piston RCs if you didn’t compensate for the upside down flight with your ailerons you would nose dive.
Because there is not up or down for the wing when its cutting through a fluid. It is not that we have seen planes flying intercontinental flights upside down.
And those upside down events do not happen at 10 feet above ground. There is plenty of fluid (air) above and below the aircraft and power (fighters jet engines are the most powerful ones on aircrafts) to be able to correct any up-downward force with flaps (basically walls to air)
This is nonsensical. Glider pilots (no engine!) fly aerobatics programs all the time, in planes with curved wings. If the pilot is a bit of a masochist, they could fly upside down for half an hour in still air given enough altitude. The various forms of air resistance will be greater and energy/altitude loss hence higher, but the wing will still be generating lift equal to the weight of the airplane.
But the "true" explanation given above is that the engine pushes the horizontally accelerated air downwards (with respect to its own orientation). Wouldn't that also lead to the conclusion that upside down flight is impossible?
I'm not immediately convinced that "effectively changing the shape" is a coherent idea. The lift effect either crucially depends on the actual, unchanging shape of the aerofoil or it doesn't. Flying upside-down proves that it doesn't. Maybe all we're disproving is a straw-man of a "Bernoulli-ist" position, but we're disproving it all right.
EDIT: trying to think what you might mean by "effectively changing the shape". Do you just mean that an upside-down aerofoil is a reflection of the aerofoil the right way up? Because that's the entire point of the argument you seem to be trying to rebut.
If you are asserting that the angle of attack does not affect the lift, you are wrong.
A plane flying upside down is most certainly not using the same angle of attack as it does right side up. The real difference in performance is efficiency, the upside down plane is burning more fuel due to the increased drag from sub-optimal operation (a high angle of attack to overcome the optimization for right-side-up flying).
Note that a right-side-up wing can easily plummet by dropping its angle of attack. That is what it's doing while upside down to generate lift.
> If you are asserting that the angle of attack does not affect the lift, you are wrong.
I am certainly not asserting that, and I'm baffled how you could have formed the impression that I was.
You appeared to be attempting to rebut an argument in favour of the significance of angle of attack. We have another pointless internet misunderstanding on our hands.
It is a lazy gotcha attempt. It is a lazy attempt at gotcha-ing someone who believes angle of attack ISN'T important. Unless you think a plane flying upside-down is somehow evidence AGAINST the importance of angle-of-attack?
Again: this entire pointless misunderstanding has arisen because you didn't see - apparently STILL HAVEN'T SEEN - which side of the debate the comment you replied to is arguing for.
I'm afraid it is most evident that you do not see. Forget trying to guess what I might or might not think about aerodynamics. Just see if you can follow the following recap of the conversation:
1) anvandare says aeroplanes can fly upside down. This is an argument AGAINST a putative person who argues that lift is entirely a function of aerofoil shape, ignoring angle of attack. In advancing this argument, anvandare implies that he DOES understand and contend that angle of attack is significant.
2) You say something unclear about "effective change of shape", apparently attempting to rebut anvandare, who, remember, contends that angle of attack is significant.
3) I say that what you said about "effective change of shape" is unclear, meaning I am rebutting you, meaning I agree with anvandare that angle of attack is significant.
4) You form the impression that I believe angle of attack is not significant, and tell me that if I believe angle of attack is not significant, then I am wrong.
Can you see where you have gone wrong there?
Having written all this, I'm come to the point of actually becoming quite concerned about your neurological state. If you've had a recent head injury or you're old enough that Alzheimers is a possibility, you need medical advice - you've failed to follow the simple thread of a conversation.
As a child I used to stick a school ruler out of the back window of the car and rotate it slightly to make it move upwards, like a plane's wing. Intuitively I felt that this happened because it was pushing some of the horizontal airflow downwards and the air was pushing back up on the ruler. Yet the books I read about aeroplanes referred to something called Bernoulli's principle which was pretty demoralising because I couldn't understand it.
I suspect, like many other things that didn’t make sense - the reason was that it wasn’t actually true.
The Bernoulli effect explains that lift is due to the design of the wing such that the path above the wing is longer than the path below the wing.
This coupled with the fact that due to the Bernoulli effect an air particle just above the wing would reach the back of the wing at the same time as an air particle just below, and that since the upper particle would therefore have to travel faster than the lower particle the pressure differential would cause lift.
The problem is the theory doesn’t hold up under testing because it isn’t true.
Isn't Bernoulli's principle only applicable when talking about the same flow? I've always found the "above path is longer than the lower path" explanation to be unintuitive because we're not talking about the same flow. They're separate flows.
That may be the correct answer here. The important point to note is that there is no physical reason why the two separate upper and lower streamlines would collude to arrive at the back of the wing at the same time and in fact they do not.
Is it possible to think of.. the roundness on the front disrupting the airflow over the top causing air to become turbulent and less dense on the top. Where as the air flow under the wing high higher relative density and the wing will rise to the less dense position?
Bernoulli's principle, the actual thing, has very strict criteria* to be applicable. People usually neglect this entirely in casually throwing the term around.
- points 1 and 2 lie on a streamline,
- the fluid has constant density (note effects of height difference > gravitational potential energy between point 1 and 2),
My point about the Bernoulli effect is that there is no physical reason why the upper flow should move faster than the lower flow and in fact testing shows that they do not.
Please explain what you mean here in simple language. I don’t understand how you conclude that the upper flow must move faster and I don’t understand how it relates to vortex production.
To get a force acting upwards on the wing, there must be a downward reaction on the airstream.
This downward force on the airstream bust change its direction.
This change of direction is a rotation about the airfoil. Specifically a downwards rotation.
The airstream moving above the centerline of this rotation is moving in the same direction, and thus will be accelerated faster.
The airstream moving below the centerline of this rotation is moving in the opposite direction, and so will be decelerated to a slower speed.
The center of this rotation happens to be along the camber line of the airfoil, so all the air above the camber line (ie over the top of the airfoil) must move faster, while all the air below the airfoil must move slower.
The vortex is just the bulk rotational movement of the airflow. In fact, the airfoil can be replaced by anything that will generate the same vortex, like a rotating cylinder.
See my sister comment. The Bernoulli effect explains the pressure differential by way of an above-wing streamline reaching the back of the wing at the same time as the below wing streamline. Since the upper wing is curved and therefore a longer path the theory claims the pressure differential is caused by the upper streamline traveling faster than the lower streamline.
The problem with this theory is that there is no physical reason why both streamlines must arrive at the back of the wing at the same time - and per experimental verification, in fact they don’t.
> The Bernoulli effect explains the pressure differential by way of an above-wing streamline reaching the back of the wing at the same time as the below wing streamline. Since the upper wing is curved and therefore a longer path the theory claims the pressure differential is caused by the upper streamline traveling faster than the lower streamline.
That's not how the Bernoulli effect explains the pressure differential. The bernoulli explanation is that air builds up in front of the airfoil, creating a high pressure region, while there is a low pressure region created in the wake of the wing. This pressure differential forces accelerates air over the wing. For an asymmetric airfoil, more of this flow is over the top than the bottom, so the airflow over the top is faster, and thus lower pressure, than the airflow under the wing.
The "equal time" thing is a pop-science misunderstanding.
That is sadly literally how it is explained both in classes and in textbooks. It’s incorrect but it’s how it’s taught which is the root of a lot of misconceptions and why this “explanation” which is not correct keeps living on.
Speaking of sailing, the angle and shape of the sails are both important to maintaining velocity. Racing boats adjust (trim) the shape of the sails all the time.
Merely deflecting the wind obeys conservation of momentum, but conservation of energy in an unpowered sailboat (overcoming losses due to friction) also means extracting energy from the airflow.
The experts weren't _wrong_ in their understanding. Bernoulli (creating lift by way of pressure differential) and Newton (reaction to redirection of the flow downwards) are different ways of describing the same thing; integrating either the pressure or velocity vector of the airflow around the wing will give you the correct results for lift.[1]
Whenever people argue about which interpretation of lift is correct I think back to this (https://xkcd.com/895/) comic about teaching how gravity works in general relativity. Only in the case of lift the explanations are actually _correct_, albeit somewhat circular. ("So the air above the wing sticks to the surface, which redirect it downwards. But _why_ does the air stick to the wing?!")
Also in no way do hummingbirds violate any known laws of physics, although they do have a pretty impressive way of harnessing them.[2]
I commented elsewhere that there is no physical reason why the Bernoulli effect would cause the upper streamline and the lower streamline to reach the back of the wing at the same time - and to my knowledge there is no experimental evidence that it does. I may be wrong about that but I have never seen an adequate rebuttal.
You're right- there isn't, and it doesn't. In fact a parcel of air moving over the top of the wing will beat its counterpart moving below it to the trailing edge. This[1] video has a very good demonstration of this. (Relevant part begins around 0:25.)
So saying that air on top of the wing moves faster, creating a pressure differential and thus lift, is absolutely correct. The problem begins when some people try to come up with an intuitive explanation for _why_ the air would need to speed up. "Because this is the lowest energy state that conserves energy, momentum, and mass" isn't a very satisfying answer; neither is "because this system of PDEs say so"; so they came up with the "equal transit time" explanation, which is simple, intuitive, and completely wrong.
Hopefully aeronautical engineers at P&W didn't actually believe that last bit?
The streamlines don't need to meet for Bernoulli to apply.
That statement, that the upper airstream flows faster because it has to meet up with the lower airstream, is wrong and easy to disprove experimentally.
So, putting aside that specific statement, re-read the post you're replying to.
The airstream above the aerofoil does travel faster, and there is a lower pressure region there (commensurate with the effect described by Bernoulli), and it turns out that this is a valid way to model the forces involved just as it is valid to model them as a redirection of the airstream.
> a re-vectoring of horizontal thrust in a downward direction. Literally the engine pushes horizontally accelerated air downward and the action-reaction mechanic causes an equal but opposite upward force lifting the plane.
You mean the wing, not the engine.
But even then that doesn't answer the question. It's just another way of looking at the effect, but it doesn't explain the cause. The question is then why/how does the wing pushes that air downward?
> The question is then why/how does the wing pushes that air downward?
Fair point. I honestly didn’t expect this quality of analysis on the topic.
I believe that another phenomenon is required to complete the explanation in addition to Newton. Couette flow explains why streamlines closest to the wing tend to follow the shape of the wing. Hence the vectoring effect. I’m sure a better article exists but Wikipedia is a bit lacking unfortunately.
Rocketry is heavier than air flight. I submit we discovered it before lighter than air flight. I will hazard that I do have a nice understanding of how it works. I learned it from media like this:
The common connotation of "heavier-than-air flight" and "lighter-than-air flight" is that air is the medium in which the flight takes place, and that air is essential for the flight to happen. The science of aerodynamics is necessary for describing how such flight works.
That's not true for rockets. Rockets can fly in air, and a rocket's fins only work in air, but rockets don't have to use fins and rockets work fine in the vacuum of space. Because air is mostly irrelevant to rockets -- and rockets [in space at least] fly by principles that have nothing to do with aerodynamics -- rockets are not typically included in discussions of how "heavier-than-air flight" works.
A hurled object would decelerate from the moment it is thrown. Its fastest speed is at launch. Rockets have their slowest speed at launch, and they continuously accelerate. So they are definitely not "hurled".
This relates to one of the reasons I gave up studying only mathematics and became an engineer. I was far more impressed by the achievement of heavier-than-air flight in 1903 than by the mathematicians who proved that it was theoretically possible - years later.
Lift is the result of induced rotation of the fluid. Both the higher speed of airflow over one side of a cambered airfoil (bernoulli) and the redirection of the airflow (newton) are results of this, not causes. This is also why you get wingtip vortices, why flettner rotors work, why curveballs curve, and many other such readily observable phenomena. This has been well understood for over a century.
This is silly. We know why: the wing pushes air down and as Newton taught us, every action has an equal and opposite reaction. To push air down the wing must be feeling a force up on it, which causes lift.
You can also see the same thing with a helicopter flying over water: the water is affected in a circular region fairly close to the rotor itself, indicating that there is a large downward force being exerted on the air and a corresponding upward force being exerted on the rotor.
One thing to add is that you can push air down in numerous ways but some ways are less efficient.
So an 45degree angled blade absolutely will give you lift but a tapered aerofoil will do the same with less energy spent pushing the air in unwanted directions (specifically fewer swirling vortexes immediately behind the wing causing drag).
So yes push air down to stay up. Don't push air sideways or in circles. The aerofoil shape and the equations that simplify the 'don't push air the wrong way' into a simple term of drag are all about doing this.
The wing directs the flow of air downwards. The flow doesn't separate except at high angles of attack (stalling). You can go into arbitrary levels of detail (why doesn't the flow separate?, how thick is the affected layer?, etc.) but it doesn't change the fact that mostly we know the answers to all those questions.
As a kid in a car, when I stuck my arm straight out the window at speed, it got pushed backwards (in my frame) by the airflow. Keeping the arm rigid took effort. If I tilted the front of my hand in a clockwise direction, my arm was pushed upwards. It took more effort (rigidity) to stop that considerable force.
That's the observation. Clearly my rigid arm/hand was accelerating some air downward. Like a gun accelerating a bullet, there's a recoil (but a continuous one). That's Newton: conserving momentum. Maybe not a complete explanation, but it's the bulk of one.
If the plane is pushing the air up, then by Newton's third law, the air is pushing the plane down. Also, pushing it up would not reduce the pressure of a parcel of air.
A great but completely wrong explanations of how a wing works is:
The cuved shape of the wing means air has to move a longer distance over the top of the wing to connect back together with the air moving under it. So since it’s moving faster it has a lower pressure and the pressure difference between the bottom and top leads to the upwards force we call lift.
It sounds like a great eli5, and it gets repeated in serious educational contexts again and again but it’s just absolutely completely wrong.
177 comments
[ 6.0 ms ] story [ 301 ms ] threadIt's not because there's a magic force that requires air particles parted be the leading edge to rejoin thier partner at the trailing edge.
The air particles on the upper surface reach the trailing edge much sooner than the ones under the wing.
Most of the lift comes from the suction side.
Actually, if you really want to test an explanation, try to apply the same reasoning to explain how a sailing boat can sail upwind (or at least up to about 45 degrees off).
Gravity or force creates the pressure differential. Wing pushes on air below it. (Why birds fly.) Additionally, for moving wing, edges create vortices that create local pressure differentials. (Why helicopters and planes and birds work better than floating pieces of paper.)
Wings work very similarly to performance ship hulls in this regard.
If I have a wing shaped like ∖, air going in -> direction, which is what you need to generate lift with a flat wing, then the air on the bottom is running into the wing and slowing down, while the air on the top is being pulled into the region the wing swept clear of particles and speeding up.
bottom air:
"bounces" down, simple enough. Force on wing up and back.
top air:
bounces up off front of wing (because it's not infinitely thin), but then is unimpeded by wing. It get's slightly more compressed at the very front, but then as the wing goes down this big gap is left. The air isn't going to bounce on the air above significantly because air compressed and this is laminar flow to boot: Viscosity > internia-ness.
The about-to-be-vacuum means the bottom air pushes the wing up more easily, usually to the point where there is no more vacuum, just low pressure. But if you go really fast (or are a hydrofoil?) then there might be an actual vacuum.
The vacuum "initially" just accelerates the air vertically, but once things get going since the airfoil "carves out a triangle", the air might speed up horizontally too. There is air behind it (front re aircraft heading) pushing on it but not air in front which is getting "untraffic jammed" away.
There we go, I think this accounts for everything in the article without any Bernoulli. Screw Bernoulli.
[0] https://youtu.be/QKCK4lJLQHU?t=834 (watch for 5 minutes to get some idea of his main points, or 35 minutes to watch in full. The link will skip the introduction.)
If you try to move a flat object through water, it creates pressure at the front and suction at the back. If you tilt it diagonally (and move it right to left), you get pressure in the bottom right and suction in the top right.
Short version is that you created a hole (lower pressure area) in air which it now tries to fill. Air and gasses have finite limited velocity known as speed of sound, which is why you get these pressure differentials while the wing is moving. With a flat wing, they're rather small and low pressure vortex is located behind the wing. In an angled wing, some of it is located below the wing and the air trying to fill the low pressure area exerts a lift force on the wing. (It's unlike a balloon. Bernoulli has very limited impact, unlike essentially wind.)
In my understanding, if you increase angle of attack sufficiently to generate vortices on the upper surface, then you aren't efficiently transferring downward momentum to the air your wing is shedding, and you lose lift, which causes aerodynamic stall. Am I missing something?
It's much simpler than that anyway. The wing forces the air downward, so the plane must be forced up.
But equally, if the plane is forced up, the air must be forced down. Cause and effect are not obvious from a force diagram.
It seems nobody really knows:
"No One Can Explain Why Planes Stay in the Air"
https://www.scientificamerican.com/article/no-one-can-explai...
Edit: Added brief from article above:
------------------------------------------------------------
- On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs.
- There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations.
- Aerodynamicists have recently tried to close the gaps in understanding. Still, no consensus exists.
------------------------------------------------------------
In order to stay at a fixed altitude upside down you have to bring the nose of the aircraft up several degrees (increasing based on air speed).
this is really all the intuition most people need to understand flight, even if it leads to an incomplete understanding. it's easy to feel the air pushing on the bottom of your hand when you tilt it up (or top, when tilted down). what's not obvious is that there is also lift created on the top side at the same time, but that can subsequently be learned in high school physics (or fluid dynamics in college, which is where it really stuck for me).
Upside down flight requires you to basically inverse this deflection, but it isn't because of Bernoulli lift.
The overwhelming majority of aircraft have an incidence angle relative to the body for the reason stated. So rather by "typically", could you name a single aircraft that doesn't have such an incidence angle? An SR-71?
As to "0 degrees angle of attack lift", such lift is close to negligible. Maybe you mean the body of the aircraft is zero degrees, but then we loop back to the core point again.
My favourite two "explanations" of flight are 1) dP/dt for air is greater down than up; and 2) Kelvin's circulation theorem, but alas that one is not very pub-friendly...
Wings need to support the weight of your aircraft while being light this means they need to be reasonably thick especially using the obvious choice of storing fuel inside them. The first obvious choice is a teardrop shape which gets lift from being angled up similarly to the way a flat wing does.
Real wings don’t quite use a teardrop shape, but if you look at the front most part of a wing you see it curves both down and up. https://en.wikipedia.org/wiki/Angle_of_attack#/media/File:Ai...
https://en.wikipedia.org/wiki/Lifting_body#Body_lift
In seriousness though, there is a big difference between "bottom up" causality-focused theories and these derived principles based on complicated notions of steady states. Even when the student is too junior not to have any choice but use the latter, I think the difference needs more emphasis.
Also the 3rd law model of flight is so much easier to understand they should teach it first.
Though I think it's more valid to think of the wing as imparting a downward momentum on the air flowing over it. Meaning it's really a reaction engine.
Never underestimate people cluelessness.
"In an analysis of the tides in Venice Lagoon, at the head of the Adriatic Sea, where the tides seem to pick up because of near-resonancy of the basin, Vittori (1992) observed that consecutive tidal maxima are highly irregular. She argued this to be indicative of low-dimensional chaos. Whether the low-order dynamics to which this is due is either inherited from the dynamics of the local wind fields or of a genuinely oceanographic nature is not clear." [1]
[1] https://journals.ametsoc.org/view/journals/phoc/32/3/1520-04...
Funny though, the guy he's interviewing said Islam is a scam and Muslims are suckers who have fallen for it. Funny what you can get away with if you couch it right.
Although the common explanations are often BS.
I learned it multiple times. In middle school, in high school, in university, on youtube explained by a quantum physicist.
Everytime I understood less of it.
So it's better to say that our models are simplified, not lies.
To be fair, for most purposes (atoms, molecules, metals) there usually (very-) technically aren't any electrons, just configurations of the relevant quantum fields (eg in the form of electron orbitals) whose asociated conserved quantities would allow them to convert into a certain number of free-flying electron particles if you dumped in enough energy to make up the difference.
You see this a bit more obviously with ('virtual'[0]) photons, where some non-particulate field configurations simply can't be thought of as particles at all (eg attactive electromagnetic forces).
0: https://profmattstrassler.com/articles-and-posts/particle-ph...
There are no quantum fields either, just as there are no atoms. Things (of the right size) behave as if there they were composed of quantum fields (or electrons or atoms), and the less we have to say "... except that ...", the more comfortable we are with the story.
I look at "quantum computer" components and go "what does a grid of wires have to do with 'computing'? And then you realize the big secret - There's regular computers that take the 'output' of the qubits/QC stuff and 'decide' what the results are, since it's all just a blob of probability anyhow...
Unfortunately I don't remember what book I used. But yeah, this definitely opened my eyes to how electricity works.
Isn’t that how to spot seniority? The junior says “I know ReactJS and SpringBoot!” The senior says: “I don’t know much…”
Unrelated, but that reminds me how my Masters Degree teachers touted the importance of their subject in their introduction course, all explaining the Ariane V explosion from a completely different angle:
- The measurements professor: “Ariane V crashed because engineers tripped themselves into different imperial/metric units, this is why Measurements & Precision is the most important topic!”
- The programming teacher: “They fit a long inside an integer and it looped to negative, which inverted the trajectory of Ariane V and triggered its destruction, this is why learning C properly is the core of your teaching this year.”
- The quality assurance teacher: “They didn’t check the contract of the component! This is why QA is the most important when creating big systems!”
- The management teacher: “It’s the story of two teams who designed two components with different assumptions, one team worked in imperial units and they didn’t communicate clearly about assumptions, that’s why management is the one topic you should really work on.”
They were all right. Or rather, they were all wrong: Everyone knows Ariane V exploded because the officer pushed a red button ;)
...and this is why learning the value of drawing the boundaries and selecting stop points in the analysis of complex topics, and the employment of humor is a powerful rhetorical tool. This is why you composition is the most important topic this semester!
Sorry... Couldn't resist. <Queue the follow up psychology is the most important topic you'll learn this semester, followed by Biology, Social Psychology, Anthropology, all getting stucktrying to get in the door.
Also, what school teaches QA These days?
Interesting question! INSA Lyon in France, but that was in 2005, you could mock that the Old Continent does a lot of V-Cycle waterfall projects and had missed the Agile turn of 2001.
BUT learning how processes help is, instead, a very important step to judge what exactly we give up with Agile.
The irony is I went on creating software for requirements, and I can testify that all of the hardware industry does QA more diligently than ever!
Software Quality Assurance is much more... Spongey.
You'll see this regularly in the series Aviation Disasters on TV. It has lessons for all engineering projects. I watch every episode :-)
They reused the launch code of Ariane 4 in Ariane 5, but Ariane 5 was much faster to take off. It was an overflow on the acceleration and bad testing because reading an Ada exception as flight data is not great.
We learn that at school in France many years later.
There are some old controversies that are largely settled. The Microsoft Flight Simulator manual in 1980 "teached the controversy" but it was really settled decades before that. People still remember the controversy from back then and keep repeating it and probably will still do it when people are living in space colonies.
The Bernoulli effect explanation is bogus.
An alternate (correct) explanation is that if you just took a piece of cardboard, held it sideways, and moved it laterally it would push the air down and thus the cardboard would be pushed up.
If you like vector fields you can show that there is a topological defect (vortex ring) that is threaded through the wings and comes around to the other side. If you do an integral around the ring you can show the vortex holds the plane up.
I hear this but never seem to get any further info. Why are wings shaped with a curved top and flat bottom? Is there a good summary I can go read to understand this all?
AFAIK the wing shape thing is about reducing drag (turbulence?)–not essential but hard to fly well without a nicely shaped wing.
https://www.nasa.gov/sites/default/files/atoms/files/foam_wi...
When learning about flight, keep asking yourself "Then how do planes fly upside down?" Any explanation which does not mesh with sustained, inverted flight is oversimplified to the point of uselessness and inaccuracy.
Most wings aren't shaped like that.
If you're in the sky and you want to stay there, you have to counteract gravity. Heavier-than-air flight does this by pushing down on air. Want to stay in the sky? Push down on enough air, fast enough, and you will stay in the sky.
Since air is a fluid, pushing down on air is equivalent to pushing air down. The lift a plane or helicopter generates is directly proportional to the amount of air it pushes down (and to a varying degree how much engine exhaust it pushes down). That is, we need something to divert air downwards and something to push us through that air. The better we can redirect air downwards, and push ourselves through the air, the easier it is to fly.
We have found many shapes that are very good at passively redirecting air when pushed through the air, we have developed engines that are good at pushing us through the air, and we have developed structures that are able to hold everything together while being light. That is why heavier-than-air flight is possible, and it's very well understood.
If anything, our understanding of why certain shapes redirect air so well is lacking, but even then not really. Experiments and modelling are really good at finding the conditions under which air stops being redirected efficiently. If we try to parameterise this airflow, and reduce it to simple equations, well maybe then the effect is not well explained. Statements like "the air moves faster on the top than on the bottom, so there is a pressure differential and hence a lifting force" may be true even if misleading, and statements like "the air moves faster on top because it is longer than the bottom side" are definitely misleading and incorrect, but just because these statements exist and some people believe them does not mean we don't understand heavier-than-air flight! Such flight is possible because we are able to push down on air with enough force to keep us flying, and so much of how that works is well understood.
[edit]
To try and say something directed more at the point the article seems to be making: confusion or misunderstanding about the technical details, or modelling, of something is very different to not understanding how that thing works.
We understand heavier-than-air flight in the exact same way we understand sailing - redirect airflow to generate a force for your own purpose - but you don't see articles about how we don't understand how sailing works.
That title doesn't get quite as many clicks unfortunately.
The fact that the second "we" doesn't really know aerodynamics, and is going to waste man-days chasing its own half-understandings round in circles in the comments, doesn't contradict that fact that the first "we" does understand aerodynamics. Planes aren't staying in the sky by accident, nor even just by the survivorship of trial-and-error engineering.
The point about "not understanding" flight is that, if we truly understood it, we could design the optimal aircraft from first principles before it ever entered a wind tunnel. Instead, we work based on incrementally improving tribal knowledge of what has worked in the past and try to make something similar to fit our desired flight envelope.
Compared to something like rocket science where the entire craft can be built on a computer and we'll know exactly how much cargo we can get to the moon without even turning a single screw, we don't understand flight.
This is getting less and less true with each generation of supercomputers though.
EDIT: There is perhaps a better way of explaining it for this crowd though. To use numerical modeling to predict performance is to take a physical problem and turn it into a computational problem. And while engineers understand physical systems pretty damn well, us computer scientists have largely failed at the objective of making software systems with hard reliability guarantees. You can write a fluid dynamics simulation to test your new wing design, but how do you know that the simulation does what you think it does? Even if the code has been tested before, how do you know you're not now hitting some sort of edge case?
At the end of the day, you have to build the damn thing to test it. Numerical simulation are used more and more these days as the codes are refined, computers get more powerful, and engineers have more trust in their capabilities. But traditionally, and still a lot of the time, they build prototypes and test in wind tunnels because reality never fails to model physics accurately.
So I would say, nobody really understands software, but many people have experience and total experience is growing. Via new languages, algorithms, patterns, etc.
The rapid experience advancement suggests there is a lot unknown and not understood.
I’ve worked with thousands who don’t and maybe two dozen who do.
Rocket engineering uses an immense amount of both modelling and physical testing. No-one says "Well I've got the Tsiolkovsky rocket equation, so let's go to the moon!"
I'm not even sure what the bar of 'understanding' is here - the fact that we have iterated and improved on powered flight as much as we have necessarily means that we understand it on a deep level, let alone the fact that we can create excellent models that predict what will happen to a wing in different situations.
At what point would you say we do understand flight?
Similarly devilishly complicated is the injector design. Obviously you want to mix the oxidizer and fuel in the optimal ratio for the highest efficiency. That’s the easy part. But then you also want to offset from this optimum near the edges to produce a colder flow near the nozzle wall to protect it from melting. Of course nowadays people do a lot of computer simulations to save on testing time, but it is still not uncommon to discover combustion instabilities or hot-spots in the engine tests.
So no, nobody can, let alone did, design a rocket entirely in a computer and then send it to the moon without many many tests, and incrementally improved tribal knowledge.
I also can’t think of a technology sector where the naive/new/low level/clueless don’t also assume this is not true.
Even manufacturing. ASICs. Software. Everything.
Planes obviously don't require that to fly. So, they're different type of beast. They somehow squeeze more from less, exploiting some nonlinearity in forces that air exhibit on wings. I can understand that too, but the nature of that phenomenon is not explained anywhere (other than in words: this is the formula. It is correct, trust us)
If a plane isn't producing more lift than weight it will fall, just like a helicopter. Planes work by pushing a wing through the air, helicopters by spinning it. In both cases the wing has to push down enough air to keep the aircraft in flight.
I am even more confused now.
Helicopters and planes are both pushing down on air to generate lift. The lift generated has to be equal or greater to the weight for the aircraft to fly.
Thrust can mean many things (at least colloquially). As discussed in the quora you linked, a helicopter will have a defined power to weight ratio that allows it to fly (maintain level flight) in its 'normal' flight envelope. There are a number of things the pilot can do that causes the aircraft to 'push down harder' on the air. One of these is flying close to the ground (the ground effect) which is sort of like pushing against the ground as well as the air, and another is by moving horizontally (usually forwards, like a plane, causing transational lift). Both of these allow the aircraft to maintain height while using less power than if it was hovering, but to do so it is still generating enough lift to counteract gravity.
The propeller doesn't contribute materially to the lifting force on an aircraft, while the rotor of the helicopter provides practically all the lifting force on the helicopter.
Both machines need to (somehow) generate lift equal to their weight to stay airborne. The airplane just does this by moving a static wing through the air, which is a much more efficient way of doing it. Its engine/propeller isn't even immediately required to move the wing through the air; once airborne, the aircraft can fly downwards at an angle without engine power to maintain its speed.
You'd say that a helicopter uses "powered lift" while an aircraft does not.
For helicopters in a static hover, the downward "thrust" is actually the lift produced by the spinning blades. The engines produce almost no forward thrust. Whereas, for an aircraft in flight, the engine thrust pushes the plane forward and the wings generate the lift that keeps it in the air.
But the explanation I can come up with is: lift is a force due to low-pressure regions caused by laminar flow over a surface. It is essentially "form drag" (caused not by the profile facing air directly but by the aft part) but the tricky part is that it is not directly parallel to the flow of air, but also depends on the orientation of the wing.
1) wings increase the surface area pushing down (gravity) on the air below, which pushes back (air pressure), and
2) as wings are falling toward ground (gravity), they create vortices above the wing, which lowers the pressure, increasing the push up effect of the air below, and
at a certain speed, the vortices are stabilized into low pressure regions above the wings, and in a certain "envelope" region, of speed, plane shape, air pressure, all of these forces are equalized to give you level flight, so long as the dial you turn to get into the envelope region, "speed", keeps up.
That's how I understand it. Happy to hear a physicist / aerospace engineer guide me in how to think about this clearly.
https://www.wolframalpha.com/input/?i=NACA+6409+airfoil&assu...
You can have lift with laminar flow. In fact, the article includes an explanation of the usage of the Reynolds number to characterize laminar and turbulent flow and how the flow around plane wings is clearly laminar (called "smooth" in the article).
- The real answer is so hard to compute that there is a million dollar prize attached to it.
- Today, we design planes using approximations and trial-and-error. It works well because we are very experienced in designing planes, sometimes at the cost of many lives, but it is not exactly a "first principles" approach.
I think what people usually mean when they say that we don't understand flight is that there are no simple equations. A lot of physical problems have elegant solutions (eg. the shape of a hanging chain is roughly the cosh function). But there are no elegant equations that describe the profile of a wing, so it's a bit unsatisfying.
As engineers we had been taught that lift was due to air above the wing traveling faster than air below the wing and thus creating lift by way of a pressure differential. The more accurate answer as seen in the article is that the effect is better explained through Newton’s laws as a re-vectoring of horizontal thrust in a downward direction. Literally the engine pushes horizontally accelerated air downward and the action-reaction mechanic causes an equal but opposite upward force lifting the plane.
Amazing how so many experts could be so wrong in their understanding while the planes continue to fly.
It reminds me of how hummingbirds don’t know that they violate the known laws of physics when they fly.
That said with my historic experience with single piston RCs if you didn’t compensate for the upside down flight with your ailerons you would nose dive.
And those upside down events do not happen at 10 feet above ground. There is plenty of fluid (air) above and below the aircraft and power (fighters jet engines are the most powerful ones on aircrafts) to be able to correct any up-downward force with flaps (basically walls to air)
Except that there obviously is. Unless you ignore gravity.
EDIT: trying to think what you might mean by "effectively changing the shape". Do you just mean that an upside-down aerofoil is a reflection of the aerofoil the right way up? Because that's the entire point of the argument you seem to be trying to rebut.
A plane flying upside down is most certainly not using the same angle of attack as it does right side up. The real difference in performance is efficiency, the upside down plane is burning more fuel due to the increased drag from sub-optimal operation (a high angle of attack to overcome the optimization for right-side-up flying).
Note that a right-side-up wing can easily plummet by dropping its angle of attack. That is what it's doing while upside down to generate lift.
I am certainly not asserting that, and I'm baffled how you could have formed the impression that I was.
You appeared to be attempting to rebut an argument in favour of the significance of angle of attack. We have another pointless internet misunderstanding on our hands.
"I usually just answer Socratically: "So how can (some) planes fly upside-down?" whenever I encounter the Bernoulli-adherents."
Which is a lazy and garbled gotcha attempt.
Again: this entire pointless misunderstanding has arisen because you didn't see - apparently STILL HAVEN'T SEEN - which side of the debate the comment you replied to is arguing for.
1) anvandare says aeroplanes can fly upside down. This is an argument AGAINST a putative person who argues that lift is entirely a function of aerofoil shape, ignoring angle of attack. In advancing this argument, anvandare implies that he DOES understand and contend that angle of attack is significant.
2) You say something unclear about "effective change of shape", apparently attempting to rebut anvandare, who, remember, contends that angle of attack is significant.
3) I say that what you said about "effective change of shape" is unclear, meaning I am rebutting you, meaning I agree with anvandare that angle of attack is significant.
4) You form the impression that I believe angle of attack is not significant, and tell me that if I believe angle of attack is not significant, then I am wrong.
Can you see where you have gone wrong there?
Having written all this, I'm come to the point of actually becoming quite concerned about your neurological state. If you've had a recent head injury or you're old enough that Alzheimers is a possibility, you need medical advice - you've failed to follow the simple thread of a conversation.
The Bernoulli effect explains that lift is due to the design of the wing such that the path above the wing is longer than the path below the wing.
This coupled with the fact that due to the Bernoulli effect an air particle just above the wing would reach the back of the wing at the same time as an air particle just below, and that since the upper particle would therefore have to travel faster than the lower particle the pressure differential would cause lift.
The problem is the theory doesn’t hold up under testing because it isn’t true.
- points 1 and 2 lie on a streamline,
- the fluid has constant density (note effects of height difference > gravitational potential energy between point 1 and 2),
- the flow is steady, and
- there is no friction.
This downward force on the airstream bust change its direction.
This change of direction is a rotation about the airfoil. Specifically a downwards rotation.
The airstream moving above the centerline of this rotation is moving in the same direction, and thus will be accelerated faster.
The airstream moving below the centerline of this rotation is moving in the opposite direction, and so will be decelerated to a slower speed.
The center of this rotation happens to be along the camber line of the airfoil, so all the air above the camber line (ie over the top of the airfoil) must move faster, while all the air below the airfoil must move slower.
The vortex is just the bulk rotational movement of the airflow. In fact, the airfoil can be replaced by anything that will generate the same vortex, like a rotating cylinder.
The problem with this theory is that there is no physical reason why both streamlines must arrive at the back of the wing at the same time - and per experimental verification, in fact they don’t.
That's not how the Bernoulli effect explains the pressure differential. The bernoulli explanation is that air builds up in front of the airfoil, creating a high pressure region, while there is a low pressure region created in the wake of the wing. This pressure differential forces accelerates air over the wing. For an asymmetric airfoil, more of this flow is over the top than the bottom, so the airflow over the top is faster, and thus lower pressure, than the airflow under the wing.
The "equal time" thing is a pop-science misunderstanding.
Speaking of sailing, the angle and shape of the sails are both important to maintaining velocity. Racing boats adjust (trim) the shape of the sails all the time.
Merely deflecting the wind obeys conservation of momentum, but conservation of energy in an unpowered sailboat (overcoming losses due to friction) also means extracting energy from the airflow.
Whenever people argue about which interpretation of lift is correct I think back to this (https://xkcd.com/895/) comic about teaching how gravity works in general relativity. Only in the case of lift the explanations are actually _correct_, albeit somewhat circular. ("So the air above the wing sticks to the surface, which redirect it downwards. But _why_ does the air stick to the wing?!")
Also in no way do hummingbirds violate any known laws of physics, although they do have a pretty impressive way of harnessing them.[2]
[1] https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html
[2] https://phys.org/news/2005-06-hummingbird-flight-evolutionar...
So saying that air on top of the wing moves faster, creating a pressure differential and thus lift, is absolutely correct. The problem begins when some people try to come up with an intuitive explanation for _why_ the air would need to speed up. "Because this is the lowest energy state that conserves energy, momentum, and mass" isn't a very satisfying answer; neither is "because this system of PDEs say so"; so they came up with the "equal transit time" explanation, which is simple, intuitive, and completely wrong.
Hopefully aeronautical engineers at P&W didn't actually believe that last bit?
[1] https://www.youtube.com/watch?v=UqBmdZ-BNig
That statement, that the upper airstream flows faster because it has to meet up with the lower airstream, is wrong and easy to disprove experimentally.
So, putting aside that specific statement, re-read the post you're replying to.
The airstream above the aerofoil does travel faster, and there is a lower pressure region there (commensurate with the effect described by Bernoulli), and it turns out that this is a valid way to model the forces involved just as it is valid to model them as a redirection of the airstream.
You mean the wing, not the engine.
But even then that doesn't answer the question. It's just another way of looking at the effect, but it doesn't explain the cause. The question is then why/how does the wing pushes that air downward?
Yes, clarifying I meant the wing not the engine.
> The question is then why/how does the wing pushes that air downward?
Fair point. I honestly didn’t expect this quality of analysis on the topic.
I believe that another phenomenon is required to complete the explanation in addition to Newton. Couette flow explains why streamlines closest to the wing tend to follow the shape of the wing. Hence the vectoring effect. I’m sure a better article exists but Wikipedia is a bit lacking unfortunately.
[1]Couette Flow - https://en.m.wikipedia.org/wiki/Couette_flow
https://youtu.be/X4iMeKif488
I do think you’re dead wrong!
That's not true for rockets. Rockets can fly in air, and a rocket's fins only work in air, but rockets don't have to use fins and rockets work fine in the vacuum of space. Because air is mostly irrelevant to rockets -- and rockets [in space at least] fly by principles that have nothing to do with aerodynamics -- rockets are not typically included in discussions of how "heavier-than-air flight" works.
Star Wars ships come out of space backwards and I don’t get it...
You can also see the same thing with a helicopter flying over water: the water is affected in a circular region fairly close to the rotor itself, indicating that there is a large downward force being exerted on the air and a corresponding upward force being exerted on the rotor.
So an 45degree angled blade absolutely will give you lift but a tapered aerofoil will do the same with less energy spent pushing the air in unwanted directions (specifically fewer swirling vortexes immediately behind the wing causing drag).
So yes push air down to stay up. Don't push air sideways or in circles. The aerofoil shape and the equations that simplify the 'don't push air the wrong way' into a simple term of drag are all about doing this.
That's the observation. Clearly my rigid arm/hand was accelerating some air downward. Like a gun accelerating a bullet, there's a recoil (but a continuous one). That's Newton: conserving momentum. Maybe not a complete explanation, but it's the bulk of one.
Here's how NASA puts it:
https://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html
It sounds like a great eli5, and it gets repeated in serious educational contexts again and again but it’s just absolutely completely wrong.