[bcaa7f3a8bbc]

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.

[Sniffnoy]

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...

[garmaine]

We understand flight perfectly well.

Although the common explanations are often BS.

[k__]

Reminds me of learning what electricity is.

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.

[laurent92]

> 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 ;)

[speedgoose]

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.

[WalterBright]

That's why proper engineering is to address all the causes, even if fixing just one of them would have prevented the accident.

You'll see this regularly in the series Aviation Disasters on TV. It has lessons for all engineering projects. I watch every episode :-)

[salawat]

>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?

[laurent92]

> 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!

[di4na]

small point: the imperial vs metric is not an Arianne V thing but a Mars Climate Orbiter one ;)

[PaulHoule]

https://en.wikipedia.org/wiki/Gimli_Glider

[handrous]

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.

[HPsquared]

For everyone other than the aforementioned physics grad student, it's a lie. For the physics grad student, it's a mystery.

[beckingz]

Turns out, the world is really really complicated.

So it's better to say that our models are simplified, not lies.

[Ericson2314]

The hydraulic analogy is at least honest!

[Judgmentality]

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.

[genewitch]

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...

[PaulHoule]

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.

[hattar]

> The Bernoulli effect explanation is bogus.

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?

[jadyoyster]

This page explain things well: https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso...

AFAIK the wing shape thing is about reducing drag (turbulence?)–not essential but hard to fly well without a nicely shaped wing.

[axiolite]

That's a terribly oversimplified explanation. May I recommend this one instead:

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.

[na85]

>Why are wings shaped with a curved top and flat bottom?

Most wings aren't shaped like that.

[tempestn]

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."

[jobigoud]

Air moves faster on the upper side, creating a pressure differential.

[belter]

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 ;-)

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.

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

[Retric]

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.

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...

[Anarch157a]

Kelly Johnson caused a stir in the engineering community when he came up with the F104 Starfighter, with it's thin and almost flat wings.

[Retric]

Yea, start throwing around enough thrust and you can fly just about anything. The F-15 got to the point where one wing was optional: https://theaviationist.com/2014/09/15/f-15-lands-with-one-wi...

[Someone1234]

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).

[sigmoid10]

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.

[clairity]

> "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).

[defaultname]

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.

[jjk166]

Wings can and generally do have zero degree angle of attack lift.

[azalemeth]

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...

[Ericson2314]

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.

[NickNameNick]

Why does the air move faster on the upper side of the wing?

It'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.

[rcxdude]

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).

[tomsto]

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).

[shadowgovt]

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.

[Ericson2314]

My Newton's-laws-only bullshit:

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.

[MaxBarraclough]

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.)

[mbrameld]

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.

[AstralStorm]

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.

[gpm]

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.

[tomsto]

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’

[jjk166]

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.

[cryptica]

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.

[tomsto]

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)

[AstralStorm]

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.)

[jameshart]

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?

[tomsto]

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)

[marvin]

I'm confused. I was in a plane that flew upside-down, and it didn't fall down. Am also a pilot.

[IshKebab]

A common and wrong explanation. The pressure differential is created by the fact that the air is being pushed into the lower side of the wing.

It's much simpler than that anyway. The wing forces the air downward, so the plane must be forced up.

[jjk166]

Another common and wrong explanation. The air is pushed down by induced rotation. An inclined wing is one way to do it, but is not necessary.

[IshKebab]

I don't know what you are trying to say.

[jjk166]

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.

[beckingz]

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.

[salawat]

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.

[Gibbon1]

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.

[S_A_P]

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.

[civilized]

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.

[varispeed]

Well, I secretly knew this video is true: https://youtu.be/JYAq-7sOzXQ?t=98