Lift: how do you explain why air accelerates over the top of the wing?
Posted by Logical-Lock8822@reddit | flying | View on Reddit | 176 comments
Studying for cfi, went down a bit of a rabbit hole on aerodynamics to try to understand the principles of flight a bit better. From what I've read, the aviation explanation for low pressure above is sort of backwards, with the acceleration of air above the wing being caused by a positive pressure gradient just after the stagnation point, instead of the increased camber of the upper half accelerating air and therefore creating low pressure through Bernoulli's principle, explaining how aircraft with symmetrical wing airfoils still produce lift.
I still dont really understand this explanation and the more I look into it the more confused I get, and I don't see any point in trying to teach a ppl student a much more complex and even contradictory explanation for lift than the FAA wants.
But now I am still stuck without a simple way to tell a student why the increased camber of the upper wing accelerates air instead of "it just does".
I've heard some people say that the air molecules on speed up to meet up with the same molecules on the bottom, but this is wrong.
How do you explain it?
TopAct9437@reddit
Think in terms of the actual airflows created in flight. A wing flies through static air. (Do NOT think in terms of the relative airflows seen in a wind tunnel experiments over a stationary wing.) As the wing moves forwards it creates a void of empty space behind it. This is the space that the wing previously occupied. Void = Low air pressure. Due to the positive AOA, this low pressure is on the topside of the wing. As a result, the low air pressure on the topside of the wing then pulls the air above the wing downwards. This action creates downwash. Simple.
Go_Loud762@reddit
Just regurgitate the FAA approved answer. After you pass the checkride you can fully dive into the reality of lift.
UnhingedCorgi@reddit
Or just disregard it as trivial information not necessary to operating an airplane
1202burner@reddit
I've been bitching about this ever since my discovery flight.
Exactly how does this help me operate an airplane? I'm not thinking about this shit at all while flying, literally zero minutes of flight time are spent thinking about how my aircraft is generating lift.
Expensive_Dig_6695@reddit
Been flying for 57 years and the explanation of the air in front and back of foil diverge in front and “re merge” in back causing the air over top of foil travel faster and creating lift has always worked for me.
Temporary_Purchase_6@reddit
Completely wrong
PinKindly7701@reddit
Except the fact that its wrong
Illustrious_Cow_4847@reddit
Stop bitching about learning things thats a terrible attitude
pisymbol@reddit
When your stall horn goes off without being in the "practice" area, I'm positive you will start thinking about it. Positive.
UnhingedCorgi@reddit
You will think about (hopefully) breaking the stall and recovering. You will not be thinking about Bernoulli or Newton.
Lumberjack-1975@reddit
You will be thinking about it when you’re pulling out of a spin, I know I sure was.
pisymbol@reddit
For sure, but these concepts help you understand why you do what you do.
1202burner@reddit
No, I'm thinking about recovering from a stall. After that I'm thinking about not going into a secondary stall.
After that comes trying to figure out what I did wrong or if something is wrong with the airplane.
Bunslow@reddit
It helps you realize just how delicate upper surfaces are, and how thoroughly you need to protect the top of your wing, fuselage, and all sides of stabilizers.
(While airborne, it helps increase fear of icing and associated preventative caution. If you're already iced up, it doesn't help much other than the standard "reduce your aoa, and be aware that your critical aoa is much lower than normal")
UnhingedCorgi@reddit
You can fully appreciate those points without a deep dive into how lift works. Plenty of accident case studies to instill some fear too.
SpartanDoubleZero@reddit
MAGIC
hunman2019@reddit
My uncle was an engineer at Boeing, specifically designing airfoils, and when I asked him this same question he said even they don’t really get it lmao one thing thats for sure is no one theory is completely wrong but none of them really give the full explanation on their own
slicktommycochrane@reddit
I've taken two different university aerodynamics classes I'm still not sure I fully understand it, so good luck.
Lv_InSaNe_vL@reddit
If people much much much much much much much smarter than me cant come to an answer what makes you think I can understand it!
Lpolyphemus@reddit
(much x 10^7) smarter
ChainringCalf@reddit
We do have an answer, just not a concise and digestible one
TopAct9437@reddit
Look at the actual airflows of a wing in flight - and not the relative airflows seen in wind tunnel experiments.
As the wing moves forward, it creates a void of empty space behind it (i.e. low-pressure zone of space). If the wing has a positive AOA, then this empty space is behind the topside of the wing.
This low-pressure zone on top of the wing then pulls the air above the wing downward and slightly forward.
Guilty-Box-7975@reddit
Bernoulli's is still just a principal.
Newton's 3rd is the LAW
Creative-Grocery2581@reddit
Remember the simple physics P1.V1 = P2.V2 Which means when speed/velocity increases, pressure decreases and vice versa. The air above the wing has to travel faster than the air below the wing due to the shape. Hence the low pressure on the top side of the wing.
If you want to think more….Assume time is a constant. The Simple observation is that there is more linear distance on the top of the wing compared to the bottom of the wing due to the shape. Air on the top has to travel faster than the bottom to cover the same distance during same time.
Hope this clarifies.
bailaowai@reddit
You are basically restating OPs question and affirming it. But this is regurgitating the debunked “equal time” theory of lift. It’s false and should be purged from our collective memory. The idea that air molecules go faster along the top of a curved airfoil because the linear distance is greater is nonsense. It implies the air molecules that are separated at the leading edge somehow “want to arrive at the same time” at the trailing edge, which is of course absurd.
Creative-Grocery2581@reddit
I see what you are saying. However, Air molecules do go faster when they have to travel a longer distance at the same time. That’s how wind is created. Your statement will be correct if the air molecules stay stationary in the relative environment. You can also think this way… Use the bottom air as stationary. In that small environment of relative space, there will be a wind on the top compared to the bottom section which is another way to think the cause of low pressure.
If you want to go deeper, a reference is to look at fluid dynamics theories. Each type of fluid or gas will always maintain a certain amount of energy under the variables of temperature, pressure and density. The mobility of the molecules are proportional directly to total energy and hence are predictable under a combination of temperature, pressure and density. Think this aspect and the air molecules tend to move as a result without staying stationary which is similar to the concept of wind.
Hope this helps. For context, I’ve an engineering background and I know it could be confusing sometimes.
bailaowai@reddit
Well, you just defined velocity. Anything that travels a longer distance in the same amount of time vs something else is by definition going faster. The point is the suggestion that somehow air molecules are going faster because the "linear distance on the top surface of the wing is longer" is fallacious. The air above the wing is indeed moving faster, but it has nothing to do with the linear distance being greater, or some concept that molecules "want" to arrive at the trailing edge at the same time. They do not in fact arrive at the same time at the trailing edge - the ones above arrive much sooner than the ones flowing below.
Creative-Grocery2581@reddit
That’s a good argument point. However, think in a three dimensional space with total volume created in a cube above the wing will be higher than the total volume below. For the same amount of relative molecules, you have more space above the wing if you don’t want to think the linear distance. This causes in creating a gap in the total energy between the two layers. To your question about why the air molecules will try to move…The air molecules will have to address the immediate energy gap. You can also think about how winds are produced irrespective of airplane wing designs. That will hopefully offer an explanation. Hope this explains or let me know if you find another explanation that’s more convincing to you.
bailaowai@reddit
Well, you’re a bot, or AI, or just very confused. Or maybe just very confused AI. But you have no idea what you’re talking about. I’m not asking you for help; I learned this is fluid mechanics when I got my engineering degree. Have a good one. 🤡
Creative-Grocery2581@reddit
Same here man. I learned the concept of fluid dynamics during my engineering time. The fact is air moves faster over the wing and that has been helping us fly for decades. It’s just what data points can help an individual to understand. Good luck on proving the fundamentals otherwise.
Wasatcher@reddit
"Due to the design of the wing, air moves faster over the top of the wing and slower across the bottom." I appreciate your desire to learn and understand but as long as you don't tell the DPE "the air moves over the top faster and meets up with the air on the bottom" you're fine. They hate equal time theory.
The detailed physics explanation you're seeking would confuse the best commercial pilot applicant. Just keep it simple for your students.
DonnerPartyPicnic@reddit
I mean, its not correct but it is a way of visualizing it
jet-setting@reddit
This is a fantastic explanation for this exact question.
https://ciechanow.ski/airfoil/
There are equally great explanations for lots of other various things on his site, particularly GPS and the internal combustion engine. Great resource for visuals to include in your lessons if nothing else.
TheOriginalJBones@reddit
We’d better keep this website to ourselves if we don’t want the Future Farmers adding “How many particles of still air are contained in an 80-nanometer cube?” to the tests.
Here’s how I explain it:
A wing makes the air passing over it turn if the wing’s at an angle. The top and bottom of the wing both make the air turn. If the top of the wing is curved, it turns the air better because Bernoulli-spoon-sink effect.
Air is kinda heavy. When it turns Newton happens.
When the angle gets too much, the top of the wing all of a sudden doesn’t turn the air so good anymore.
probablyinahotel@reddit
Wow. 30+ years of flying and I've never seen a more detailed explanation of lift. That was a fun rabbit hole to go down.
Logical-Lock8822@reddit (OP)
now this is sick
jet-setting@reddit
I like to use this page as a case study with CFI students, pay attention to how it develops through the levels of learning, from known to unknown and from simple to complex.
cazzipropri@reddit
The ground truth is Navier-Stokes. You can't teach Navier-Stokes to someone who didn't take differential equations first.
Excet92@reddit
You can also consider the theory of circulation (kutta joukowski theorem)
TheOldBeef@reddit
Ugh. The navier stokes equations are hard to solve, not hard to understand qualitatively. All they do is describe conservation of mass and momentum.
cazzipropri@reddit
But practically, it is out of the question to introduce them to the standard 19yo PPL student.
It's just not feasible from a lesson planning point of view.
Admirable_Meet_931@reddit
“Assume a spherical cow…”
TheOldBeef@reddit
Well yeah, it's not worth it introducing the specific equations to most flight students, but it wouldn't be hard to explain the basic principles of them. Instead we have do drone on about Newton's third law and Bernoulli's because that's what DPEs want to hear
7layeredAIDS@reddit
And energy
Vihurah@reddit
half the time you cant even explain it to someone who did (i got an engineering degree and still have no idea what im looking at outside the variables)
InvestmentGuilty8736@reddit
There’s also no solution to navier stokes 👀 so I stand on ….lift=magic. (Aerospace engineer and pilot so I think I know a thing or two about magic(I barely passed engineering school))
Mission-Wasabi-7682@reddit
As a fellow aerospace engineer and pilot I politely disagree. The Navier-Stokes-Equation has no analytical solution. But we are perfectly capable of solving it numerically. So at least we know how to simulate the magic ;)
coldnebo@reddit
meh, simulation is still pretty expensive. it’s much easier to flight test.
the only sim I heard that has 100% fidelity is the F-22 Raptor Demo. they apparently get the same behavior out of the sim as irl, aerobatics and everything.
although I have my doubts that NS is a factor if you can be sandwiched between laminar flow and the rocket equation. 😅
Mission-Wasabi-7682@reddit
Sure thing, simulation of complete flight dynamics is expensive (but a lot easier for commercial aircraft due to much easier flight envelope), but when it comes to pure aerodynamic it’s standard procedure. Every wing profile went through extensive CFD before any test flight, commercial or mil.
coldnebo@reddit
yes, but as we know, CFD on the parts does not necessarily equal CFD on the whole. we still simplify expediently.
that’s primarily because the CFD analysis is used to optimize parts for laminar flow regimes and there is an expectation that jets are not used intentionally outside those regimes.
so optimizing parts and then recombining the linear models makes sense and saves time… but I don’t know if any company is completely skipping flight test based on their CFD models yet?
TgardnerH@reddit
Its always a fun to ask "how would this be different if it was actually magic," and in this case, as in quantum physics, I always come around to there being no difference. The physics are governed by arcane rules that no human has a full understanding of, our best tools can only approximate, and ultimately you kind of just have to try it and see what happens. It's amazing, and magic.
coldnebo@reddit
I mean yes, but also no.
this was the argument put forth on social… since equal transit is wrong and NS doesn’t have an analytical solution, we must not know how we fly.
But this overlooks statistical arguments where Bernoulli is pretty good on average and polars are fairly accurate in laminar regimes.
The numbers in your POH come from two sources: 1), the polars give the theoretical limits in laminar flow, 2) flight test refines those numbers if real behavior differs. These numbers are perfectly good, we don’t throw them out because they aren’t absolutely correct. They are “good enough”.
Furthermore, the regimes where these calculations don’t work (stalls, slow flight, wake turbulence, wind shear and airshow aerobatics) are either trained to be avoided or recovered.
this is similar to weather prediction. we may complain about precise weather predictions being inaccurate, but the general statistical physics of stable vs unstable air-masses plays out with remarkable consistency. so we teach to that instead of throwing up our hands and saying we can’t understand anything about weather.
Mission-Wasabi-7682@reddit
I’m no expert in quantum mechanics, but aerodynamics, Navier-Stokes, even turbulence and boundary layers are pretty well understood by now. There is no trial and error. Even for the most complex airflows we can simulate it to virtually any required accuracy.
Own_Leadership7339@reddit
I struggled with differential equations so much, lol. I think it'd be easier to memorize the answer rather than understand it.
InvestmentGuilty8736@reddit
We called diff-eq, diff-e-screw, back when I was in school.
Mission-Wasabi-7682@reddit
Yes you can. The solution might be hard, the principal is relatively simple.
britishmetric144@reddit
Meteorologist here; those equations are complex. To the point that almost no one solves them anymore by hand, even in class. You really need to understand what a partial derivative is for them.
Electrical-Fee5127@reddit
“We observe that air moves more quickly over the more curved upper part of the wing/airfoil, and Bernoulli’s principle states that as a fluid, like air, increases in velocity it decreases in pressure. Thus we are left with higher relative pressure under the wing and lower relative pressure above. The high seeks the low, causing the wing to lift.”
Electrical-Fee5127@reddit
And then the FAA explanation for Newton’s third law, even though nasa has a presentation on specifically why the FAA’s explanation is incorrect.
ArmadilloNo6976@reddit
Air flow at the intake of a tube = airflow at the exhaust of the tube. Tube becomes thinner = less room for the air to pass trough : so the air has to speed up to flow at the same rate. Air moves faster = less time to press on the tube wall = less static pressure = lift. Now cut the tube in half and you have a wing. All this considering the air is uncompressible, at higher flow the compression changes the game.
Being_a_Mitch@reddit
Without getting in the weeds, I like a basic "get the idea" explanation.
Take a bowl of water. Put a flat hand fingertips-down in the bowl. Now move your hand across the bowl towards your palm and observe how as you push water to one side of the bowl, a 'gap' opens behind your hand that water rushes to fill. You also feel extra pressure on your palm from the water you're pushing out of the way.
Air is the same way. As an airfoil moves with some angle of attack, air is split into flow over and below the airfoil. Air below the airfoil is like the water you're pushing with your palm. As the airfoil forces it out of the way downward, it is initially slowed down, and the pressure is increased as the wing is pushing air out of the way. Air over the top of the airfoil is just like water behind your hand. As the airfoil slices the airflow in half, as long as there is some angle of attack, there will be a "gap" above and behind the wing where the air has essentially been pushed out of the way. The air along the top of the airfoil will be accelerated towards that low pressure zone. Think of it like the air over the top is always rushing to try to fill that 'pocket' above the wing.
This has nothing to do with "air molecules meeting back up", camber length, or anything like that. "Equal Transit Theory" is the catchall term for those incorrect lift explanations, and NASA has a great website that explains why it's wrong. Googling "NASA Equal Transit Theory" will get you there.
Nice_Cellist_7595@reddit
Explore the Bernoulli principle a bit more. Due to the lower pressure, velocity increases. https://en.wikipedia.org/wiki/Bernoulli%27s_principle Also note that for a long time it was thought that the flow would reconverge at the trailing edge of the wing but this has been experimentally proven to be false.
lnxguy@reddit
Bernoulli. His theories suck.
pmmeuranimetiddies@reddit
Yeah imma be honest as an airspace engineer you reach a point where qualitative explanations stop being comprehensive and you kinda just crunch the equations
It doesn’t really matter if a partial vacuum causes air to accelerate or if it’s the bernoulli principle causing the pressure to drop… because bernoulli becomes the equation that describes the relationship in either case.
Nerds in STEM like playing the “well actually” game so you end up with many competing quantitative explanations but at the end of the day, lift can be accounted for solely by Navier-Stokes, the more general form of Bernoulli, or solely by Newton’s third.
Qualitatively, the best explanation I can offer is that airfoils are cambered to deflect airflown downwards more efficiently to produce the most lift for the least drag
Mission-Wasabi-7682@reddit
I also think the “air goes down, plane goes up” explanation is the most accurate and tangible of all the simplified explanations.
But actually, neither is the Navier-Stokes-Equation a more general form of Bernoulli’s equation nor is it solely Newton’s third.
Navier-Stokes-Equation has three components: * conservation of mass * conservation of momentum (Newton’s second law) * conservation of energy (which is somehow related to Bernoulli’s law for certain simplified conditions)
In real life you would also want to add a turbulence model, since Navier-Stokes strictly speaking only applies to laminar flows.
pmmeuranimetiddies@reddit
We’re really not beating the “well actually” allegations, are we?
Anyways, Bernoulli is literally the steady, inviscid case for the NX x-momentum equation.
Mission-Wasabi-7682@reddit
Sure we do. Yes, if you take NSE and omit conservation of mass and conservation of energy and if you assume incompressibility and no viscosity and then finally drop two dimensions you end up with Bernoulli’s principle. Never said it’s not in there. But if you want to explain lift for an aircraft you need at least: * a second dimension to account for wing geometry * compressibility because of air is compressible
Therefore, Bernoulli’s principle is not fitting to explain lift and it drives me nuts that this is still taught this way.
But the point is Navier-Stokes is not a generalized form of Bernoulli’s principle. Neither was it historically derived that way, nor would you teach it so. Also Newton’s third law is not in there.
pmmeuranimetiddies@reddit
This is exactly what I mean by the “ well actually” game, you’re splitting hares so fine Lola Bunny gets body image issues.
Big_Marzipan_405@reddit
airspace engineer
HawaiiClipper@reddit
You seen the TERPS? Forget Navier Stokes, THAT shit is complicated!
pmmeuranimetiddies@reddit
Damn autocorrect
scootty83@reddit
Newton’s 2nd Law applies to fluids just like anything else:
Air accelerates when there is a pressure gradient force acting on it.
The air accelerates from a higher pressure to a lower pressure.
So, think of what the air is doing as it strikes an airfoil:
At the leading edge, air stagnates and the flow splits. A pressure field develops, including a lower pressure over top of the foil.
This lower pressure creates a favorable pressure gradient which accelerates the air over the foil.
The air speeds up not just because the foil is forced through it, but because it is also being pulled along by lower pressure ahead of the air being forced over.
itszulutime@reddit
In my aerospace physics class way back when in college, my professor made an analogy with a harden hose. Compress the hose and the water moves faster. The shape of the top of the wing is curved in the same way the hose is when you compress it with your finger, so you get some compression of the air molecules as they move over the top of the wing, causing them to accelerate.
Big_Marzipan_405@reddit
they teach this incorrect/incomplete explanation at universities?
itszulutime@reddit
I posted a very basic explanation of the Venturi effect, which is part of the equation for Bernoulli’s principle, since OP was looking for a simple explanation for primary students. Has the science changed in the last 25 years? I haven’t kept up with physics since then, but that is absolutely what major universities were teaching back then.
Big_Marzipan_405@reddit
a wing is not a venturi
Rainebowraine123@reddit
It's a comparison to help understand the basic principle, not a scientific explanation.
poser765@reddit
The certainly did 20 years ago
Rainebowraine123@reddit
I dont know why you are getting downvoted. Its like the Venturi effect.
rustypilot66@reddit
Give the standard FAA answer. If the examiner asks you for more, get a strip of paper, let it hang down for your hand, then blow over the top of the paper. The strip of paper will rise due to less pressure on the side you are blowing - just like the air moving over the top of the airfoil. Then ask the examiner “any other questions?”
CreakingDoor@reddit
Just tell them that it’s because Bernoulli said so, and he seems like a trustworthy guy you know?
SRM_Thornfoot@reddit
If your student is familiar with engines, more specifically carburetors, or if they have ever used a paint gun then you have a point of knowledge you can use because they should be familiar with a venturi valve. A paint gun and a carb use a venturi valve that has a hole in the middle of it attached to a tube that sucks either paint or gas up due to the low pressure created inside the venturi when air passes through it. A wing is simply a venturi valve cut in half and the same principal still applies that there is lower pressure created above the wing than there is below the wing. This alone does not make the wing fly but makes it much more efficient. If you have a newspaper page, you can demonstrate this by holding it up in front of you by the top two corners and blowing first below the top of the paper directly onto the paper which just moves the paper back a little bit, and then blowing over the top of the paper simulating faster moving air over the top of the wing which will lift the paper nearly horizontally straight out. Note though, that a plane can also fly upside down because it also are forces a significant amount of air in a downward direction which it does both right side up and upside down.
TheOldBeef@reddit
None of this explanation is correct
SRM_Thornfoot@reddit
Are you unfamiliar with a carburetor or were you just never taught it this way? Because this analogy is pretty spot on. All three operate on the exact same principle, which is known as Bernoulli's Principle.
TheOldBeef@reddit
Yes, I know what a venturi is, and what it isn’t, and it isn’t an airplane wing. See Nasa and various other sources for why the wing is not a venturi
SRM_Thornfoot@reddit
I'm not convinced that you do. A wing is essentially one half of a venturi. Is it a true venturi? No. It is half a venturi and therefore not as efficient, but the principle is the same. The air speeds up over the top of the wing and the pressure drops creating some lift. All of the lift? No. But it makes the wing more efficient. Using a known device, like a venturi makes for an excellent way to explain how a curved wing helps create lift.
TheOldBeef@reddit
Nooooooooo. Search nasa venturi lift or something similar ffs
SRM_Thornfoot@reddit
I did. I read that a wing is not an actual venturi since it is not fully enclosed. Nothing new there.
TheOldBeef@reddit
https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong3.html
TheOldBeef@reddit
Yep, the wing is not a venturi. There aren't half venturis. A thumb over a garden hose is not analogous to an airfoil and has very little explanatory power in regard to lift, instead obfuscating what is actually happening with the pressure differences created by an airfoil.
braided--asshair@reddit
You said it yourself. A wing is half of a Venturi. So where is the other half? A plane isn’t gonna fly with only half an engine, or just the lower torso of a pilot in the seat.
Rainebowraine123@reddit
https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/venturi-theory/
aviatortrevor@reddit
I'll give it a shot to explain.
Imagine a large cube moving through the air. The air is hitting the cube perpendicular to one of its sides. That leave the opposite side being shielded from the air flow. What happens in that shielded area of the cube? Initially, as the moving cube gains speed from a stand-still, a vacuum starts to form on that downwind side because you just pushed all the air out of the way with the cube itself, and left a void. Due to air molecules bouncing around off each other (aka "air pressure"), the air flowing near this void will appear to be attracted to this void/vacuum as speed picks up. You'll see the air curl back into this zone. Objects don't change direction or move unless a force is applied to them, and in this case, it's the vacuum that is the force. (Technically, it's the air molecules bouncing off each other that is the force, but the vacuum facilities an area where there is no resistance or forces, so the air flows towards it and away from where it's bumping into other air molecules that pushed it towards the vacuum).
The same thing happens with a wing. It begins to move from a stand still, and because the front side of the wing is thicker/bigger than the trailing-edge of the wing, it leaves a little vacuum near the surface of the rear-part of the wing (initially as the wing begins to move through the air). But soon after beginning to move, the air starts to "bend" over the top of the wing, and almost appear to "follow" the bend/shape of the wing back towards the wing's trailing edge. Remember, that bending of the air to follow the shape of the top of the wing is the result of some force applied to the air to make it change course from a straight-line. So, that rear-side of the top of the wing initially started out having a vacuum when movement through the air began at low speed, and now the air on the top of the wing that is closer to the peak/crest of the wing's hump is being *pulled* towards that area of low-pressure near the tail-end of the wing, which accelerates the air that is closer to the front/hump of the top of the wing, which *lowers* the pressure there too.
Compared to the bottom of the wing, not a whole lot is happening down there on the bottom. It's flat down there for a lot of airplanes. We call it "high pressure" under the wing, but it's really just "high" in comparison to the top of the wing, which had its pressure drop.
salty_greek@reddit
This is trivial. Top of the wing is usually higher than bottom. As the air gets higher, it tends to become careless and lightheaded, irresponsible and dangerous, and accelerates to enjoy the speed.
acniv@reddit
It's like saying the word Bernoulli, the further into pronouncing the word you get the faster it rolls off the tongue ; )
Mazer1415@reddit
My first ground instructor argued Bernoulli doesn’t even apply since there is no upper surface to cause the acceleration. Air molecules aren’t married, so why do the same two have to meet again at the rear of the wing?
It really came down to giving the rote answer during my oral. Add in his hair splitting of the difference between centrifugal and centripetal forces and there was a lot I needed to re learn in the approved manner.
M2K-throwaway@reddit
Low pressure is created by deflection/redirection of the incoming air over the top of the wing, causing lower pressure which then pulls air in and accelerates it over the remainder of the top of the wing. If anything, the low pressure causes increased velocity, not vice versa.
littlepenisbigheart1@reddit
It’s fizziks man. The distance over the top is farther, iso the air has to speed up to meet with the air that went under the wing. See? Fizziks
OzrielArelius@reddit
magic
skyrider8328@reddit
That's complete BS!! Come on man! The forces that make an aircraft fly is money!!
OzrielArelius@reddit
sorry wasn't trying to get too into the weeds about it but yea
SMELLYJELLY72@reddit
pfm
SaltyCAPtain1933@reddit
I like the way Rod Machado teaches lift. Dumbed down to make it easy to understand but also complex enough to make it feel like he's not talking down to you.
https://youtu.be/ObEM46KPuiw?si=C-cpIlRWtB_EvkEc
braided--asshair@reddit
This explanation by NASA is probably the most complete one you’ll get, if you click “next slide” it’ll go into incorrect theories on lift as well.
It’s not super necessary to know this, but I would be an engineer if I couldn’t get a medical so this stuff interests me. This is how I would personally explain it to students if they really wanted to know.
Velocity = direction x speed Acceleration = any change in velocity
Air acts as an incompressible fluid (when subsonic) and is somewhat “sticky” as in it will want to follow the curvature of the top of the wing - so long as the critical AOA is not exceeded (This is important later). This air that follows and “sticks” to the wing is called the boundary layer.
As the airflow follows the curvature of the wing, it must be turned - this creates a change in direction of the airflow which is a change in velocity, AKA an acceleration. The technical term for this is angular acceleration.
Bernoulli’s principle states that as a fluid accelerates in velocity - its pressure will decrease proportionally.
So if you take an airfoil that has a curve on the top, the air will follow it, which causes the air to angularly accelerate and then proportionally decrease in pressure.
Now the for the critical AOA. As you increase your AOA, amount that the air needs to turn is increased as well. More flow turning -> more angular acceleration -> more lift. This is why you climb more as the AOA is increased. However, once you reach that critical AOA, the air physically cannot “stick” to the wing anymore as it is unable to turn or angularly accelerate enough. This causes flow separation from the wing.
When you see that flow separation from the wing (boundary layer separation), the air is no longer being turned, so it’s no longer accelerating, and no longer producing lift.
I hope this helps.
Nui-_-Nui@reddit
There is a book about aerodynamics and it talks about continuity which yes theoretically it’s about encased fluid but it can be translated into lift as air flows like a fluid so it wants to stay together in the way it was before it separated and that’s all I say for my lesson basically Bernoulli’s then continuity and Newton’s third law I can explain it more or send you a screen shot of my lesson just dm me
Frederf220@reddit
A symmetrical aerofoil is not symmetrical when the incipient flow is not along the geometric axis of structure symmetry. Of course a symmetrical aerofoil doesn't produce lift when the flow is along that axis. If it wasn't you could mirror the picture for an identical setup producing a different result.
Anyway why does any molecule flow around a structure? Forget faster or slow, why at all? A structure in a vacuum impacted by a single molecule is going to have that molecule ricochet like a billiard ball, not a flow around the structure.
What gives rise to flow around a structure? Other molecules. Imagine not 1 but 2, 10, a million, a trillion bumping off each other. A cubic centimeter of air has on order 10^19 molecules, ten million trillion. The reason air flows around a wing faster is that the intrusion of the wing onto the volume means necessarily more bumping. More bumping is more opportunity for changing direction. Its path between bumps is longer in areas where there are fewer molecules to bump into.
Increased flow speed necessarily involves a sort of density gradient (temperature gradient would also work but not applicable). The molecules pile up in front of and above the wing. There's a sort of venturi tunnel above the wing of less hardness than either the wing (highly rigid) or the denser air (relatively more rigid).
Bowzy228@reddit
Pressure of the atmosphere squeezing the upper camber causing a bottleneck of airflow. Just like having your thumb on a garden hose, water shoots out faster
Big_Marzipan_405@reddit
this is not right
Frederf220@reddit
Yeah it is. In a rarified atmosphere with just a few molecules wouldn't demonstrate this behavior. They would just bounce off the leading edge like pool balls in straight lines.
It is the interaction of molecules with other molecules that causes accelerated flow over the top surface.
SumOfKyle@reddit
People forget the atmosphere above us has weight!
Juggles_Live_Kats@reddit
Read:
Stick and Rudder: An Explanation of the Art of Flying
by W Langewiesche. The first chapter or two he breaks it down. Best aviation read you'll ever have. Every pilot should read this.
Petey3@reddit
One way to visualize it is to use Newtons Third Law. The redirection of air downward is a large mass flow and results in a physical reaction - this downwards thrust results in upward thrust. According to NASA, LIFT IS GENERATED BY TURNING A MOVING FLUID.
rhapsodydude@reddit
I have to agree with other comments about pampering to whatever the FAA wants you to regurgitate because for FAA first do they even have a real aerodynamicist to write this stuff and second it’s very difficult to connect the two worlds without launching into a detailed discussion. So sometimes white lies will have to do.
My own take is this mental picture. First you accept that an airfoil shape immersed in the air will force the air to try to follow the shape of the airfoil. In practice, fluid collects on a stagnation point somewhere around the leading edge and then flows off the trailing edge rather than curving around that shape edge. Nature abhors abrupt changes. Second, the curvature of the airfoil( along with AOA) means local air flow will also be curved and the degree of curvature indicate a required pressure gradient to sustain this flow curvature with the required centripetal pressure force. Third, you start with the stagnation point and observe the airfoil curvature map on both sides, given an angle of attack. Then you develop an idea that flow accelerates significantly around the top of the leading edge m, much more than the lower side of the leading edge. Not much happens in low speed regimes aft of the so-called suction peak on the upper leading edge. This is the most significant factor behind positive lift generated by a cambered airfoil or symmetrical at positive AOA. in transonic conditions pressure changes are modified by compressibility and appear somewhat different but the basic mental picture is the same.
I’ve omitted pressure gradient along the direction of travel but it’s actually the controlling factor behind separation and stall. You need to know that curvature must be moderate otherwise the fluid has insufficient energy to follow the curvature.
High lift is complex. With multiple element airfoil, ie your flaps slats out, the upwasg and down wash of each element influences neighboring elements in achieving favorable interference. Suppressing undesirable high suction peaks, rounding out pressure distribution to extract max energy outs of fluid parcels, etc. it’s not “more camber more area”.
Think about this, then go online and search for Cp pressure coefficient diagrams around airfoils in both subsonic and transonic conditions. Only visit websites from reputable university/NASA type orgs. Notice the suction peak on the top of LE and contrast it with the lower LE. Think about the relationship between local curvature and the rest of the Cp plot. Look at the streamline and check the change in flow curvature in response to AOA. Then observe again the different Cp response at higher Mach around airfoils designed specially for that regime, ie the transport airplanes you’ll eventually fly.
It’s a long struggle between the academia and operational worlds. What do pilots actually need to know? Speed, AOA, airframe contamination that’s probably all there is to know up to CPL. if you pull out a real aerodynamics textbook during training I don’t really think most students are inclined to follow thru. There’s a lot more to being a commercial pilot than knowing the aerodynamics of lift. What you need to prevent is the kind of pilot who doesn’t even know the difference between IAS, TAS, GS, ask ATC to check their airspeed (you have your IRS GNSS GS), underestimate contamination, and disrespect envelope limits. In most cases you’ll have to leave it to experimental test pilots and real aerodynamicists to figure these things out.
Mission-Wasabi-7682@reddit
Because that explanation is wrong.
Simplified: lift is generated by deflecting an airstream downward, therefore, due to conservation of momentum, an equal amount of momentum upwards is generated.
Air goes down, plane goes up.
Not so simplified: in the vector field of airmass around your plane, conservation of mass, momentum and energy has to apply (Navier-Stokes-Equation). Any changes in that vector field, e.g. due to shape of wing and aoa of wing moving through that vector field will result in forces acting on said wing.
Really not simplified: turbulence enters the chat…
The pressure differential is a result and the acceleration of airflow again a result.
Aerospace engineer with a degree in aerodynamics and fellow pilot, who is always annoyed by the exam questions 😂
Mikegfx4@reddit
This is only a portion of how lift is generated. This is lift from newtons 3rd law. There is also a lot of lift generated from Bernoulli’s principle. Some wings rely more on one or the other. The wing on a fighter jet for example relies a lot on newton, the wing on a C172 relies a lot on Bernoulli.
Mission-Wasabi-7682@reddit
All the lift (and drag) generated is in the Navier-Stokes-Equations. They consist of: * conservation of mass * conservation of momentum (Newton’s second law) * conservation of energy (Bernoulli’s principle is somehow related to that part)
There is no one or the other, it’s all interconnected. You cannot just solve conservation of momentum or energy separately.
Mikegfx4@reddit
Sure thing, the main point is that just air goes down and plane goes up is a tiny part of how the plane flies.
Mission-Wasabi-7682@reddit
Agreed. For me it’s just the best simplified explanation that is at least somewhat correct. Also works for helicopters (downwash approximately proportional to weight). But again simplified. That’s why I included the second not so simplified answer ;)
mvpeav@reddit
This is why I got my degree in Astrodynamics, much less of this pesky fluid medium for me to have to worry about 😂
Mission-Wasabi-7682@reddit
Well, your MHD equations are no piece of cake either as far as I can remember.
Well-Pitter-Patter@reddit
The way I’ve always tried to explain it was this: put a balloon right next to an open door. Now slam the door shut. The balloon will inevitably get sucked in behind the door. This is the concept of lift. The balloon represents the pressure differential of the air above and below the wing. Above the wing, a sort of vacuum is created, essentially pulling the wing upwards into the void, while the air below the wing pushes upwards into an effort to equalize the pressure. This creates your lift.
justvims@reddit
Interesting. I always just thought it was because air wants to stick together (e.g. like laminar layers) and the top side of the wing is longer than the bottom. Thus air over the top needs to travel faster than across the bottom if it’s going to stay together at the trailing edge. Basically it.
Any_Purchase_3880@reddit
Like everyone said for all intents and purposes just keep it simple stupid.
However being just like you I wanted to know the nitty gritty details also. The simple answer is the "conservation of mass"
Draw the Venturi that old man Bernoulli used to discover his principles, the bottom of the Venturi (bottom bottleneck) is the top of an airfoil. The top of the Venturi (top bottleneck) is the weight of the atmosphere. That explains why amigo behaves similar to the Venturi Bernoulli was using. Conservation of mass explains why it accelerates.
Valid__Salad@reddit
Think of it in terms of Bernoulli’s principle. The passage way narrows (changes direction) and so air has to speed up. Same thing on top of the wing, despite that it’s a convex and not concave shape.
Recent-Day3062@reddit
If you really want to understand it, ask yourself this: how can a plane with a cambered wing fly upside down, with the camber on the bottom? I’ll give you a hint: aero engineers always say you can make a barn door fly with enough angle of attack.
I find it easiest to think about a rowboat. When you row one, you displace water. After the boat passes, the water fills back in where the boat was. And, if you look over the back, you see the water being @sucked” back in behind the boat.
When a wing with AOA passes by, it sort of pushes the air out of the way too. So the air creates a vacuum on top of the wing
BalladOfALonelyTeen@reddit
Pilots dont need to know why. They just need to know it does. Stick to the PHAK and that’s all you need.
tangowhiskeyyy@reddit
I don't know what it's called I just know the sound it makes when it takes a man's life
ronerychiver@reddit
I just know the sound it makes WHEN IT LIES!
JustAnotherDude1990@reddit
Beds give me nightmares.
propell0r@reddit
if you have 20mins, here’s a great video I found that dives deep into your questions: https://youtu.be/yjX7H2EN8Gs?si=uAF9he47QE9v38dZ
Basically boils down to the way air acts at the trailing edge and the concept of angular momentum needing to be conserved. Those two combined cause a circulatory flow around the airfoil which slows down the flow underneath and speeds up the flow on top of the wing.
snailmale7@reddit
Lift is generated by money ... Sometimes it costs $200 an hour , sometimes $1500 dollars an hour ... Regardless of the amount , someone pays the bird tax ... To fly with the birds ..
The Newtons 3rd law and B principle are polite ways of dancing around the money conversation :)
pilotlife@reddit
Below is a video I found interesting during my CFI training, giving a slightly more in depth look without getting too in depth and over the head into the "dragons nest" that is the theory of lift. We really don't know exactly how it happens, but we know it does and we can calculate it mathematically.
https://youtu.be/E3i_XHlVCeU?si=eNUrEgQRNg-DXoIu
gforero@reddit
I was asked about aerodynamics pretty heavily on my CFI oral and when you teach it just teach like you’re teaching a student. All you really should cover is Newton’s 3rd Law and Bernoulli’s Principle to explain how lift is generated.
charlespigsley@reddit
It’s Bernoulli’s principle. It’s basically creating a Venturi. The air is forced to compress when going overtop of the wing (look up wind tunnel examples. There are good car wind tunnel pics that show the bottom air streams being forced up and over the top of the car. Same thing with a wing.) When the air compresses - just like in a carburetor or any other Venturi - the static pressure decreases because the dynamic pressure (speed/velocity) increases.
Basically, you’re forcing the same amount of air through a smaller space, so in order for the air to move at the same rate then it must speed up.
HornetsnHomebrew@reddit
In reality you’re asking what conceptual device your student can take away to describe lift. All of these devices are going to be incorrect by both conceptually leaving stuff out and not allowing the calculation of accurate flows, forces, momenta, etc. The best practical advice I see in these comments focuses on your goal: getting a student through a checkride. The contents of the PHAK really must be sufficient for a DPE, with anything beyond that really serving only as extra credit for somebody (the student or the examiner’s ego). Specifically, you can offer the student some options:
1) lift is magic that cannot be accurately explained without resorting to quantative methods attacking your friends Navier and Stokes. Counterintuitively this is the most correct answer but doesn’t demonstrate the understanding we want of students pilots.
2) Conservation of momentum. Air go down so wing go up. Steeper AOA means more air go down. This is true but incomplete.
3) Stream tubes on an infinitely long wing. At some distance from the wings, the streamlines are effectively straight, forming a virtual pipe. The air is locked in this pipe (molecules can’t cross stream lines), so the air molecules actually do act like they are making appointments. Two molecules that enter the tube at the leading edge and take different paths have to meet at the other end in order to have zero net mass gain in the tube, so different velocities and pressures result on the top and bottom of the wing. Like all of the conceptual devices, true (in the limit for some of the ideas, see calculus 101) but incomplete.
4) circulation: complicated. I understood that lift cannot be fully explained without accounting for circulation on a finite wing for 5-10 seconds in undergrad. That was in the first millennium AD.
5) compressibility: opens the can of transonic and supersonic flow. Also your boys N&S. Good luck.
Good for you being prepared when the student asks WHY. Also good advice to eventually revert to the mission of the training here of getting a DPE to say “pass.”
shhbedtime@reddit
The answer you are required to teach is wrong. It is a gross over simplification that doesn't come close to explaining what happens. But that is irrelevant, just teach them what they need to parrot to pass the test. In almost all other cases i wouldn't say this, but for this unfortunately that's what you need to do
htnut-pk@reddit
Fluid dynamics. People can relate to water, and why a flat object will not drop vertically to the bottom of a pool. Apply that reasoning to air, a fluid.
JJohnston015@reddit
Picture pouring water out of a glass. If you pour slowly, the water clings to the glass and falls off the bottom (the Coanda effect). If you tilt the glass far enough, the flow can't take that much change of direction and breaks away from the glass and falls in a separate stream. Likewise, the air clings to the top of the wing until you tilt the wing up far enough that the airflow can't make that fast a change of direction and breaks away from it. When pouring water, this is what you want. In flying, you don't.
As long as the airflow clings to the top of the wing, and the wing is at a positive angle of attack, the air comes off the trailing edge at a slight downward angle. That means the wing is pushing (or sucking) the air down, and the constant downward push is what generates lift. In short, the wing makes lift by pushing air down. That's all a pilot needs to know. Same principle as rocket thrust. Conservation of momentum. The cambered shape increases the wing's efficiency in making lift, but strictly speaking, it's not necessary. A flat slab would also generate lift.
A symmetric wing makes its lift the same way: by having a positive angle of attack. A cambered wing can also make lift inverted, but it would have to be at a much higher angle of attack (I can tell you from personal experience that a Decathlon flies noticeably nose higher inverted than upright). The advantage and the tradeoff of the symmetric wing is that, while it flies at a higher angle of attack than a cambered wing, it flies at the same angle of attack at the same airspeed inverted and upright.
drrhythm2@reddit
I just tell me students to stick their hand out of a car window parallel to the ground, then see what happens as they slowly tilt it up.
It wants to go up (lift) and back (drag).
TheOldBeef@reddit
That’s not the coanda effect
No_Advice_9017@reddit
Centripetal Acceleration
Big_Marzipan_405@reddit
I'm an aerospace engineering student taking an aerodynamics class and there's not an amazing way to teach the 'correct' reason(s) that lift is a thing to someone with zero background in fluid mechanics. I'd just stick to explaining the two types of lift (newtonian and bernoullian) even though they are 'wrong'. a DPE doesn't care (or honestly even understand himself) how lift generation works. My favorite niche video I've found on the internet explaining lift/circulation is this one though: https://youtu.be/qg8JetYv_cM?si=5e01K9s9_tVxNl3M
even though circulation isn't really the "full" explanation for lift...
Go_Loud762@reddit
That's a terrible video and explaination.
Big_Marzipan_405@reddit
elaborate?
Go_Loud762@reddit
Go watch it. The video is 14 years old, for one.
They don't talk about symetrical airfoils or airloils that are perfectly flat. Their example shows a plane that is nearly perpendicular to the fluid flow.
All they show is vortices, which is a turbulent flow. That neither proves or disproves lift.
AWACS_Bandog@reddit
do you think the answer actually changed in 14 years?
I have some background in Electrical Engineering, and outside of humans getting 50/50 shot wrong, there really isn't much Amper, Maxwell, Ohm, or Watt described thats changed.
Big_Marzipan_405@reddit
I've watched that video multiple times which is why I recommended it in the first place lmao, yes it's 14 years old so quality ain't great. I think the video is a good short demo of how circulation and vortices work, helped me have a eureka moment when i was taking fluid mechanics
Go_Loud762@reddit
Explain how a flat piece of wood produces lift, according to that video.
srkjb@reddit
brennen.caltech.edu/fluidbook/externalflows/lift/flatplateairfoil.pdf
Big_Marzipan_405@reddit
i'm beginning to doubt this go_loud dude is even an engineer lol
srkjb@reddit
Certainly seems that way 😂
Big_Marzipan_405@reddit
A flat piece of wood can produce lift because lift depends on circulation and how the flow is turned, not on curvature alone. When the plate is set at a small positive angle of attack, the airflow cannot pass straight through and must split and move around it. As the flow starts, a starting vortex is shed from the trailing edge, and by conservation of vorticity an equal and opposite bound vortex forms around the plate. This bound circulation increases velocity over the top surface and decreases it underneath, creating a pressure difference according to Bernoulli’s principle. The pressure is lower above and higher below, generating an upward force. At the same time, the circulation causes the wake behind the plate to be deflected downward, meaning the plate imparts downward momentum to the air and experiences an upward reaction force. Read up on thin airfoil theory bud.
now answer my question, how long has it been since you took fluids? you learn this stuff in like week 2.
srkjb@reddit
Vortices are essential to the formation of lift and are not inherently indicative of turbulent flow, in fact the high viscosity of the fluid in the video is probably keeping the flow mostly laminar. Lift theory is a lot more complicated than what any pilot needs to know but the video is trying to show the basic principles behind some of the more accurate models of lift formation.
https://en.wikipedia.org/wiki/Horseshoe_vortex
Inevitable_Mix_455@reddit
Bernoulli and Newton's third law done.
primalbluewolf@reddit
Camber isn't a relevant part of the explanation.
The same thing happens on a zero camber wing i.e. a flat plate.
EngineerFly@reddit
Correct. In fact, symmetrical wings fly just fine. And cambered wings can fly inverted.
RogLatimer118@reddit
Air flows straight over the bottom of the wing. But on the top it curves up over the front of the wing, and a vacuum is created on the upper surface of the rear part of the wing because the air has to accelerate to rejoin the airflow past the wing. This vacuum creates the lift, and it also causes the air to accelerate as it tries to rejoin the airflow behind the wing. Basically the Bernoulli effect.
EngineerFly@reddit
This is not true. The air above and below the wing have no “appointment” to rejoin at the trailing edge. In fact, they don’t.
Big_Marzipan_405@reddit
equal transit theory is incorrect.
IchBinKagy@reddit
What you described is called equal transit time theory and has been proven incorrect. Air does accelerate and become lower pressing per Bernoulli's principle but it's not because air needs to "rejoin" at the back end.
KrynnAgain@reddit
I'll just drop this here and show myself out. 🤣🤣🤣 https://xkcd.com/2678
TheOldBeef@reddit
Best explanation yet
classysax4@reddit
I just think of lift as a downdraft created by the passing of the wing through stationary air.
salajander@reddit
This has a good explanation of lift, in my experience. I'm not an aerospace engineer. See How It Flies
Tuckboi69@reddit
Anything beyond what we need to know as pilots quickly turns into fluid mechanics, and I dealt with enough advanced math in college.
SumOfKyle@reddit
Read “aerodynamics for naval aviators”.
flyghu@reddit
The simplified version is, by nature of simplification, incomplete. But the complete version is not very simple and more than fits discussion in this forum.
tempskawt@reddit
Look at you, man. The most correct answer of all.
This guy explains it well.
Pilot-Imperialis@reddit
Everyone’s answer here is correct. As a CFI, you’ll be training pilots not engineers, so give pilot level answers.
If you’re really curious, start with the law of continuity. It’s the missing part you’re looking for although there is more to it admittedly.
Anyway one look at it should convince you why you don’t need to worry about it.
kevinossia@reddit
Newton’s Third Law, my friend. All you need.
Working_Football1586@reddit
Just read the phak and push the i believe button. Leave the engineering to the engineers.
VileInventor@reddit
You don’t you tell them what the FAA wants them to know. That said, pressure doesn’t change, it’s either static or dynamic. If the total pressure is 12 and the plane flys through it causing the static pressure to turn into dynamic pressure over the wings then the static pressure is 4 and the dynamic pressure is 8. It still = 12 but the kinetic energy is now in use.
datcrazybro@reddit
Just say Bernoulli’s principle and Newton’s third law. You don’t need to get that in depth unless you’re genuinely curious
IdahoAirplanes@reddit
There aren’t two types of lift. Wings deflect air down and the wing moves up in response. That deflection sets up a circulation of air around the airfoil which in turn produces the lower pressures because of the faster moving air at larger radii.
x4457@reddit
It doesn’t. “The air molecules split and have to meet at the same time at the back” is bullshit. Completely incorrect.
Even NASA doesn’t know exactly how lift is produced. Hope this helps :)
rFlyingTower@reddit
This is a copy of the original post body for posterity:
Studying for cfi, went down a bit of a rabbit hole on aerodynamics to try to understand the principles of flight a bit better. From what I've read, the aviation explanation for low pressure above is sort of backwards, with the acceleration of air above the wing being caused by a positive pressure gradient just after the stagnation point, instead of the increased camber of the upper half accelerating air and therefore creating low pressure through Bernoulli's principle, explaining how aircraft with symmetrical wing airfoils still produce lift.
I still dont really understand this explanation and the more I look into it the more confused I get, and I don't see any point in trying to teach a ppl student a much more complex and even contradictory explanation for lift than the FAA wants.
But now I am still stuck without a simple way to tell a student why the increased camber of the upper wing accelerates air instead of "it just does".
I've heard some people say that the air molecules on speed up to meet up with the same molecules on the bottom, but this is wrong.
How do you explain it?
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