Airfoils - How do they work?

No that is autistic. Just correct them. Or don't bother, few people really need to know the reason.

This comes up rather frequently here. The thing is you can derive the fact that aerofoil geometries cause pressure differences just even from incompressible non viscous irrotational flow theory (also known as potential flow) Although this model has shortcomings (e.g. it predicts 0 drag which is plain false) it can help you get a first look into things.

Look for potential flow around an aerofoil, conformal mappings and Joukowski transformations for a start and derive it yourself.

If you want a more advanced treatment, look up the Prandtl-Meyer shock expansion theory for an explanation.

Just a figure of speech. I don't enjoy disparaging people. One thing, though: the simulation I posted shows air moving faster over the top. Is it not correct to say the curvature of the air reduces pressure, thereby also increasing air speed? Or does a reduction in pressure not necessarily create an increase in air speed?

Wing shaped objects moving though air cause an air flow which accelerates air downwards.

At leas thats an explanation I personally think is plausible.

In 3rd grader terms, are you saying it has as much to do with the effect of the air in the vicinity of the air foil and not just directly along its surface?

I know all this. The question I was asking is why the particular shape of a cambered airfoil (as in the original picture) creates lift. Any flat surface can create lift at an angle of attack. Why does a cambered air foil generate lift at zero AoA is what I'm wondering about.

youtube.com/watch?v=aFO4PBolwFg

I've seen this before. In fact, I'm subscribed to Veritasium. Much as like the guy, his video didn't explain anything new to me and didn't answer my question as why the cambered air foil at zero angle of attack, speeds up the air above it. He just states that it does.

Well what it probably comes down to is the question why a certain diferential equation (the differential equation modeling the pressure and continuity conditions around a wing) has a certain solution, i.e. the actual airflow around that wing.

>why the cambered air foil at zero angle of attack, speeds up the air above it.
Uhhh....because the distance over the wing is more the distance below?
There's no way it couldn't speed up if the tails were meeting at the end.
Conservation of mass and that.

Conventional slow moving airplanes have this shape because it decreases pressure underneath the wing relative to above. Higher pressure means the wing is more likely to move to that side, since pressure is NOT downwards, but parallell to the wing shape.

It means the plane has higher lift rate when flying level, but needs a much sharper AoA to fly inverted.

High velocity airplanes like fighter jets do not, have the same shape. They need to trim pitch more than conventional airfoils to keep altitude at low velocity, and do not fly perfectly level (absolute 0 pitch), but are in return much more maneuverable and able to fly equally stable and at the same AoA inverted.

It also means the airplane behaves more predictably at supersonic speeds, which as we all know no passenger aircraft operates at. Regular aerodynamics do not suffice to explain fuselage design at supersonic speed.

It's less fuel intensive to use a "built in" AoA like with airfoils than to constantly be trimming. This is relevant for personal airplanes where fuel is time, and commercial airplanes where fuel is money.

>fas.org/man/dod-101/sys/ac/intro.htm

From that site, before anyone chops my head off for not being specific enough:
>Most of the time, the top of the wing does the majority of the "pushing" on the air (actually, in this case, "pulling" the air down). The top and the bottom of the wing combine to produce a force, and the part of this force perpendicular to the relative wind is lift. Since the wing not only pushes the air down but slows it down as well, some drag (induced drag) is caused.

>Schools teach the wrong shit so people are helpless and must be consumers.
There's a reason there's a home economics class.

As has already been well established in this thread and in the scientific community, the tails do NOT meet up at the end.

Your link provides the erroneous explanation that "air passing over the top and bottom must reach the rear of the wing at the same time." Once again, this isn't true.

Its a bumby ride

How can it be a good model if it isn't true? I'm not trying to be obtuse, but your explanation doesn't make much sense to me.

The "path of least resistance" is still under the airfoil, since there's literally the least resistance there.

The air hitting the wing in front does so at the same constant speed as the air under, it doesn't slow down, as all the surrounding air is still moving along. Instead it increases in pressure, kinda like if you ride your bike and whistle, the speed of the whistle wind is the combined speed of your bike and your blowing.

In the same way, the speed of the air going above the wing is higher, as there is literally more molecules trying to fit, kinda like a bar of soap being squished in wet hands.

Following, the air expands slightly after crossing the most resistance, and since the wing is angled down, follows the path of least resistance relative to being above the wing, meaning along the airfoil, downwards.

If you fold paper airplanes and use flaps to increase their lift, and you understand which way to bend the flap to trim it up or down, you already understand the reason why airfoils are shaped as they are. It's just a little difficult to explain all the time the wing isn't just a cross section.

> How can it be a good model if it isn't true?
Because it works and is close enough to the right answer?
Newtonian physics isn't "true" but is a good model.

I actually like the Euler explanation better. So I deleted my post.
Pressure increases across curved streamlines.

> The "path of least resistance" is still under the airfoil, since there's literally the least resistance there.
I meant to a point near the tail on the top surface.
There should be a low pressure there, right?

> If you fold paper airplanes and use flaps to increase their lift, and you understand which way to bend the flap to trim it up or down, you already understand the reason why airfoils are shaped as they are. It's just a little difficult to explain all the time the wing isn't just a cross section.
Haha. I've been making paper airplanes forever without actually thinking about trim.
I just copied ideas.

Wings work using the coanda effect

Lift is generated by displacing air downwards as they fly along

?
The pressure of the air is irrelevant, its the displacing of the air that matters.
You can measure it with pressure of the air on the wing, but its not the low pressure vs high pressure itself lifting the plane.

That only applies to internal flows. The eye of a storm is low pressure yet slow moving air no?

Why don't you just go read the very well written article over at:
en.wikipedia.org/wiki/Lift_(force)
instead of making this thread every fortnight?

It explains everything well, and describes several of the models which are used in calculating it.

>Bernouli's principle.

to add to this anons post:
the subarticle "Circulation and the Kutta-Joukowski theorem" in the wikipedia article is a pretty intuitive way of describing why the air actually moves at different velocities on the upper and lower side of the airfoil
the circulation adds velocity onto the free stream velocity at the top and subtracts it from the free stream velocity at the bottom

>What causes the air to move faster over the top than bottom at zero AoA
"Zero AoA" is an arbitrary datum.
>The air passed the wing is traveling at the same speed above and below.
No, it didn't. Are you blind? Air moved over the upper surface nearly twice as quickly as across the bottom surface.
It does travel faster - often MUCH faster. But not because "the path is longer" or any bullshit like that.
It applies, but it's more of a result than a cause.
>Look for potential flow around an aerofoil, conformal mappings and Joukowski transformations for a start and derive it yourself.
This, really. It really comes down to "because of its shape and orientation," answering it any more extensively than that requires complicated analysis.
>Uhhh....because the distance over the wing is more the distance below?
No.
>the subarticle "Circulation and the Kutta-Joukowski theorem" in the wikipedia article is a pretty intuitive way of describing why the air actually moves at different velocities on the upper and lower side of the airfoil
This is good advice as well.

Pilot here

Lift is a result of several properties. First, Bernoulli's principle. Second, Newton's 3rd law. Third, air moves from high pressure to low pressure.

A wing is an aerodynamic surface that creates lift when moved through a gas. Lift is defined by the equation L=Cl•Q•S, where Cl is coefficient of lift, Q is dynamic pressure, and S is the wing area. Cl consists of many things, but essentially it defines the lift potential at a given angle of attack for a wing. Q defines the air pressure and density, which varies with temperature and velocity. S is simply the area of the wing.

The reason a wing produces lift at all is because of the pressure difference between the upper and lower surfaces of the wing. On a standard high-camber wing, the air flowing over the top will be accelerated due to Bernoulli's principle. Essentially the air is forced to accelerate, which produces a low pressure area. The air on the lower surface is at a higher pressure, relative to the upper surface. This pressure disparity imparts a force on the wing, as defined by both Newton's 3rd law and the law that defines air flows from high pressure to low pressure (forget the name).

This is the basics of how a wing generates lift.

>>?
>The pressure of the air is irrelevant,
jesus fuck

if you have very low pressure, your wing won't even stay in the air

>the air flowing over the top will be accelerated due to Bernoulli's principle.
can you explain this?

Aero engineer here. Bernoulli's equation tells you Static pressure+0.5*density*velocity*2 is constant if you assume incompressible inviscid flow but it doesn't tell why the air above is faster and hence lower pressure.

Go look up Kutta-Joukowski circulation theorem and potential flow theory for a better explanation as mentioned previously in the thread.

>the air flowing over the top will be accelerated due to Bernoulli's principle. Essentially the air is forced to accelerate, which produces a low pressure area
bernoulli does not cause air to accelerate, it causes accelerated air to experience a pressure drop

When the air flows over the top of a wing, it has the same effect as flowing through a constriction. Bernoulli's principle states that as a gas flows through a constriction, both the pressure drops and the velocity increases simultaneously (assuming incompressibility).

>When the air flows over the top of a wing, it has the same effect as flowing through a constriction.
except that it doesn't
vorticity causes airfoil top/bottom surface speed difference, everything else, including the generation of lift, is a byproduct of that

>Bernoulli's principle states that as a gas flows through a constriction, both the pressure drops and the velocity increases simultaneously (assuming incompressibility)
lol, no
fluid velocity increase within a constriction is governed by continuity and conservation of mass
bernoulli has fuck all to do with it until you want to look at the resulting pressure difference

>vorticity causes airfoil top/bottom surface speed differences
It isn't any one property that causes lift. Circulation about a wing plays a part, but the velocity difference can also be attributed to the camber of the wing. The camber of the wing has a direct impact on the velocity difference of the fluid above/below the wing, which can be attributed to Bernoulli's principle.

faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80t-80.pdf

>bernoulli has fuck all to do with it
"Bernoulli’s theorem is the principle of energy conservation for ideal fluids in steady, or streamline, flow.."

Why don't you go research what you're going to refute. Bernoulli's theorum has literally everything to do with it

britannica.com/science/Bernoullis-theorem

>quoting the definition of bernoulli as if it somehow negates your asinine statement of bernoulli having something to do with the speed increase within a constriction
stop backpedaling into weblinks just because you're not capable of wording your shit correctly
conservation of mass dictactes fluid speed change within a constriction
bernoulli relates this speed change to a change in pressure
these two things are entirely seperate
bernoulli says nothing about how a fluids speed will increase when passing through a constriction, only how that speed increase will affect its other energetic properties

the only way to even get any information about the speed increase is via continuity, which, once again, has FUCKALL to do with bernoulli's theorem itself
you would know this if you'd have done a single problem sheet about fluid mechanics in your life

also adding weblinks that noone will bother to read anyway, because apparently this is a perfectly viable substitutes for any self-constructed verbal argument
physics.bu.edu/~duffy/py105/Bernoulli.html
sci-culture.com/physics/Bernoulli-Equation.html

I find the most helpful thing is to imagine the simplest wing possible: a flat, rigid surface held at an angle to the direction of motion.

Why it produces lift is exactly the same reason as why an airfoil produces lift. The ways in which it would cause unnecessary drag, unwanted torque, and variation of forces (oscillating or chaotic) explain the shapes of good airfoils, which are designed to prevent those problems.

If you keep looking at it from this angle, you can also see why very small wing-flapping creatures which don't glide (especially insects) tend to have simple planar wings rather than elegantly curved ones: the physics of their lift generation are dominated by vortices rather than laminar flow.

Why so heated? Its a discussion on aerodynamics on an image board, not a fight. Don't get so upset.

>conservation of mass [continuity equation] dictates fluid speed change within a constriction
>Bernoulli relates this speed change to a change in pressure
While they are not the same, they are absolutely related. The relation of fluid speed change and fluid pressure change is the practical application of these properties. Without the pressure change, there would be no lift. Without the speed change, there would be no pressure gradient. My point is that BOTH of the concepts play an important role, but to discount the applicability of Bernoulli's principle would be sheer ignorance.

Are you by chance an AE?

Pressure differences cause a net force on the wing.

>Bernoulli's equation tells you Static pressure+0.5*density*velocity*2 is constant if you assume incompressible inviscid flow
That's for a steady flow, airflow over a wing is unsteady so you need to use the unsteady form of Bernoulli's equation.

they don't. Airplanes are in fact a hoax.

Gas is compressible, like a springs pushing on the wing from above and below. The front of the wing splits the airflow with more going to the top, so the top has to flow faster to fit under the spring so its Bernoulli push is smaller.

>When the air flows over the top of a wing, it has the same effect as flowing through a constriction. There are parallels between internal flow through a constriction and external flow around objects in general; air accelerates to "squeeze past" the obstruction in both cases. But at "zero AoA," this generally applies to the bottom surface as well (to a lesser extent than the top, but nevertheless), not just the upper surface. So you need a bit more to explain why there is an IMBALANCE in these velocities and pressures that results in lift. That's where circulation comes in, as [] stated.
>bernoulli says nothing about how a fluids speed will increase when passing through a constriction, only how that speed increase will affect its other energetic properties
This is true, and a very good point.
>the physics of their lift generation are dominated by vortices rather than laminar flow.
You have it completely backwards. Viscosity and laminar flow dominate at small scales. Inertia and turbulence dominate at large scales.
>airflow over a wing is unsteady
No... it's not. Not enough for the steady-state assumption to be invalid, anyways. The unsteady form is for analyzing waves and shit.

>explain why there is an imbalance in these velocities
That depends entirely on the shape of the wing. At zero AoA, high-camber airfoils will have a large disparity of pressures between the upper and lower surfaces, which creates lift at α=0. A symmetrical airfoil at 0 AoA will not produce any lift, because there is no disparity of pressures at α=0 because the upper and lower surfaces are identical. The pressure differential is a result of the shape of the airfoil. A high camber airfoil will have a larger disparity between upper and lower surfaces than a low camber airfoil will (at the same free stream air velocity). This interaction between the free-stream air and the shape of the airfoil will determine BOTH the amount of circulation AND the pressures on the various surfaces. That is why there is an imbalance.

>explain why there is an imbalance in these velocities
That depends entirely on the shape of the wing. When the free stream air interacts with the airfoil, the air is forced around different parts of the object at different speeds. When the air is forced around the upper surface, it acts as a constriction. Due to the continuity equation, if the area is 'reduced', the velocity increases, and the pressure decreases. The lower surface of the airfoil has less of a curvature, and therefore interacts differently. It remains at a higher pressure relative to the upper surface, varying with the AoA of the airfoil as a whole. Along with circulation about the airfoil, Bernoulli's principle is observed (greatly so) on the upper surface of the wing, accounting for the pressure drop/velocity increase.

Now, do not confuse a positive camber airfoil with a symmetrical airfoil. A symmetrical airfoil will not produce any lift at 0 AoA because no pressure disparity occurs when the surfaces are identical. This does not mean that either the top or bottom surfaces produce a force (which they do), but rather that they cancel each other out. A positive camber airfoil will produce lift even at 0 AoA because of the difference in curvature between the upper and lower surfaces.

>Bernoulli's principle is observed (greatly so) on the upper surface of the wing, accounting for the pressure drop/velocity increase.
uh
no

>Pilot here
just because you know how to fly a plane doesn't mean you know how a plane works

there's plenty of pilots who think a plane can't take off from a treadmill

all these pointless arguments on Veeky Forums lately...

can we at least agree that, no matter how planes fly, they couldn't have brought down the towers?

Nice bait

Great refutation, I like your explanation.

how's it bait. being a pilot doesn't mean knowing how a plane works any more than the existence of cars implies the average person could tell you how an internal combustion engine and a transmission work.

The treadmill part is bait and half, and you know it

I'll agree with you on the other part though, I know a lot of pilots who don't understand the aerodynamics of their aircraft.

offtopic:

quick question grammar nazis: ath the beggining of the sentence " First group was etc. etc." do I have to write "The first", or is it fine without the? (It is to be written in an article if it changes anything)

A lot of the bogus explinations around air travel are to hide the fact that air vehicles are THROWN toward their destination and use the fins to guide them around

Saying it without the "the" makes me feel as if I'm omitting it to speak faster

All these people frantically scouring Wikipedia and Google searches

Just to call each other names

And so few Mechanical Engineers.

not him. but any real world plane would take off from a treadmill. You're either stupid or baiting others by calling bait.

I see, thanks.
/offtopic

ok mech eng here

the way I like to think about is is there are more atoms / molecules with relative momentum hitting the underside of the wing than are the the top of the wing. A result the forward velocity of the plane. and angle of attack. So the resultant upwards force is due to the m(v1.-v2) cos 0 (OR sine 0 or tan) I can't be bothered thing about if it is cos or sin. This way of thinking means I don't have to worry about Bernoulli etc etc, laminar flow, turbulent flow or why a plane can fly upside down. Stick your hand out side the car window some day , angle it a bit and see it get pushed up.

It's that simple.

>That depends entirely on the shape of the wing.
Orientation (AoA) is just as important - moreso, really. But yes.
My point was that on most airfoils, including moderately-cambered ones, flow over the bottom surface is still accelerated and suction occurs on both the top and bottom surface at zero (or negative) AoA. For example, the cambered example I posted has overunity v/V and negative Cp from about 5% to 70% of the lower surface. But as you pointed out, it's the imbalance of these pressures that results in net lift, and with a cambered airfoil such imbalance still exists at zero AoA. That's not to say you can't cancel out the lift of a cambered airfoil by pushing it to negative AoA - that you most certainly can do.
Sad but true. Not saying anything about that user - his explanation was perfectly passible - but I've been flying since I was 15, and just from being part of the community, it's clear that the prevalence of stupid misconceptions amongst pilots - even professional ones - is outrageous. "Aerodynamics for Naval Aviators" should be mandatory reading for all licensed pilots IMO.

>there is no physical principle or rule that necessitates that the molecules must "meet up."
horror vacui

>a flat, rigid surface held at an angle to the direction of motion.
That works under a completely different principle

The flat plane you described sort of "pushes" the air downwards

Whereas a wing is being "pulled" by the air

Sails work exactly like wings, so maybe looking at those will help you.

False dichotomy. The principles at work are exactly the same. Both deflect air downwards.

I think we're about on the same page here. Aerodynamics for Naval Aviators is an excellent book. Both my Aerodynamics and Aircraft Performance professor used it religiously.

Go away, and take your horseshit with you

whatever you say bro
youtube.com/watch?v=YORCk1BN7QY

That shit has nothing to do with a treadmill and you know it. Take your Discovery Channel physics lesson and fuck off

youtube.com/watch?v=ZgcvBK2YJM0

>It's that simple.
except that it's not that simple and your explanation ignores the vast majority of actual science behind it

>a real world example of something I claimed to be impossible doesn't count
fuck off

The fact that they took off from a moving piece of carpet has absolutely nothing to do with it. That aircraft is a purpose-built short takeoff and landing light sport kit plane, meant to take off at very low airspeeds and high angle of attack. The speed at which the wheels spin have no bearing on the lift of an aircraft.

The pilot of those planes don't even require a private pilot's license, so I wouldn't expect the pilot to know much about the aerodynamics of the aircraft.

>Pilots
>knowing shit about aircraft or mechanics of flight
I'm yet to meet a pilot who could explain properly how slotted flaps work, for example.

Except that most of the lift is created on top of the wing, not bottom, so your way of thinking is wrong.
And no, it's far from being simple.

Don't take us all for idiots; some of us aren't stupid.

I'm sure, but as I said - PPL is given out to everyone these days.

That is dependent on the AoA and Reynolds number. In a simple case, the boundary layer will be attached and the flow will be steady.

>air molecules
Kill yourself.

To differentiate between "air particles" and the air itself, what the fuck is your problem, shitstain? Go hang yourself in your mom's closet.

Seriously, some people have no tolerance.

>I'm yet to meet
I think you mean "I've yet to meet" as in "I have yet to meet"

That pilot got most of it correct, you are all being assburgers attacking him for not knowing that Bernoulli is technically wrong. It's like a F1 driver will probably be aware that his engine works due to compressing and exploding the gas in a cylinder but he isn't going to know anything about thermodynamic cycles. All he would need to know is higher compression = more efficient just like our pilot here only needs to know higher airspeed over wing = more lift.

In any case the Bernoulli explanation is the most widely taught. In my 1980's encyclopaedias they say exactly what he said about lift.

>The fact that they took off from a moving piece of carpet has absolutely nothing to do with it.
why? because you don't like it? you're being autistic. saying something is wrong without saying why does not make it so.
>That aircraft is a purpose-built short takeoff and landing light sport kit plane
why is this a point? a plane on a treadmill should generate no lift, according to you people. So the fact that it easily generates lift means nothing.
>The speed at which the wheels spin have no bearing on the lift of an aircraft.
so you agree that a plane would take off then?
>The pilot of those planes don't even require a private pilot's license, so I wouldn't expect the pilot to know much about the aerodynamics of the aircraft.
he doesn't have to know anything about it to disprove it. he took off from a treadmill, so it's disproven no mater what he does or doesn't know.

The way I always think of it as the airfoil is getting slammed against the wind, and thus gets pushed.

When they talk about pressure they mean effective pressure... it's weird, I have a hard time describing it.

How is Bernoulli technically wrong

if wings wouldn't work without the stickyness factor, then this is an underrated post.

A couple posts in this dread hint that the wing pressure is pushing out on the air. Those are two different basins, and cannot be compared. Even on loose skinned fabric planes, the moving air pushes the fabric into the skeleton on both sides, just slightly more so on the bottom.

>the moving air
what i the model for this moving air ? is it just particles /

all air, both in and out of the wing is the same composition, but they are different testbeds in terms of bernoufag, and cannot be compared. the moving air is the blowjob outside of the wing.

but mathematically, what is the model for air in the system air+wing

re: this diagram...

test an idea with me. lets narrow the bottom half's fatness AFTER the initial vertical velocity has been placed on the incoming air.

Same force imparted on the air to kick it out of the wing's way, so same change in airmass momentum BUT less wing there to fill the gap. More suction?

Also, that minuscule backforce on the trailing edge could not overcome the dragforce of the green lifting arrows on the downramp and the plane would experience about a g and a half of retardation.

there is none. they are incomparable. in a fabric plane the air is free to bleed through (some), bud that doesn't mean their is a bridge of reset between the two testbeds.

The shape creates a partial vacuum above and behind the wing when it moves through the air. The airfoil experiences a force due to the pressure differential which are referred to as lift and drag. You cannot create lift without creating some drag.

Drag that unrelated to the creation of lift is referred to as parasitic drag.

cambered airfoils experience less drag relative to lift at the chosen airspeed and air density. It just works better.