Why we don’t understand heavier-than-air flight

https://www.ribbonfarm.com/2021/04/08/heres-why-we-dont-understand-heavier-than-air-flight/

by sillybilly • 5 years ago

133 168 5 years ago

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.

tempestn• 5 years ago

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• 5 years ago

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

Gibbon1• 5 years ago

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• 5 years ago

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.

salawat3• 5 years ago

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.

Sniffnoy• 5 years ago

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...

garmaine2• 5 years ago

We understand flight perfectly well.

Although the common explanations are often BS.

varispeed• 5 years ago

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

Cogito5• 5 years ago

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.

armada651• 5 years ago

"We know how heavier-than-air flight works, but I want to be pedantic and nerd about some physics"

That title doesn't get quite as many clicks unfortunately.

carl_dr• 5 years ago

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.

parineum• 5 years ago

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.

garmaine2• 5 years ago

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.

jessedhillon• 5 years ago

Almost nobody understands software then, by this definition.

Cogito5• 5 years ago

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?

krisoft• 5 years ago

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.

hamilyon22• 5 years ago

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)

Cogito5• 5 years ago

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.

blueblisters• 5 years ago

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.

exporectomy• 5 years ago

Seems like rockets and bullets somehow don't count as being heavier-than-air flight. Maybe they should say we don't understand aerodynamic lift?

trhway• 5 years ago

rockets and bullets don't fly. They [free]fall.

LinAGKar• 5 years ago

Rockets fly, although they use thrust rather than lift.

raverbashing• 5 years ago

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.

graderjs• 5 years ago

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.

tomsto6• 5 years ago

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.

bbojan• 5 years ago

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.

robotresearcher• 5 years ago

A defining characteristic of a plane is that it is not continuously falling. Falling can’t really be the explanation, since they don’t.

oolonthegreat• 5 years ago

I didn't understand what we don't understand about hta flight but maybe that's just me.

progfix• 5 years ago

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.

yiyus• 5 years ago

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).

GuB-42• 5 years ago

- 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.

scarier• 5 years ago

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.

yarcob• 5 years ago

I have the same feeling.

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.

stensonb• 5 years ago

Would be curious to get a physicist's explanation of how the Coanda effect relates: https://en.m.wikipedia.org/wiki/Coand%C4%83_effect

jjk1669• 5 years ago

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.

lowkey8• 5 years ago

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.

ghosty141• 5 years ago

As a layman this explanation makes more sense than the "fast air over the top of the wing" one.

anvandare• 5 years ago

I usually just answer Socratically: "So how can (some) planes fly upside-down?" whenever I encounter the Bernoulli-adherents.

chasd00• 5 years ago

Has someone taken an rc airplane and turned the wing upside down and flown it? It should be pretty easy to demonstrate.

naves• 5 years ago

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)

rkique• 5 years ago

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?

afterburner6• 5 years ago

Changing the angle of attack effectively changes the shape of the airfoil. No gotcha here either.

twirligigue• 5 years ago

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.

lowkey8• 5 years ago

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.

afterburner6• 5 years ago

Causing a pressure difference is the same as causing a force vector. There's no gotcha here, it's just two ways of looking at the same thing.

lowkey8• 5 years ago

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.

lowkey8• 5 years ago

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.

formercoder• 5 years ago

Same as sailing, does the airplane have an equivalent of the keel?

mr_toad• 5 years ago

Yes, the engine.

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.

slavak2• 5 years ago

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]

[1] https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html

[2] https://phys.org/news/2005-06-hummingbird-flight-evolutionar...

lowkey8• 5 years ago

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.

benhurmarcel2• 5 years ago

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?

lowkey8• 5 years ago

You mean the wing, not the engine.

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

tdy7213• 5 years ago

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:

https://youtu.be/X4iMeKif488

I do think you’re dead wrong!

dreamcompiler• 5 years ago

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.

tdy7213• 5 years ago

Technically correct. Hey it’s only mostly flight until max Q, then it’s... spaceflight?

Star Wars ships come out of space backwards and I don’t get it...

JasonFruit• 5 years ago

Many people would object that rockets don't fly, they are hurled.

perryizgr8• 5 years ago

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".

pansa2• 5 years ago

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.

jjk1669• 5 years ago

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.

jeffreyrogers2• 5 years ago

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.

AnotherGoodName• 5 years ago

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.

benhurmarcel2• 5 years ago

Obviously lift is produced by pushing air down. The question is why/how is the air pushed down.

jeffreyrogers2• 5 years ago

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.

8bitsrule• 5 years ago

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.

Here's how NASA puts it:

https://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html

ec109685• 5 years ago

That only (intuitively) explains lift generated by a positive angle of attack. As shown here, you can get lift with a flat angle of attack: https://www.wolframalpha.com/input/?i=NACA+6409+airfoil&assu...

smusamashah• 5 years ago

The upside is curved pushing air upwards reducing the pressure hence generating lift from below. Isn't it just that?

boublepop3• 5 years ago

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.

Why we don’t understand heavier-than-air flight

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