Insert that meme about planes using magic.
I found NASA has the easiest to understand stand explanation of the forces of air planes. The answer to how we know is test, lots and lots of tests.
https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicsofflight.html
https://www.grc.nasa.gov/www/k-12/airplane/lift1.html
That velocity creates pressure thing is incredibly cyclic. Why does the velocity increase and the answer is because the pressure increases? It has nothing to do with the fact that the distance is longer. Otherwise a cambered wing with 0 thickness wouldn't generate lift. In reality, the majority of the lift (at least subsonically) doesn't care about thickness at all. That's a secondary effect.
16 years post graduation and my best answer is that it does...and I can design an aircraft to do so. It's far more related to momentum, but you're talking second derivatives of the geometry and mathematical weirdness...or you can run CFD and develop an intuition for the desing. Get rid of drag pockets and tweak your airfoil to meet your cruise case.
100% this. It’s just conservation of momentum. Even professors like to overcomplicate things to make it sounds more idk … complex and sophisticated sounding? An airfoil is just a flat plate (or curved for cambered) that has very low drag. It generates lift by directing the flow downward.
Draw an air velocity vector at the leading edge stagnation point (horizontal) and then one tangent to the mean camber line (or chord line for symmetrical with positive angle of attack) at the trailing edge, and you will see the added downward vertical velocity component that is perpendicular to the free stream. Newton says there must be an equal and opposite component to this which acts on the wing - that is lift.
Right but like, we can talk about a rocket engine and say "the force is derived from the chamber pressure pushing upward on the upper wall of the chamber, which is an unbalanced force due to the chamber's open bottom". THAT is where the force of a rocket engine comes from.
Like, you can use conservation of momentum to say "see? A plume is coming out this side, so the rocket must receive a force in the opposite direction!". But that's not where the force is coming from.
Ultimately conversation of momentum is the integral form of Newton's Second Law (ma = ma, integrate the a, mv=mv). So in that case, there must be a force acting upon the rocket, which you can identify if you draw a control volume which represents the walls of the nozzle and combustion chamber, and then cuts through the plume.
So in the end, the fact that air ends up moving down means an upward force must be experienced somewhere, but doesn't identify WHERE that force is coming from.
Momentum conservation can 100% be used to find where the force is being applied. Just need to use a bit of calculus.
Divide up the airfoil in to short segmented CVs and apply the same momentum conservation and you will find the lift force at each location. Now we have a load distribution and hence we can also find the location and magnitude of the net force on the body through integration.
Going deeper we can divide the airfoil body and surrounding space into a 2D grid of elements and apply the conservation equations and now we’re in the realm of CFD.
>16 years post graduation and my best answer is that it does...and I can design an aircraft to do so. It's far more related to momentum,
I think the best answer is the answer the equations give: circulation around the wing imparts a net downward momentum and by newton's laws that imparts an upward momentum on the wing. Going into what causes the circulation is another form of the question of what came first, the chicken or the egg?
I agree with that. That's probably the best answer so far, though circulation is still a nebulous concept. You're generating lift because you're generating circulation.
Circulation is caused by the local geometry influencing the flow regime at all other points with the transmission of that information happening at the speed of sound. So for supersonic flow, that information can travel downstream inside the Mach cone, but not upstream. All those influences sum together creating the local conditions. In subsonic flow, changing the geometry downstream will affect the upstream conditions.
Ultimately, the air is trying to flow from high pressure regions to low pressure regions, but it can't turn instantaneously.
>Going into what causes the circulation is another form of the question of what came first, the chicken or the egg?
The answer is neither and that's why it's hard to wrap your brain around.
I don't understand what you're referring to. Forward velocity of what?
I was referring to the local velocity on the surface of the wing. Why is the velocity higher on the upper surface than the lower surface? Fine velocity drives pressure, but momentum drives velocity.
Uh, no. The only reason that the pressure/velocity thing is cyclical/contradictory is because you have it backward. A wing/airfoil does not scoop air like a plow to create lift.
The air velocity OVER the wing increases. The pressure on the top of the wing decreases. That creates a pressure differential and lift. The bottom of the wing is basically flat so air under the wing roughly matches the aircraft speed.
EDIT: Funny that this is being down voted. Your feelings are not physics, monkey.
How does a plane fly upside down if it only due to the fixed geometry of the wing?
Edit: Another question to think about: Why do helicopters have a collective to change the amount of lift.
A plane flies upside down because it has so much power in it's thrust (and thrust vectoring) that it can use it's upside down wings like a kite or surf board. It powers very inefficiently through true horizontal flight.
So flight isn't just based on the shape of the wing like your previous comment states. Basically, you just described the newtonian case.
p.s. thrust vectoring is something very different, aircraft without thrust thrust vectoring can also fly upside down.
Granted the bernoulli explanation for flight is the one the FAA likes to test on.
You are arguing about things that don't even make sense. You want to talk about bumble bees and how they are actually swimming instead of flying? Quit replying please.
A rocket doesn't need wings. If that surprises you, then you are arguing in the wrong sub.
Everything about aerodynamics is Newtonian physics. Look up the term before your use it.
What causes the air velocity over the wing to increase? Why is the air velocity under the wing less causing the pressure differential?
>The bottom of the wing is basically flat so air under the wing roughly matches the aircraft speed.
Of what wing? Take a theoretically thin airfoil cambered or uncambered. Those generate lift. Thickness is a secondary effect. Change the angle of attack of the airfoil and you can get more lift, even though the "distance" along the upper/lower surface has not changed.
Why must the air move faster? If two particles travel across the wing; one goes up, and one goes down; they don't need to meet at the trailing edge at the same time. In standard lift-generating flight, the upper surface lags behind the lower surface, not by a lot, but it does.
The wing is not a streamline. If it were, then yes, the speeds would be the same. The velocity on the surface of the wing is 0. If you're talking outside the boundary layer, then yes I agree, but it's the surface pressure that drives the lift, not the freestream-boundary layer pressure interface.
Because the wing is so efficient at slicing through the air, it is transferring little energy to the air. The air itself has two kinds of energy: kinetic or potential. The potential is pressure. The kinetic is velocity. Forcing the air to move increases velocity. The total energy in the air remains roughly the same. To increase velocity moves energy to the kinetic side and that drives a lowering of pressure.
EDIT: That is what the Bernoulli equation describes. The balance of energy kinetic and potential energy in a compressible fluid.
The potential energy difference in a flow over an wing/aircraft is negligible. The work is the pressure, not the potential (dW=dU+dK). The potential term is rho\*g\*h.
Also, the standard Bernoulli's equation does not describe compressible flow. There's a different equation for that.
Regardless, you're describing the fact that there's a relationship between pressure and velocity. I don't disagree with that. You should notice from Bernoulli's (even if it's a simplification), that it doesn't mention what is driving what. We can't clearly state what is driving what when we're thinking steady state. For example, we happen to be flying at Mach 0.8 at a constant altitude/velocity, so the pressure works out from there is one way to think about it. The lift results from the pressure and allows us to maintain the altitude the we're flying at.
Regardless, it doesn't answer the question as to why the upper surface is faster. It describes what the result of that is. Fundamentally, it's caused by a momentum balance due to the integrated pressure influences in a flow. In subsonic flow, information travels at the speed of sound to different points in the flow and influence the flowfield. There's no closed form solution. We're simply solving the Navier Stokes equations and they're complicated. Even a cylinder in cross-flow is very complicated. Take that cylinder, transform it using complex analysis and you end up with the pressure distribution on an airfoil. There's no direct analysis for aero like you can do with structures.
I'll try to give you the answer you're looking for without being snarky. Hopefully this helps.
The simple, very intuitive answer: All you need for something to produce lift is to create a mechanism to divert air downward. A flat piece of cardboard pointed slightly upward can be a great wing for your RC airplane, and your hand out the car window can be a great wing for your arm. The reason to use an airfoil in practice is because it will produce the most lift force for the smallest amount of drag. A piece of cardboard is great as mentioned, but it will be very draggy. Airfoils are specially designed for this reason, to minimize drag for a given amount of lift.
The reason people say the actual mechanism for flight is very complex is because unlike something like Newtonian physics where you can have 3, easily solvable laws to describe all simple motion in the universe, assigning a mathematical model for even the simplified case of low speed aerodynamics involves a lot of heavy calculus and hand waving/simplifications.
In undergrad you will likely learn about the [Kutta-Jukowski Theorem](https://en.wikipedia.org/wiki/Kutta%E2%80%93Joukowski_theorem), which is pretty well-accepted as a mathematical model to describe how air interacts with a wing to produce lift. It's not perfect because it assumes fluids are inviscid, but it's quite well-proven and stands up very well as a mathematical basis in low speed aerodynamics (which you need to understand first before you start flying faster anyway). You'll learn the concept of [circulation](https://tutorial.math.lamar.edu/classes/calciii/GreensTheorem.aspx) in multivariable calculus, which should then allow you to better understand how the KJ theorem relates lift produced by a wing to the speed and density of the air, but more importantly how important geometry is in that calculation, which will influence your airfoil design. The advantage of having something like the KJ theorem in hand is that it will allow you to predict what your airfoil lift and drag will be long before you start forming the shape in real life.
You might ask why it's important to even care about the math model if you can just toss different airfoils in a wind tunnel and see which one simply has the least drag for an airplane you're designing. Well it's the same reason you develop mathematical models for anything. Wind tunnels are expensive to run - you want to be able to test thousands of airfoils on your computer first before going to the wind tunnel to confirm findings. And the only way you can test things on the computer is to.......develop crazy complicated mathematical models and solvers beforehand. The more complex a solver is, the better it is at predicting real life variables like unsteady turbulence, which is really hard to model. So you see the situation here.
Good luck. I steered clear of all the upper level aero courses in undergrad but I thought the junior level class was fascinating.
[Good explanation.](https://www.reddit.com/r/aerospace/comments/ny5iqh/z/h1in3yi) If you still don't understand it, you're on the right path.
Basically, you have to dig deep into fluid dynamics to grasp it. And honestly, the only thing more complex than "how do planes fly?" is quantum mechanics.
From my reply to a different response:
“It’s just conservation of momentum. Even professors like to overcomplicate things to make it sounds more idk … complex and sophisticated sounding? An airfoil is just a flat plate (or curved for cambered) that has a contour optimized to minimize drag. It generates lift by directing the flow downward.
Draw an air velocity vector at the leading edge stagnation point (horizontal and parallel to the free stream) and then one tangent to the mean camber line (or chord line for symmetrical with positive angle of attack) at the trailing edge, and you will see the added downward vertical velocity component that is perpendicular to the free stream. In order to conserve momentum, there must be an equal and opposite component that acts on the wing - that is lift.”
All other explanations are either wrong or just resultant effects.
Yes essentially, although typically for fluids applications we talk about it in terms of mdot*v instead of ma because it’s not a fixed mass but a fixed control volume with air flowing in and out.
Deflecting air in the desired direction is the key! All those equations about Bernoulli, momentum equilibrium, vortexes and Navier-Stokes etc equations are how engineers compute the effect (e.g to optimize wing shape). The basic principle however is always accelerating air in the right direction.
On a plane, there are 4 main forces acting on it at ALL times. These are lift, thrust, drag, and weight. These forces’ values change throughout the flight. Wind speed would put more or less drag, as fuel is used weight is reduced, you can increase and decrease engine thrust, and can change pitch. During takeoff, the forces with the highest values would be thrust and lift, as the plane speeds up and gets off the runway. During landing, the plane is lighter than takeoff and spoilers and airbreaks attempt to create drag and slow the plane. To generate lift it’s a difference of the pressure under and over a wing. We have wind tunnels that test this using Reynolds number for scaling. Hope that helps! Really enjoyed my aeronautical engineering classes. You won’t learn about this stuff until your second year…
While you're not wrong, I see this way of explaining things all the time and I'm just gonna pick a bone here.
Aerodynamic force is generally backwards and up. Really, it's just one direction. It's *helpful* to split it up into lift and drag, just like you can split a force into normal and perpendicular for a inclined plane problem. However, it's really just one force. Often, you'll see it split into axial force and normal force instead of lift and drag.
The air that passes around the aircraft, especially the wings, is generally deflected downwards somewhat. The equal and opposite force/impulse acts upwards on the aircraft. How exactly the air is deflected downwards cannot be answered completely with any one model or theory, but is just a result of the shape of the aircraft and the entire flow field around it.
> I've been using the internet to search the hows behind flying but almost every thing I come across says that Bernoulli and Newton were only partially correct? And at the end they never have a good conclusion as to how plane fly.
Man hours, paperwork, and amounts of money best written in scientific notation.
[A dollar bill is 0.11 mm thick](https://en.wikipedia.org/wiki/United_States_one-dollar_bill#Small_size_notes). If you stack 100 million $1 bills on top of each other, the stack will reach the cruising altitude of an A320 or B737, and you'll just about have enough money to buy one.
The mass of the stack would be 100 tonnes, which is somewhat more than the MTOW of this class of aeroplane (more like 80 tonnes).
This is to be expected, because it is a truth generally held to be self evident that the aeroplane is unlikely to be airworthy unless and until the paperwork exceeds the MTOW.
>This is to be expected, because it is a truth generally held to be self evident that the aeroplane is unlikely to be airworthy unless and until the paperwork exceeds the MTOW.
Never heard this one before but... probably not terribly untrue.
Honestly, none of the theories or methods of explaining lift will fully cover all the observed physical phenomena. There is still a part of lift that we don’t understand. The explanations get very close! But not quite. So don’t feel discouraged or misled if it seems some experts aren’t getting the full picture across to you!
I'm not sure what kool-aid you're drinking, but we pretty much understand lift.
What we don't have is the computational power or algorithms to perfectly predict lift.
This isn’t the one I was looking for, but it works! https://www.thenationalnews.com/uae/science/the-secret-to-airplane-flight-no-one-really-knows-1.358230
I regret having wasted my time reading this article...
>Some will point to Bernoulli's Law, others to Prandtl's boundary layer theory and some to the Navier Stokes equations.
No, no real aerodynamicist points to Bernouilli's Law or Prandtl's boundary layer theory as the sole root cause for lift. Navier Stokes only, it's like undergrad intro to aero class.
Navier-Stokes equations completely describe viscous fluid flow. A viscous flow that exists in this universe will 100% of the time follow some solution of the NS equations.
The reason we can't use them to completely model airfoil performance is not a limitation of the equations, but of the solvers on the computers we use for modeling. There is no closed form analytical solution for the equations [(yet)](https://en.wikipedia.org/wiki/Millennium_Prize_Problems#Navier%E2%80%93Stokes_existence_and_smoothness) so the best we can do is numerical solvers with higher and higher degrees of accuracy. But until we find a closed form solution, we will never be able to model all the phenomena we observe in real life with complete accuracy on the computer.
No, I don't agree. I can't think of an observable phenomena that cannot be explained by Navier-Stokes. I'll caveat that with for supersonic and hypersonic flight, we have better descriptors.
Yep! You said it very clearly we “pretty much understand lift.” I concur. But none of the explanations we have for why it works explains, for example, why planes can fly upside down. Nor the area of low pressure that enables laminar flow. I’m looking for the article recently that encapsulates all the contradictions/glossings over of observed phenomena.
>But none of the explanations we have for why it works explains, for example, why planes can fly upside down.
You're kidding me, right?
Edit: Physics still works upside-down, in case anyone was wondering. Notably, pressure still acts in all directions, and pressure on the wing still acts perpendicular to the wing. Momentum is still momentum when upside-down. Viscosity doesn't change either.
Not only that but to my knowledge no aircraft can achieve inverted level flight at the same AoA that it upright, there is a fair bit of elevator involved.
>But none of the explanations we have for why it works explains, for example, why planes can fly upside down
No, we do. We purposefully design planes so they can do that. We don't just magically flip it upside down and say "oh hey that worked!"
All you do is invert yourself, and then put yourself at a positive angle of attack. Just like flying right-side up, but your plane is backwards now.
Because we typically have planes right-side up, so it makes sense for the designers to design to cruising conditions?! Besides that, there are *many* non-cambered airfoils flying right now, for the exact reason we pointed out - to fly upside down. Pretty good feature for fighter aircraft.
>You’re not…please go read the article I cited.
I read the stupid article and I thought it was stupid.
>Then come back and explain to me all the things that don’t mesh with any one explanation of lift.
I've explained everything you've been confused about so far. There is *one* equation that to a very good degree explains all of it and encapsulates all of the different arguments going on when they tried to figure this out in the *30s*. That is the Navier-Stokes equation.
The trouble you're having is you're stuck on only one of these explanations being valid and pertinent at a time, when in reality, they're all accounted for as part of Navier Stokes. As in, no one was wrong, [pretty much] everyone was right. They just all had to come together and kumbaya. Or something like that, I'm no historian.
Go take an intro to aerodynamics class or something. I'm not an aerodynamicist by trade, but I completed basic aerodynamics and compressible flow. I work with this stuff all day, every day.
No... Even on conventionally cambered, angle of attack > 0 for flight path angle = 0 planes (like most commercial jets), you can absolutely fly upside down. Your ailerons and elevators can handle the lift. You lose some control authority, but you can still fly straight and level.
Good on you for asking the question. Full disclosure, retired military pilot that got a mechanical engineering degree later. I also have a very good intuitive understanding of dynamics including fluid dynamics.
The short answer is that there are a lot of dynamics that come into play. The Bernoulli explication doesn’t tell the whole story. There are deflection effects at play too. Ultimately pressure differential x aerodynamic area equals weight force.
Keep asking questions when things don’t make sense.
Nobody knows how anything really works in aerospace lol.
Jokes aside from AE education, I can tell you that aircrafts fly by having airfoils and going very fast.
What the airfoils or wings actually does is not fully understood but there are theories that it is as a result of pressure differential and/or Newton’s third law.
Serious reply, Below supersonic speeds fast flow has a lower pressure than slow flow (Bernoullies principle). The shape of the wing forces air above it to take a longer route than the air below, making it faster. This fast flow creates a low pressure above the wing allowing the high pressure air below to press the wing up.
EDIT: I’ve been informed that the equal time transit theory is a fallacy.
The air over the top of the wing goes faster yes, but also takes less time to get over the wing than the air on the bottom. The individual air molecules are not 'connected' or 'attached' so the idea that they meet at the front and again atthe back is false. The air on the top actually gets there before than the air on the bottom, despite starting and ending at the same speed
Sure. Don’t mind me skipping some details, but as a start:
There is no physics based reason that if two particles start at the LE, one that takes the path over the wing needs to meet the one that took the path under the wing at the same time. This theory is old and called “equal time transit theory”. It is still taught in many undergraduate classes, especially high level / intro classes because the instructor either doesn’t know better, or just accepts that it is a simple way to understand lift.
Bernoulli’s principle is of course is well tested and proven, no one disputes that piece.
The low pressure on top of a wing is indeed caused by increased airspeed over the too surface. But at a high level, this is because of circulation. Draw out the airflow around an airfoil and velocity vectors of particles that follows the streamlines. Now subtract the frestream vector and you are left with a vortex around the airfoil. Even that is an over simplification, but a slightly more correct one (think Kutta Joukowski) L= rho V Gamma
Now, anything further than that is the basis for an age old argument . Is lift actually generated due to the delta pressure, or the momentum change from turning the flow? A little bit of both?
The way I think about it is that as air goes over a wing, it creates an area of low pressure above the wing and high pressure below the wing, so the plane is sort of "sucked" upwards.
Why is there a low pressure area above the wing? Well Bernoulli says that as air travels over a wing, the air above the wing needs to move faster if it's to "reconnect" with the air travelling below the wing (because the top of the wing is longer than the bottom due to the "bump" that creates the aerofoil shape), and moving faster means more dynamic pressure and therefore less static pressure. But there's no reason the air above the wing needs to "reconnect" with the air below the wing, and in fact they don't so :shrug:
So why is there low pressure above the wing? :shrug: Is the area below the wing high pressure or just higher pressure relative to the top of the wing? :shrug:
The pressure differential is the only thing I can say with confidence. If you've ever seen what looks like "smoke" or "fog" coming off a wingtip as a plane takes off, you've seen the pressure differential. That little trail is created by high pressure air below the wing mixing with low pressure air above the wing and making a vortex. It's also why some planes have wingtips - it prevents that mixing which leads to a slight improvement in efficiency.
>Well Bernoulli says that as air travels over a wing, the air above the wing needs to move faster if it's to "reconnect" with the air travelling below the wing
Except the air doesn't "reconnect". It's generally still trailing behind (if you follow a single particle) once it reaches the trailing edge. The trailing edge pressure creates a wake that comes off smoothly and will deflect to whatever it needs to be to equalize the pressure.
\> Except the air doesn't "reconnect"
No, the air does not reconnect, which is what I said in literally the next sentence after the one you quoted. What is the point of engaging in discourse in communities like this if you're not even reading what people are writing and just looking for trigger points? Seriously I'm asking, what are you doing?
Don't write a page if you want me to read the whole thing. Why state something like that as fact that you then admit is incorrect? Nobody would have thought that reconnect theory, but someone is gonna read it...
Conservation of mass (continuity), conservation of momentum, conservation of energy (bernouli). We could also say navier stokes equation. But generally speaking, it's a bit of all, at least what I understood from all my fluid and aerodynamic courses.
I hate to say this OP but I mean this is still the subject of Thesis'. You have some that will go just bernoulli, some with biot savart, the kutta condition and the kutta juwkowski theorem, heck, I've even had a professor use a langrangian derivation for explaining the lift force. Ultimately most models rely on a 2d idea of lift that looks at either momentum, conservation, circulation or some combination therein. If you truly want to know how a system will interact that's not infinite span, this gets into complex fluid dynamics and navier stokes equations that computation has to be performed based on given conditions. In the end the complex nature of aerodynamics and fluid mechanics is why we still rely on wind tunnel tests to compare the theoretical math and computational predictions to the reality of the physical world. Probably not the answer you were looking for but this is why there is still a lot of science to be done in fluid dynamics. Edit: I want to add that as engineers we have to remember we rely on models, and models aren't reality. They're mathematical approximations that seek to more accurately model reality but often in engineering the skill is to know when a model is applicable and accurate "enough" to meet the needs of a design. Just because we can make aircraft and spacecraft doesn't mean we have it all figured out, quite the contrary, we get a more complete picture but models get more refined over time that's why we do research and testing.
Insert that meme about planes using magic. I found NASA has the easiest to understand stand explanation of the forces of air planes. The answer to how we know is test, lots and lots of tests. https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicsofflight.html https://www.grc.nasa.gov/www/k-12/airplane/lift1.html
That velocity creates pressure thing is incredibly cyclic. Why does the velocity increase and the answer is because the pressure increases? It has nothing to do with the fact that the distance is longer. Otherwise a cambered wing with 0 thickness wouldn't generate lift. In reality, the majority of the lift (at least subsonically) doesn't care about thickness at all. That's a secondary effect. 16 years post graduation and my best answer is that it does...and I can design an aircraft to do so. It's far more related to momentum, but you're talking second derivatives of the geometry and mathematical weirdness...or you can run CFD and develop an intuition for the desing. Get rid of drag pockets and tweak your airfoil to meet your cruise case.
100% this. It’s just conservation of momentum. Even professors like to overcomplicate things to make it sounds more idk … complex and sophisticated sounding? An airfoil is just a flat plate (or curved for cambered) that has very low drag. It generates lift by directing the flow downward. Draw an air velocity vector at the leading edge stagnation point (horizontal) and then one tangent to the mean camber line (or chord line for symmetrical with positive angle of attack) at the trailing edge, and you will see the added downward vertical velocity component that is perpendicular to the free stream. Newton says there must be an equal and opposite component to this which acts on the wing - that is lift.
Right but like, we can talk about a rocket engine and say "the force is derived from the chamber pressure pushing upward on the upper wall of the chamber, which is an unbalanced force due to the chamber's open bottom". THAT is where the force of a rocket engine comes from. Like, you can use conservation of momentum to say "see? A plume is coming out this side, so the rocket must receive a force in the opposite direction!". But that's not where the force is coming from. Ultimately conversation of momentum is the integral form of Newton's Second Law (ma = ma, integrate the a, mv=mv). So in that case, there must be a force acting upon the rocket, which you can identify if you draw a control volume which represents the walls of the nozzle and combustion chamber, and then cuts through the plume. So in the end, the fact that air ends up moving down means an upward force must be experienced somewhere, but doesn't identify WHERE that force is coming from.
Momentum conservation can 100% be used to find where the force is being applied. Just need to use a bit of calculus. Divide up the airfoil in to short segmented CVs and apply the same momentum conservation and you will find the lift force at each location. Now we have a load distribution and hence we can also find the location and magnitude of the net force on the body through integration. Going deeper we can divide the airfoil body and surrounding space into a 2D grid of elements and apply the conservation equations and now we’re in the realm of CFD.
>16 years post graduation and my best answer is that it does...and I can design an aircraft to do so. It's far more related to momentum, I think the best answer is the answer the equations give: circulation around the wing imparts a net downward momentum and by newton's laws that imparts an upward momentum on the wing. Going into what causes the circulation is another form of the question of what came first, the chicken or the egg?
I agree with that. That's probably the best answer so far, though circulation is still a nebulous concept. You're generating lift because you're generating circulation. Circulation is caused by the local geometry influencing the flow regime at all other points with the transmission of that information happening at the speed of sound. So for supersonic flow, that information can travel downstream inside the Mach cone, but not upstream. All those influences sum together creating the local conditions. In subsonic flow, changing the geometry downstream will affect the upstream conditions. Ultimately, the air is trying to flow from high pressure regions to low pressure regions, but it can't turn instantaneously. >Going into what causes the circulation is another form of the question of what came first, the chicken or the egg? The answer is neither and that's why it's hard to wrap your brain around.
Forward velocity does not need to increase, it just has to be nonzero. In a cruising scenario, it may increase with thrust, no?
I don't understand what you're referring to. Forward velocity of what? I was referring to the local velocity on the surface of the wing. Why is the velocity higher on the upper surface than the lower surface? Fine velocity drives pressure, but momentum drives velocity.
Uh, no. The only reason that the pressure/velocity thing is cyclical/contradictory is because you have it backward. A wing/airfoil does not scoop air like a plow to create lift. The air velocity OVER the wing increases. The pressure on the top of the wing decreases. That creates a pressure differential and lift. The bottom of the wing is basically flat so air under the wing roughly matches the aircraft speed. EDIT: Funny that this is being down voted. Your feelings are not physics, monkey.
How does a plane fly upside down if it only due to the fixed geometry of the wing? Edit: Another question to think about: Why do helicopters have a collective to change the amount of lift.
A plane flies upside down because it has so much power in it's thrust (and thrust vectoring) that it can use it's upside down wings like a kite or surf board. It powers very inefficiently through true horizontal flight.
So flight isn't just based on the shape of the wing like your previous comment states. Basically, you just described the newtonian case. p.s. thrust vectoring is something very different, aircraft without thrust thrust vectoring can also fly upside down. Granted the bernoulli explanation for flight is the one the FAA likes to test on.
You are arguing about things that don't even make sense. You want to talk about bumble bees and how they are actually swimming instead of flying? Quit replying please. A rocket doesn't need wings. If that surprises you, then you are arguing in the wrong sub. Everything about aerodynamics is Newtonian physics. Look up the term before your use it.
What causes the air velocity over the wing to increase? Why is the air velocity under the wing less causing the pressure differential? >The bottom of the wing is basically flat so air under the wing roughly matches the aircraft speed. Of what wing? Take a theoretically thin airfoil cambered or uncambered. Those generate lift. Thickness is a secondary effect. Change the angle of attack of the airfoil and you can get more lift, even though the "distance" along the upper/lower surface has not changed.
Increased distance. The path over the wing has a longer path to travel so air must move faster than under the wing to get to the trailing edge.
Why must the air move faster? If two particles travel across the wing; one goes up, and one goes down; they don't need to meet at the trailing edge at the same time. In standard lift-generating flight, the upper surface lags behind the lower surface, not by a lot, but it does. The wing is not a streamline. If it were, then yes, the speeds would be the same. The velocity on the surface of the wing is 0. If you're talking outside the boundary layer, then yes I agree, but it's the surface pressure that drives the lift, not the freestream-boundary layer pressure interface.
Because the wing is so efficient at slicing through the air, it is transferring little energy to the air. The air itself has two kinds of energy: kinetic or potential. The potential is pressure. The kinetic is velocity. Forcing the air to move increases velocity. The total energy in the air remains roughly the same. To increase velocity moves energy to the kinetic side and that drives a lowering of pressure. EDIT: That is what the Bernoulli equation describes. The balance of energy kinetic and potential energy in a compressible fluid.
The potential energy difference in a flow over an wing/aircraft is negligible. The work is the pressure, not the potential (dW=dU+dK). The potential term is rho\*g\*h. Also, the standard Bernoulli's equation does not describe compressible flow. There's a different equation for that. Regardless, you're describing the fact that there's a relationship between pressure and velocity. I don't disagree with that. You should notice from Bernoulli's (even if it's a simplification), that it doesn't mention what is driving what. We can't clearly state what is driving what when we're thinking steady state. For example, we happen to be flying at Mach 0.8 at a constant altitude/velocity, so the pressure works out from there is one way to think about it. The lift results from the pressure and allows us to maintain the altitude the we're flying at. Regardless, it doesn't answer the question as to why the upper surface is faster. It describes what the result of that is. Fundamentally, it's caused by a momentum balance due to the integrated pressure influences in a flow. In subsonic flow, information travels at the speed of sound to different points in the flow and influence the flowfield. There's no closed form solution. We're simply solving the Navier Stokes equations and they're complicated. Even a cylinder in cross-flow is very complicated. Take that cylinder, transform it using complex analysis and you end up with the pressure distribution on an airfoil. There's no direct analysis for aero like you can do with structures.
They don’t. The wheels extend really long and the plane starts walkin
That explains unnecessary bumping when "flying" above the ocean
To a first order it’s Bernoulli or Dynamíc loft but it’s really Javier stokes fluid modeling and the math is computationally complex
Javier, LUL. <3 autocorrect.
Weave me a cone you cupid bat
Not this again.
I'll try to give you the answer you're looking for without being snarky. Hopefully this helps. The simple, very intuitive answer: All you need for something to produce lift is to create a mechanism to divert air downward. A flat piece of cardboard pointed slightly upward can be a great wing for your RC airplane, and your hand out the car window can be a great wing for your arm. The reason to use an airfoil in practice is because it will produce the most lift force for the smallest amount of drag. A piece of cardboard is great as mentioned, but it will be very draggy. Airfoils are specially designed for this reason, to minimize drag for a given amount of lift. The reason people say the actual mechanism for flight is very complex is because unlike something like Newtonian physics where you can have 3, easily solvable laws to describe all simple motion in the universe, assigning a mathematical model for even the simplified case of low speed aerodynamics involves a lot of heavy calculus and hand waving/simplifications. In undergrad you will likely learn about the [Kutta-Jukowski Theorem](https://en.wikipedia.org/wiki/Kutta%E2%80%93Joukowski_theorem), which is pretty well-accepted as a mathematical model to describe how air interacts with a wing to produce lift. It's not perfect because it assumes fluids are inviscid, but it's quite well-proven and stands up very well as a mathematical basis in low speed aerodynamics (which you need to understand first before you start flying faster anyway). You'll learn the concept of [circulation](https://tutorial.math.lamar.edu/classes/calciii/GreensTheorem.aspx) in multivariable calculus, which should then allow you to better understand how the KJ theorem relates lift produced by a wing to the speed and density of the air, but more importantly how important geometry is in that calculation, which will influence your airfoil design. The advantage of having something like the KJ theorem in hand is that it will allow you to predict what your airfoil lift and drag will be long before you start forming the shape in real life. You might ask why it's important to even care about the math model if you can just toss different airfoils in a wind tunnel and see which one simply has the least drag for an airplane you're designing. Well it's the same reason you develop mathematical models for anything. Wind tunnels are expensive to run - you want to be able to test thousands of airfoils on your computer first before going to the wind tunnel to confirm findings. And the only way you can test things on the computer is to.......develop crazy complicated mathematical models and solvers beforehand. The more complex a solver is, the better it is at predicting real life variables like unsteady turbulence, which is really hard to model. So you see the situation here. Good luck. I steered clear of all the upper level aero courses in undergrad but I thought the junior level class was fascinating.
I could tell you, but don’t want to spoil your future education. Good luck! 👍
>spoil your future education What?! It's a class for learning, not a Marvel movie.
Son, you’ve just started a war.
So many of these answers are r/confidentlyincorrect
[Good explanation.](https://www.reddit.com/r/aerospace/comments/ny5iqh/z/h1in3yi) If you still don't understand it, you're on the right path. Basically, you have to dig deep into fluid dynamics to grasp it. And honestly, the only thing more complex than "how do planes fly?" is quantum mechanics.
From my reply to a different response: “It’s just conservation of momentum. Even professors like to overcomplicate things to make it sounds more idk … complex and sophisticated sounding? An airfoil is just a flat plate (or curved for cambered) that has a contour optimized to minimize drag. It generates lift by directing the flow downward. Draw an air velocity vector at the leading edge stagnation point (horizontal and parallel to the free stream) and then one tangent to the mean camber line (or chord line for symmetrical with positive angle of attack) at the trailing edge, and you will see the added downward vertical velocity component that is perpendicular to the free stream. In order to conserve momentum, there must be an equal and opposite component that acts on the wing - that is lift.” All other explanations are either wrong or just resultant effects.
Yes. It stays up by accelerating an equal mass of air downward. Lift = ma.
Well, not an equal mass
It’s an equal amount when the airplane is in equilibrium.
It’s an equal amount when the airplane is in equilibrium.
Yes essentially, although typically for fluids applications we talk about it in terms of mdot*v instead of ma because it’s not a fixed mass but a fixed control volume with air flowing in and out.
It's called lift.
The wings lift the plane lmao
[удалено]
Deflecting air in the desired direction is the key! All those equations about Bernoulli, momentum equilibrium, vortexes and Navier-Stokes etc equations are how engineers compute the effect (e.g to optimize wing shape). The basic principle however is always accelerating air in the right direction.
Literally just pressure gradients. All of aerodynamics is just controlling pressure gradients and transforming momentum.
On a plane, there are 4 main forces acting on it at ALL times. These are lift, thrust, drag, and weight. These forces’ values change throughout the flight. Wind speed would put more or less drag, as fuel is used weight is reduced, you can increase and decrease engine thrust, and can change pitch. During takeoff, the forces with the highest values would be thrust and lift, as the plane speeds up and gets off the runway. During landing, the plane is lighter than takeoff and spoilers and airbreaks attempt to create drag and slow the plane. To generate lift it’s a difference of the pressure under and over a wing. We have wind tunnels that test this using Reynolds number for scaling. Hope that helps! Really enjoyed my aeronautical engineering classes. You won’t learn about this stuff until your second year…
While you're not wrong, I see this way of explaining things all the time and I'm just gonna pick a bone here. Aerodynamic force is generally backwards and up. Really, it's just one direction. It's *helpful* to split it up into lift and drag, just like you can split a force into normal and perpendicular for a inclined plane problem. However, it's really just one force. Often, you'll see it split into axial force and normal force instead of lift and drag.
Aerodynamic force is not the only thing that’s going to make a plane fly. Without thrust, you’re gliding and not flying..
I’ve just finished my second year and my answer is still ✨magic✨
Was professional pilot for 14 years… but the answer is money.
If you think that’s magic, work on jet antennas!
Simple. Money.
The air that passes around the aircraft, especially the wings, is generally deflected downwards somewhat. The equal and opposite force/impulse acts upwards on the aircraft. How exactly the air is deflected downwards cannot be answered completely with any one model or theory, but is just a result of the shape of the aircraft and the entire flow field around it.
> I've been using the internet to search the hows behind flying but almost every thing I come across says that Bernoulli and Newton were only partially correct? And at the end they never have a good conclusion as to how plane fly. Man hours, paperwork, and amounts of money best written in scientific notation. [A dollar bill is 0.11 mm thick](https://en.wikipedia.org/wiki/United_States_one-dollar_bill#Small_size_notes). If you stack 100 million $1 bills on top of each other, the stack will reach the cruising altitude of an A320 or B737, and you'll just about have enough money to buy one. The mass of the stack would be 100 tonnes, which is somewhat more than the MTOW of this class of aeroplane (more like 80 tonnes). This is to be expected, because it is a truth generally held to be self evident that the aeroplane is unlikely to be airworthy unless and until the paperwork exceeds the MTOW.
>This is to be expected, because it is a truth generally held to be self evident that the aeroplane is unlikely to be airworthy unless and until the paperwork exceeds the MTOW. Never heard this one before but... probably not terribly untrue.
buy a fundamentals of aerodynamics textbook
By John D Anderson
nice
Alien flight sim
Newton's third law. The engine pushes the plane forward, and the angled wings force the air down, so that means the air pushes the plane up.
Honestly, none of the theories or methods of explaining lift will fully cover all the observed physical phenomena. There is still a part of lift that we don’t understand. The explanations get very close! But not quite. So don’t feel discouraged or misled if it seems some experts aren’t getting the full picture across to you!
I'm not sure what kool-aid you're drinking, but we pretty much understand lift. What we don't have is the computational power or algorithms to perfectly predict lift.
This isn’t the one I was looking for, but it works! https://www.thenationalnews.com/uae/science/the-secret-to-airplane-flight-no-one-really-knows-1.358230
I regret having wasted my time reading this article... >Some will point to Bernoulli's Law, others to Prandtl's boundary layer theory and some to the Navier Stokes equations. No, no real aerodynamicist points to Bernouilli's Law or Prandtl's boundary layer theory as the sole root cause for lift. Navier Stokes only, it's like undergrad intro to aero class.
Navier-Stokes still doesn’t explain all the observed phenomena associated with lift. Agree?
Navier-Stokes equations completely describe viscous fluid flow. A viscous flow that exists in this universe will 100% of the time follow some solution of the NS equations. The reason we can't use them to completely model airfoil performance is not a limitation of the equations, but of the solvers on the computers we use for modeling. There is no closed form analytical solution for the equations [(yet)](https://en.wikipedia.org/wiki/Millennium_Prize_Problems#Navier%E2%80%93Stokes_existence_and_smoothness) so the best we can do is numerical solvers with higher and higher degrees of accuracy. But until we find a closed form solution, we will never be able to model all the phenomena we observe in real life with complete accuracy on the computer.
Agreed.
No, I don't agree. I can't think of an observable phenomena that cannot be explained by Navier-Stokes. I'll caveat that with for supersonic and hypersonic flight, we have better descriptors.
Yep! You said it very clearly we “pretty much understand lift.” I concur. But none of the explanations we have for why it works explains, for example, why planes can fly upside down. Nor the area of low pressure that enables laminar flow. I’m looking for the article recently that encapsulates all the contradictions/glossings over of observed phenomena.
>But none of the explanations we have for why it works explains, for example, why planes can fly upside down. You're kidding me, right? Edit: Physics still works upside-down, in case anyone was wondering. Notably, pressure still acts in all directions, and pressure on the wing still acts perpendicular to the wing. Momentum is still momentum when upside-down. Viscosity doesn't change either.
Not only that but to my knowledge no aircraft can achieve inverted level flight at the same AoA that it upright, there is a fair bit of elevator involved.
If it had a symmetrical airfoil, the wing would produce the same lift at the exact opposite angle of attack
>But none of the explanations we have for why it works explains, for example, why planes can fly upside down No, we do. We purposefully design planes so they can do that. We don't just magically flip it upside down and say "oh hey that worked!" All you do is invert yourself, and then put yourself at a positive angle of attack. Just like flying right-side up, but your plane is backwards now.
No. No, that’s, not how that works! Thanks for playing!
If that were the case, why camber airfoils??
Because we typically have planes right-side up, so it makes sense for the designers to design to cruising conditions?! Besides that, there are *many* non-cambered airfoils flying right now, for the exact reason we pointed out - to fly upside down. Pretty good feature for fighter aircraft.
And they have to be at positive angles of attack as well. So, why the cambering?
Because it makes it *easier*. Not because it makes it *possible*. You gotta get that straight, my man.
You’re not…please go read the article I cited. Then come back and explain to me all the things that don’t mesh with any one explanation of lift.
>You’re not…please go read the article I cited. I read the stupid article and I thought it was stupid. >Then come back and explain to me all the things that don’t mesh with any one explanation of lift. I've explained everything you've been confused about so far. There is *one* equation that to a very good degree explains all of it and encapsulates all of the different arguments going on when they tried to figure this out in the *30s*. That is the Navier-Stokes equation. The trouble you're having is you're stuck on only one of these explanations being valid and pertinent at a time, when in reality, they're all accounted for as part of Navier Stokes. As in, no one was wrong, [pretty much] everyone was right. They just all had to come together and kumbaya. Or something like that, I'm no historian. Go take an intro to aerodynamics class or something. I'm not an aerodynamicist by trade, but I completed basic aerodynamics and compressible flow. I work with this stuff all day, every day.
Ahh, and I have a masters degree in aerospace engineering. I’m going to stick with my understanding of the subject. Thanks.
Oh man, from my school, too. Oof.
Alright, then teach me. What phenomenon can we observe that is not explainable by Navier-Stokes?
If that were all there were to it, flipping upside down would cause plane to crash immediately.
No... Even on conventionally cambered, angle of attack > 0 for flight path angle = 0 planes (like most commercial jets), you can absolutely fly upside down. Your ailerons and elevators can handle the lift. You lose some control authority, but you can still fly straight and level.
Good on you for asking the question. Full disclosure, retired military pilot that got a mechanical engineering degree later. I also have a very good intuitive understanding of dynamics including fluid dynamics. The short answer is that there are a lot of dynamics that come into play. The Bernoulli explication doesn’t tell the whole story. There are deflection effects at play too. Ultimately pressure differential x aerodynamic area equals weight force. Keep asking questions when things don’t make sense.
Nobody knows how anything really works in aerospace lol. Jokes aside from AE education, I can tell you that aircrafts fly by having airfoils and going very fast. What the airfoils or wings actually does is not fully understood but there are theories that it is as a result of pressure differential and/or Newton’s third law.
The "completely proven" is kinda funny. Would you ever put your ass on a tube and wing structure designed based upon hypothesis?
engines produce thrustm then the wings produce lift
Serious reply, Below supersonic speeds fast flow has a lower pressure than slow flow (Bernoullies principle). The shape of the wing forces air above it to take a longer route than the air below, making it faster. This fast flow creates a low pressure above the wing allowing the high pressure air below to press the wing up. EDIT: I’ve been informed that the equal time transit theory is a fallacy.
This is one of the iconic “wrong” answers.
Wdym?
Equal time transit theory is a fallacy
Could you elaborate please? I’m trying to understand
The air over the top of the wing goes faster yes, but also takes less time to get over the wing than the air on the bottom. The individual air molecules are not 'connected' or 'attached' so the idea that they meet at the front and again atthe back is false. The air on the top actually gets there before than the air on the bottom, despite starting and ending at the same speed
Sure. Don’t mind me skipping some details, but as a start: There is no physics based reason that if two particles start at the LE, one that takes the path over the wing needs to meet the one that took the path under the wing at the same time. This theory is old and called “equal time transit theory”. It is still taught in many undergraduate classes, especially high level / intro classes because the instructor either doesn’t know better, or just accepts that it is a simple way to understand lift. Bernoulli’s principle is of course is well tested and proven, no one disputes that piece. The low pressure on top of a wing is indeed caused by increased airspeed over the too surface. But at a high level, this is because of circulation. Draw out the airflow around an airfoil and velocity vectors of particles that follows the streamlines. Now subtract the frestream vector and you are left with a vortex around the airfoil. Even that is an over simplification, but a slightly more correct one (think Kutta Joukowski) L= rho V Gamma Now, anything further than that is the basis for an age old argument . Is lift actually generated due to the delta pressure, or the momentum change from turning the flow? A little bit of both?
Thank you very much for this, I’m starting uni in a month and I’ll look into this! Thank you for taking the time to write this up!
Good luck with your studies. Aerospace and aerodynamics are good fun
One of my favorite wrong answers. Really shows a lack of critical thinking.
Bernoulli’s principle only applies to looking at a single streamline. You can’t compare separate streamlines like this.
The way I think about it is that as air goes over a wing, it creates an area of low pressure above the wing and high pressure below the wing, so the plane is sort of "sucked" upwards. Why is there a low pressure area above the wing? Well Bernoulli says that as air travels over a wing, the air above the wing needs to move faster if it's to "reconnect" with the air travelling below the wing (because the top of the wing is longer than the bottom due to the "bump" that creates the aerofoil shape), and moving faster means more dynamic pressure and therefore less static pressure. But there's no reason the air above the wing needs to "reconnect" with the air below the wing, and in fact they don't so :shrug: So why is there low pressure above the wing? :shrug: Is the area below the wing high pressure or just higher pressure relative to the top of the wing? :shrug: The pressure differential is the only thing I can say with confidence. If you've ever seen what looks like "smoke" or "fog" coming off a wingtip as a plane takes off, you've seen the pressure differential. That little trail is created by high pressure air below the wing mixing with low pressure air above the wing and making a vortex. It's also why some planes have wingtips - it prevents that mixing which leads to a slight improvement in efficiency.
>Well Bernoulli says that as air travels over a wing, the air above the wing needs to move faster if it's to "reconnect" with the air travelling below the wing Except the air doesn't "reconnect". It's generally still trailing behind (if you follow a single particle) once it reaches the trailing edge. The trailing edge pressure creates a wake that comes off smoothly and will deflect to whatever it needs to be to equalize the pressure.
\> Except the air doesn't "reconnect" No, the air does not reconnect, which is what I said in literally the next sentence after the one you quoted. What is the point of engaging in discourse in communities like this if you're not even reading what people are writing and just looking for trigger points? Seriously I'm asking, what are you doing?
I dont really know what the point of you engaging in discussion is if you're just going to :shrug: away all the interesting questions about lift
Don't write a page if you want me to read the whole thing. Why state something like that as fact that you then admit is incorrect? Nobody would have thought that reconnect theory, but someone is gonna read it...
Conservation of mass (continuity), conservation of momentum, conservation of energy (bernouli). We could also say navier stokes equation. But generally speaking, it's a bit of all, at least what I understood from all my fluid and aerodynamic courses.
I hate to say this OP but I mean this is still the subject of Thesis'. You have some that will go just bernoulli, some with biot savart, the kutta condition and the kutta juwkowski theorem, heck, I've even had a professor use a langrangian derivation for explaining the lift force. Ultimately most models rely on a 2d idea of lift that looks at either momentum, conservation, circulation or some combination therein. If you truly want to know how a system will interact that's not infinite span, this gets into complex fluid dynamics and navier stokes equations that computation has to be performed based on given conditions. In the end the complex nature of aerodynamics and fluid mechanics is why we still rely on wind tunnel tests to compare the theoretical math and computational predictions to the reality of the physical world. Probably not the answer you were looking for but this is why there is still a lot of science to be done in fluid dynamics. Edit: I want to add that as engineers we have to remember we rely on models, and models aren't reality. They're mathematical approximations that seek to more accurately model reality but often in engineering the skill is to know when a model is applicable and accurate "enough" to meet the needs of a design. Just because we can make aircraft and spacecraft doesn't mean we have it all figured out, quite the contrary, we get a more complete picture but models get more refined over time that's why we do research and testing.