[HN Gopher] Why we don't understand heavier-than-air flight
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Why we don't understand heavier-than-air flight
Author : sillybilly
Score : 121 points
Date : 2021-06-26 07:20 UTC (15 hours ago)
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| raverbashing wrote:
| 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 wrote:
| 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.
| tomsto wrote:
| 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 wrote:
| 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.
| tomsto wrote:
| For a symmetric aerofoil, 0 angle of attack is 0 lift. For
| an asymmetric - there is (some) lift, which comes entirely
| from the suction side.
|
| https://www.wolframalpha.com/input/?i=NACA+6409+airfoil&ass
| u...
| robotresearcher wrote:
| A defining characteristic of a plane is that it is not
| continuously falling. Falling can't really be the explanation,
| since they don't.
| Cogito wrote:
| 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.
| [deleted]
| armada651 wrote:
| "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 wrote:
| 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.
| zazen wrote:
| 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.
| [deleted]
| parineum wrote:
| 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.
| jessedhillon wrote:
| Almost nobody understands software then, by this definition.
| foobiekr wrote:
| I can't tell if this is meant as a counter or not.
| Literally the pool of people who really understand capital
| S Software is tiny.
|
| I've worked with thousands who don't and maybe two dozen
| who do.
| Nevermark wrote:
| 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.
| Cogito wrote:
| 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 wrote:
| 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.
| foobiekr wrote:
| I can't think of a technology sector where this logic isn't
| true.
|
| 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.
| garmaine wrote:
| 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.
| meheleventyone wrote:
| 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.
| hamilyon2 wrote:
| 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)
| Cogito wrote:
| 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.
| hamilyon2 wrote:
| So, people answering on quora for example, are wrong?
| https://www.quora.com/Can-a-helicopter-having-a-power-to-
| wei...
|
| I am even more confused now.
| DangitBobby wrote:
| 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.
| afterburner wrote:
| 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.
| DangitBobby wrote:
| 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.
| marvin wrote:
| 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.
| Cogito wrote:
| 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.
| blueblisters wrote:
| 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.
| stensonb wrote:
| 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
| jjk166 wrote:
| 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.
| jjk166 wrote:
| 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.
| exporectomy wrote:
| 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 wrote:
| rockets and bullets don't fly. They [free]fall.
| LinAGKar wrote:
| Rockets fly, although they use thrust rather than lift.
| technothrasher wrote:
| Bullets fly too, ballistically. As do balloons, bouyantly.
| This is all kind of a pointless semantic eddy though.
| oolonthegreat wrote:
| I didn't understand what we don't understand about hta flight but
| maybe that's just me.
| yarcob wrote:
| 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.
| GuB-42 wrote:
| - 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 wrote:
| 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.
| progfix wrote:
| 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 wrote:
| > 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).
| lowkey wrote:
| 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.
| twirligigue wrote:
| 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.
| lowkey wrote:
| 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.
| adtac wrote:
| 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.
| drwiggly wrote:
| 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?
| lowkey wrote:
| 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.
| Serow225 wrote:
| 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),
|
| - the flow is steady, and
|
| - there is no friction.
| benhurmarcel wrote:
| > 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?
| slavak wrote:
| 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...
| lowkey wrote:
| 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.
| afterburner wrote:
| 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.
| lowkey wrote:
| 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.
| jjk166 wrote:
| > 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.
| lowkey wrote:
| 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.
| jjk166 wrote:
| The upper flow does move faster, and must for the sake of
| vortex production.
| lowkey wrote:
| 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.
| jjk166 wrote:
| 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.
| formercoder wrote:
| Same as sailing, does the airplane have an equivalent of the
| keel?
| ghosty141 wrote:
| As a layman this explanation makes more sense than the "fast
| air over the top of the wing" one.
| anvandare wrote:
| I usually just answer Socratically: "So how can (some) planes
| fly upside-down?" whenever I encounter the Bernoulli-adherents.
| rkique wrote:
| 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?
| [deleted]
| chasd00 wrote:
| Has someone taken an rc airplane and turned the wing upside
| down and flown it? It should be pretty easy to demonstrate.
| dogma1138 wrote:
| 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.
| afterburner wrote:
| Changing the angle of attack effectively changes the shape of
| the airfoil. No gotcha here either.
| zazen wrote:
| 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.
| afterburner wrote:
| 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.
| zazen wrote:
| > 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.
| afterburner wrote:
| I am replying to this:
|
| "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.
| zazen wrote:
| 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.
| afterburner wrote:
| Oh I see. You think the Bernoulli approach is independent
| of angle of attack? It's not.
| zazen wrote:
| 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.
| naves wrote:
| 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)
| marvin wrote:
| 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.
| jeffreyrogers wrote:
| 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.
| benhurmarcel wrote:
| Obviously lift is produced by pushing air down. The question is
| why/how is the air pushed down.
| jeffreyrogers wrote:
| 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.
| AnotherGoodName wrote:
| 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.
| bcaa7f3a8bbc wrote:
| 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 wrote:
| 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 wrote:
| Air moves faster on the _upper side_ , creating a pressure
| differential.
| NickNameNick wrote:
| 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.
| mbrameld wrote:
| 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.
| Ericson2314 wrote:
| 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.
| rcxdude wrote:
| 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 wrote:
| 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 wrote:
| 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.
| AstralStorm wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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)
| IshKebab wrote:
| 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 wrote:
| 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 wrote:
| I don't know what you are trying to say.
| jjk166 wrote:
| 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 wrote:
| 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.
| belter wrote:
| 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 wrote:
| 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 wrote:
| 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.
| [deleted]
| Retric wrote:
| 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 wrote:
| 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).
| defaultname wrote:
| 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 wrote:
| Wings can and generally do have zero degree angle of
| attack lift.
| defaultname wrote:
| The 747 wing is at a 2deg 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 5deg at the body,
| twisting to -0.5deg 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.
| mbrameld wrote:
| 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.
| sigmoid10 wrote:
| 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 wrote:
| > "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).
| azalemeth wrote:
| 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 wrote:
| 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.
| marvin wrote:
| I'm confused. I was in a plane that flew upside-down, and
| it didn't fall down. Am also a pilot.
| Gibbon1 wrote:
| 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.
| salawat wrote:
| 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.
| S_A_P wrote:
| 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 wrote:
| 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.
| skywal_l wrote:
| 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.
|
| Never underestimate people cluelessness.
| exporectomy wrote:
| 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]
|
| [1] https://journals.ametsoc.org/view/journals/phoc/32/3/1520
| -04...
| tsimionescu wrote:
| 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...
| exporectomy wrote:
| Oh, well that's different.
|
| 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.
| Sniffnoy wrote:
| 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 wrote:
| We understand flight perfectly well.
|
| Although the common explanations are often BS.
| PaulHoule wrote:
| 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 wrote:
| > 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?
| na85 wrote:
| >Why are wings shaped with a curved top and flat bottom?
|
| Most wings aren't shaped like that.
| jadyoyster wrote:
| This page explain things well: https://www.grc.nasa.gov/w
| ww/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.
| k__ wrote:
| 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.
| handrous wrote:
| 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 wrote:
| For everyone other than the aforementioned physics grad
| student, it's a lie. For the physics grad student, it's a
| mystery.
| beckingz wrote:
| Turns out, the world is really really complicated.
|
| So it's better to say that our models are simplified, not
| lies.
| a1369209993 wrote:
| > 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...
| PaulDavisThe1st wrote:
| 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.
| zamfi wrote:
| All models are wrong, but some are useful. (George Box)
| Ericson2314 wrote:
| The hydraulic analogy is at least honest!
| Judgmentality wrote:
| 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 wrote:
| 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...
| laurent92 wrote:
| > 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 ;)
| salawat wrote:
| >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 wrote:
| > 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!
| salawat wrote:
| 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.
| PaulHoule wrote:
| https://en.wikipedia.org/wiki/Gimli_Glider
| di4na wrote:
| small point: the imperial vs metric is not an Arianne V
| thing but a Mars Climate Orbiter one ;)
| speedgoose wrote:
| 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 wrote:
| 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 :-)
| varispeed wrote:
| Well, I secretly knew this video is true:
| https://youtu.be/JYAq-7sOzXQ?t=98
| 8bitsrule wrote:
| 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
| tdy721 wrote:
| 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 wrote:
| 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.
| tdy721 wrote:
| 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 wrote:
| Many people would object that rockets don't _fly_ , they are
| _hurled_.
| pansa2 wrote:
| 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.
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