Airflow across a wing
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- Опубліковано 19 січ 2012
- "It is often said that the lift on a wing is generated because the flow moving over the top surface has a longer distance to travel and therefore needs to go faster. This common explanation is actually wrong." Here, aerodynamics expert Professor Holger Babinsky from the University of Cambridge's Department of Engineering debunks a popular, yet misleading, explanation of how wings lift.
For more information, read the accompanying story published by the University of Cambridge www.cam.ac.uk/research/news/ho...
This doesn't really explain how it works though...
Different local velocities in uper and lower location.
@@liuqm5324 ok but why
Do the same thing with the smoke at a flat level angle of attack.
Lift is caused by turning a fluid. The 'speed' difference between the upper and lower surfaces is not generated in the way most people think. The wing lower surface pushes the lower air forwards (in the direction of the the wing's movement) and downwards. his is on component of lift and the reason for a slightly increased pressure compared to static on the lower surface. Watch the pulsed smoke.
The pressure bubble or bow wave created by this lower surface is what pushes the air up and over the wing. As the air 'turns the corner - as it must -(there are no vacuums in normal space) the rapid change of direction as it follows the upper surface causes a slight pressure drop but the lift force is created by changing the direction of the air. The pressure change is a RESULT not a cause of lift.
If you bend the air down the wing goes up. Simple. It works perfectly well with a flat plate so wing shape is not the primary cause.
All wing shape does is dictate the angular change of the air. Less camber less angular change and vice versa. Remember: Supercritical wigs as used on all modern airliners and curved on the lower surface and flatter on top.
Also the air does not speed up or accelerate (using the common definition) over the wing. It actually slows down when it comes in contact with the wing surface. This is called form or profile drag and results in the boundary layer.
Again watch the pulsed smoke. You can see the streamlines closer to the wing get pulled forwards while the ones near the wind tunnel wall continue on relatively undisturbed and transit the space more quickly.
The primary cause of lift changes is change of AOA as AOA is the primary cause of the angular change of the air. Zero effective AOA = no lift.
When trying to understand aerodynamic principles or flow around an aircraft the most effective way to visualise it is to imagine the aircraft under water like a submarine. Water behaves exactly the same as air (at subsonic speeds) but is more dense so the effect are easier to understand. This is why aerodynamics belongs to the field of fluid dynamics. Same rules apply.
+Complete Aerogeek Lets say the weather is calm and there is no wind or whatsoever. Wouldn't that mean the air molecules are almost at rest. So, how would they slow down further as you claim.
There is a pressure differnece between the lower and upper surface of wing. Thats the main causes of lift.
Adjust the chord line so that it is parallel to the relative airflow and post a video on the results of that test... This wing section in this video is at such a steep attack it is resulting in a near stall condition.
So the higher speed over the top is still the cause of the pressure difference. Meaning Bernoulli's principle is still at play, the explanation for why the speed difference occurs is the only problem?
Here is how it works according to Bernoulli's principle, where the fluid speed is high the fluid pressure will be lower at that region and where the fluid speed is slow air pressure will be higher. This pressure difference above and below the aerofoil generates the force called lift that flies the airplane upwards...
I think most people are wondering why the airflow actually speeds up over the wing as opposed to understanding how it generates lift.
was that airfoil symmetric? if so how would a typical asymmetric airfoil differ? and how about lessening the angle of attack?
Regarding the lift equation, the V Squared will it then refer to the relative air speed below or above the aerofoil?
I was never taught that the air sped up because it had a longer distance to travel. I was always taught that the lower pressure was caused by the mass flow and the creation of a venturi, essentially, on top of the wing. the increase in kinetic energy leads to a decrease in static pressure as explained by the bernoulli principle. in addition to pressure difference, air is deflected by the wing, also creating an upward force on the airfoil
So lift is actually push? That's to say, it's the backed up air flow under the wing creates the upward force called lift. How about if the angle of attack is decreased and the flow is more laminar? I guess the effect is just less pronounced, but still exists.
As long as you keep a positive angle of attack to the airflow, enough to create the airflow distortion and thus the speed difference and concequently the pressure differential you should be able to fly just fine. But it just wouldn't be so efficient for an assymetric airfoil, so perhaps you would need more power, or airspeed to keep the plane flying...
is there a reverse flow on the wing?...perhaps sublaminar? thanks
@lglglggl If you look closely the airfoil in the video is symmetrical, or nearly symmetrical. What's to say that the airplane that it's attached to isn't already upside down?
While the shape of the airfoil contributes to lift, another (often larger) contributor is the angle of attack (in the video the airfoil has an AOA, about 15-25 degrees). Airplanes flying inverted have at least some downward pressure on the elevator to create a negative angle of attack, creating negative lift.
What time difference is there between the snapshots at 0:31 - 1:08?
That's never been the explanation at all. The explanation was always that the air travels faster over then it does under.
This creates a pressure drop as air density is directly related to speed. It's the reduction in pressure above the wing that creates lift.
@lglglggl it doesn't matter for an airplane to fly upside down or normally, as long as the angle of attack remains positive. When the angle of attack is negative, you'll have a spoiler, like the one used in F1 to keep the cars on the ground..
Any reason for the high AoA?
Lift comes from 2 things:
1. angle of attack
2. shape of wing
In this video, the angle of attack is so extreme that most of the lift comes from there.
If they used a 0 degree angle of attack, then the shape of wing and what they're visualizing would actually make sense.
@Iglglggl, wings cause the pressure under the wing to increase relative to the air above the wing. This video illustrates Bernoulli's principle, which shows how a wings shape does lower the air pressure above the wing. But this isn't the main reason wings generate lift. Really it is the higher pressure of the under the wing that is the main factor.
@lglglggl If you notice the video, the cross-section of the airfoil shown is fully symmetrical (it's not more curved on the top). Flipping it over and pointing it upward slightly (as aerobatic planes do when they fly interved) would generate the same lift.
What do you use to get the smoke to do that? I want one.
Actually this is a very good example to explain why it can also fly upside-down. You can see that the wing profile is symmetrical so it makes no difference if you turn the plane upside-down. All you need is airspeed and angle of attack.
It is also possible with an asymmetrical profile but you would have some differences in efficiency
@lglglggl Those are aerobatic maneuvers, and usually involve the aircraft having substantial initial velocity, absent which they would stall during the maneuver. The main thing is having sufficient velocity to operate the control surfaces - you haven't seen them flying upside down for a very long time, have you?
Excellent visualisation, will there be a follow up to explain how lift is generated?
Awesome vid! Can you post a vid using a flat plate and also a sail profile with same angle of attack as the wing in this vid pls.
BABINSKI best lecturer ever =)
My comments aren't posting or are not visible when I am not logged in?
The angle of deflection is also part of creating lift. When inverted, the pilot controls the the pitch of the aircraft to keep the nose up, thus giving the wings sufficient angle of attack to deflect air downwards. But Bernoulli isn't helping out as much so more thrust is required. Pretty cool!
you state that the air above the wing speeds up, is it not the air below slowing down?
Doesn't look like the air above the wing is speeding up, but staying the same speed. It looks like the air below the wing is slowing down. Just my observation.
@lglglggl It doesn't matter which way up the fuselage is! It's relative to the wing's AoA.
This model demonstrates a simple symmetrical wing... What do you think it would look like upside-down? What requires explaining?
@lglglggl
The wing works the same way upside down when the angle of attack is increased to a certain point, but cambered airfoils aren't as good at creating lift upside down because the laminar flow separates at a higher airspeed and a lower angle of attack in this configuration. This is why many aerobatic aircraft use a more symetrical airfoil because while it creates a lower coefficient of lift, it stalls at a higher AOA and it acts uniformly regardless of which side is up.
@lglglggl it's really falling very slowly, not 'flying' when upside down ?
yes BUTTTTT.. a perfectly straight surface will do the same thing..
so its the pressurization on the underside of the wing that causes lift not the speed difference by itself
Ailerons are for controlling bank. Are you referring to the elevators? they control pitch not the ailerons.
@krispykreme82603 ... yes, yes it will. It will react *exactly* the same. There are differences, such as the 'smoothness' of the flow and the effect the walls will have on the air, but these are generally small and accounted for.
the flow on the top is accelerated because it's being sucked forward, the same vacuum that lifts the plane wing is also pulling air, I would think the extra distance the top flows could signify strength of the vacuum/negative pressure zone?
the curvature of the wing causes the change in air pressure because **it pulls some of the air upwards, which reduces pressure, and forces the rest beneath it, creating higher pressure**.
can u explain how it pulls the air upward???
i await your response...
Yes a plane flying upside down is the perfect demonstration as are wings with symmetric profile, as to why Bernoulli is NOT responsible for flight. In reality lift is generated by causing a deflection in the air stream, just bending it downwards. This is achieved because of the Coandă effect which shows us that a fluid flow will adhere to a surface even when that surface curves away from the direction of fluid flow, Also Brute force deflection of airflow (called angle of attack) works too.
I noticed you conveniently used a symmetrical airfoil with a positive angle of attack for your model, thus producing a lift coefficient, thereby complicating the discussion. The same complication exists when using a sail example. Your symmetrical airfoil would produce zero lift in a wind tunnel with zero angle of attack. Just as a paper (symmetrical wing) airplane would produce no lift with a zero angle of attack. However, an airfoil such as a Clark-Y would indeed produce lift in a wind tunnel with zero angle of attack. Why is that? What principle would be responsible? The upper surface would be accelerated and there would be lower pressure. The air molecules would NOT meet at the same time at the trailing edge, but who (relevancy) cares? Decreased pressure = increased velocity, in that order, due to shape alone. Personally, I would call this lift Bernoulli's Principle. Please correct me if I am wrong. I am assuming all things being equal for both tests, such as air density, air velocity, wing area and no lift coefficient (zero angle of attack) .
Considering the lift equation: L = (1/2) d v2 s CL, Bernoulli's Principle plays a small part in making up the Coefficient aspect and can be explained referencing wind tunnel tests as you have shown, provided one assumes a zero angle of attack for the explanation of Bernoulli's small role in the overall scheme of the overall total lift equation.
I have never considered Bernoulli's Principle as to the sole explanation as to why wings fly, but merely a rudimentary way to explain what the shape aspect (looked at alone) partially does without the other factors of the lift equation. With this small part of the explanation expressed with an assumed zero angle of attack.
Awaiting response.
Great teaching aid. Thank you
Therefore the lift effect is amplified? The time that the air reaches the end on the top of the wing is shorter. Hence the speed is faster than originally thought. Therefore pressure is even lower and the lift higher? Is this a correct assumption?
@lglglggl
It can because when flying upside down one flies at such angle of attack that the lift is still produced.
There is nothing new in this video and its explained nicely at NASA's site.(equal transit time theory) Please explain why at all the speed of the airflow increases over an aerofoil.What makes the aerofoil speed up when it passes over the aerofoil.
What I see here is that the lower side of the wing slows the air down more while the upper side of the wing does not necessarily increase the speed. Just compare the points of smoke as they are closer to the wing versus farther away the upper section of the wing stays relatively all the same speed where is the lower section of the wing seems to be slow down more the closer it gets to the wing
This would also appear to be true on the upper side of the wing just at a much closer location
@ubellubo This isn't necessarily true - the size of the contributions of the upper and lower surfaces is different for each wing, angle of attack and flight regime. To pick one extreme example, a supercritical wing (an aerofoil designed to fly just below the speed of sound) actually has a lower pressure on the bottom surface than in the free stream, but the pressure on the upper surface is even lower still, generating the lift force.
@lglglggl angle of attack... foil shape of a wing just makes flight more efficient. Flying upside down powers the angled wing through the air with a resultant up force. Kind of like sticking your angled hand out the car window at speed makes it lift from the wind pressure.
@lglglggl
Because of negative angle of attack. the lift generated is negative, and when the aircraft is upside down, the negative lift cancels out the weight
@lglglggl Same reason a bumble bee can fly. Shear power. Look at the video again. It seems to me another factor is the attitude of the wing. Most text books I remember from high school science show a perfectly horizontal wing. This one is tilted to attack the wind with the force hitting the underneath. That alone will create lift if the air is pushing on the bottom.
@lglglggl Because while the shape of a wing is important, more lift is produced by the wing forcing air downward than by the pressure difference caused by the shape of the wing. At a high enough speed and with the right angle of attack, a sheet of plywood would produce sufficient lift, albeit with much less efficiency.
airplanes dont fly in a vacuum so air is displaced with more air. if all things were to be equal, and you only had 2 molecules of air one above and one below they would start and end together. depending on the shape of the airfoil and the direction it faces the molecules would start separation and end together at different locations.
These streamlines, which are visualized in the video, are so far from the boundary layer. However, theory is valid at the wall. And there are streamlines very close the wall. Only those streamlines which are very close to wall will meet up at the back of the wing. When you get far from the wall, they will not meet up.
Thanks for debunking that myth, but why not take an extra 20 seconds to explain how it actually works?
@lglglggl It's the exact same thing, only now the top side of the wing is now the bottom. If you were to perform this same smoke tunnel test with the wing upside down, it would look identical to the video above.
Nice way to deflect the wing in an upwards attitude to manipulate the airflow.
Why is the difference in air pressures not mentioned. which is why the actual wing rises.
@surindertiwari You'd have to attend a few of the second year thermofluid lectures to understand that.
This video is biased. The airfoil used is symmetrical therefor Bernoulli principle does not apply. Also the air is flowing slower on the bottom because the airfoil is angled down. It should be level.
Ok, seriously, in order for the airflow to speed up over the top of the airfoil, something has to impart energy to it. Why cant it be that the airflow is being physically impeded on the bottom surface and extracting energy from the air? If the airflow over the top appears to be going faster, isnt it possible that its speed didnt change, but rather the velocities on the bottom side have been reduced?
This a great video, but you say nothing as to how lift IS created. You simply debunk the common myth that it is related to the speed of the flow over the top of the wing. It is really air pressure difference that causes the lift, but I wouldn't have known that if I hadn't been lead to this video via the Gizmodo link that includes the explanation.
@lglglggl angle of attack, thrust and wing shape.
I've always thought the original idea made no sense. Another test that could have been shown in this video is Ground Effects, which would further prove this theory correct.
Some of the comments here don't make sense. Subsonic Lift is not terribly complex. if you can make a paper plane you can debunk most silly theories.
Here is a resource to start with:
www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html
Forget what air is moving how fast. That is a secondary issue and a result of AOA. It also brings forth all kinds of nonsensical explanations like equal transit time.
Also remember that in fact: the air is not moving the wing is. By thinking the air is moving we ascribe all kinds of things to it like momentum which it does not have. It is static. and so Newtons 1st Law applies. As soon as the wing bashes into it Newton's 2nd and 3rd laws apply. Still air also has inertia and viscosity. (it is not inviscid)
If you watch the pulsed smoke approach the wing-you will see that in the correct perspective (still air-wing moving) The wing begins to bend the vertical stream before it even reaches the leading edge.
This is a pressure wave (bow wave if you like) being pushed ahead of the wing.
If we isolate the upper and lower surfaces, we can clearly see that as the wing moves forward it pushes the lower surface air forwards (i.e. to the left of screen and downwards.) This is the primary reason for the pressure increase under the wing.
Now to the upper surface: The wing due to form drag (viscosity) PULLS the air forwards in its direction of travel.
If you were to imagine the air moving - wing still scenario: It means that the air is SLOWING DOWN NOT SPEEDING UP as many people claim..
Kinda messes with some people's thinking...
As the air is forced upwards by the pressure wave that lives in front and below the leading edge, it then splits (you can see the streamline turn into a Y shape at the transition point) turns the corner and rapidly changes direction. This creates a momentary drop in pressure/density and the volume of space increases behind the leading edge.
In effect the wing rides up on the pressure wave caused by AOA much like a water skier or a boat on the plane.
This and rapid change of direction above the wing and the attending pressure/density drop is what causes the pressure differential, not wing shape. The effect on both surfaces creates a total reaction force and this is lift. F=Ma. The pressure differential is a by product of AOA + upper surface camber (if any) not wing shape.
Don't believe me?
The simplest kind of wing is a flat plate inclined to the relative airflow-just like you hand out of a car window.
A flat plate has a superb L:D ratio but has an earlier stall angle because the air cannot turn the corner as easily as on a curved plate.
This is the same reason aircraft have leading edge slats.. ( NASA's Foilsim is a fun tool to experiment with wing shapes, stall angles, camber and L:D ratios)
If this were not so, how would hang gliders, kites, flat plate wings (missiles and some high sped aircraft) paper planes sailboats fly?
Most military and medium speed aircraft have had symmetrical wings since the 1940s.
ALL airliners today have supercritical wings (flatter on top and more curved underneath. i.e. inverse camber).
In summary: Always start at the most basic exemplar: What you can prove for a flat plate wing is also true for any other kind of wing.
The primary reason for lift is that the wing mechanically interacts with the static air 'bending it. Bend the air down the wing goes up. (no different to a ceiling fan, or a kite, or a hang glider, or a sailboat etc etc.)
And the best part? You can prove it practically and mathematically without ever having to hear of Bernoulli!
Enjoy!
that wing is about to stall, lower the angle of attack and do it again
Are you so concerned about that tiny laminar bubble? It won't stall until there's backflow
So lift is not caused by the airflow having to move faster over the top, it's caused by airflow moving much faster over the top. I think post production forgot to include the explanation on how lift is actually caused.
Ok....so what is the point exactly? I have never heard that lift is caused by air having a longer distance to travel over the top of the wing. I have only heard that lift is caused by air moving faster over the top of the wing, which the video proved. So I am confused to what this is really about.
Hmmm, let's see. The airflow above the wing is faster than the airflow below the wing. Yet dear professor says that this does not generate lift because the air above wing gets to the back -before- the air below the wing. If I run to the toilet faster than the professor, will I not be having a leak before him? If I got there at the same time, would I have run faster? What does this prove anyway?
I have always been annoyed by the conventional explanation. What's even more annoying is the idea that the wing is "sucked" upwards by the low pressure. The shape of the wing is also irrelevant for basic flight, a plank will work with enough air pressure. This smoke test appears to illustrate how the energy from the lower airflow is reduced and therefore absorbed by the wing.
The "actual force upward" is caused by air molecules (mass) colliding with the wing and causing lift due to an equal and opposite reaction. Lift is not created by something happening somewhere "beyond the trailing edge". This does not in any way serve to deny that a part of the lift is attributable to the "shaped wing" concept, explainable by bernoillis principles.
By using the dynamic surfaces of the wing (Ailerons), you can create 'lift' in either direction as you alter the effective profile of the wing. What the Prof (IMHO) is actually saying above is that the wing lifts by slowing down the air underneath it rather than accelerating the air above. This creates a higher pressure zone underneath, driving the wing upwards.
You are 1/2 way there! You discovered problem #1, but not problem #2 which is inherent in wind tunnels and in assumptions about "wing" shapes. You must take force measurements on the "wing" in multiple configurations to say how much force is due to which effects. Remember the wing moves through the air which is problem #2. A static wing in a wind tunnel is a problem because it doesn't actually show the impact of the wing's movement at the point of wind impact. Start with a flat surface being hit by air as in down-wind sailing or as in a road-sign being blown over by high winds. Change the angles. Alter the airflows around the "wing" by using shapes which prevent the air from flowing around the back-side of a traditional wing. Measure the forces in all configurations. Nobody can proclaim they know what is causing a majority of lift when they don't show the effect of the wind both "with" a traditional wing shape and "without". We need to see the force of wind deflection, for instance.
It's all about angle of attack.
@lglglggl the long answer, get a book on aerodynamics and read it.
or the very dumbed down answer... err... some aerofoils are symmetric, and others are asymmetric. passenger aircraft have asymmetric aerofoils, they dont fly well upside down, if they can at all. fighter/acrobatic aircraft have symmetric (or close to) aerofoils; the wing is the same shape top and bottom. the angle of attack is what produces the lift
it can't
"this shows that we're showing you a very non-standard airfoil as if a typical "wing", plus let's not omit to take notice of the severely exaggerated angle of attack which we are deliberately suggesting, without clarification, as if it's a typical AoA of an airplane wing in flight. If you are sufficiently confused and misled by this point, remember you've enjoyed this presentation in a potentially authentic British accent, discourtesy of none other than Cambridge, mind you. Good day, fine fellows."
@lglglggl because the shape of the wing doesnt change when you flip it upside down. It still generates lift.
Regardless....the air over the top of the wing moves faster, thus creating a lower pressure compared to the pressure under the wing which in turns generates lift.
This will always be a sore topic. I don't think anything is debunked from this video, since the wing here is clearly much higher angle of attack than the 2-5 degrees a wing on a plane would usually have when flying efficiently. The wing in the video is stalling or near to it, is probably fixed in place, and there is no indication of how much lift is generated. Planes especially passenger planes also have a million moving parts that change the shape of the wing when flying at lower airspeeds for takeoff and landing. I am fully confident Boing, Airbus etc. know exactly what they are doing, and there is probably an explanation why doctor Holger is not cashing in millions working for those.
The classical explanation is much closer to reality, when you think about how big an area wings are, and do the physics calculations. A cutout doesn't do it justice.
"This common explanation is actually wrong." Well? Don't leave us hangin', Babs; finish what you started. Here I'll help you begin: "The correct explanation is..."
because it has a great pilot :P
I've been wondering about this. So then, why DO wings lift??
@lglglggl
Angle of attack is the explanation.
"This common explanation is actually wrong." Well? Don't leave us hangin', Babs; finish what you started. Here I'll help you begin: "The correct explanation is..."
Does the air above the wing slow down/speed up/stay the same as it travels over the wing?
Does the air below the wing slow down/speed up/stay the same as it travels beneath the wing?
Why do each of the above occur?
Is it this difference which creates higher pressure below and lower pressure above which in turn causes lift?
The wing in your video appears to be close to stalling, at this angle the air beneath the wing is being compressed by the wing and therefore slowed down, resulting effect is to push up on the underside of the wing. If you level out the wing to an angle more akin to flying flat and level you would see that the air moving over the top of the wing is traveling further than the air underneath as it needs to rise and fall over the same distance. faster moving air = less pressure = lift
@lglglggl Symetric airfoil
I'd always suspected it's more of an angular deflection thing more than a low pressure zone doing the real lifting.
What about the conservation of mass that is unaccounted for in your explanation. There would be a severe degradation (drag) of the flow over the wing if your explanation was correct. There are binary layer control devices that make up for the highly energetic flow this slowing it down and reducing it's energy so that it can meet on the other side at a similar time span thus causing less vacuum pressure drag than in your simple flow isn't symmetrical explanation. Imagine a plane flying very fast with a very curved wing for a great distance, in your explanation does the split up air remain displaced forever? If not, when is it accounted for?
You would get a different result if you did this in liquid helium. The non-superfluidity of our atmosphere is the beginning of the explanation of lift.
@lglglggl Well...because air moves faster over the top of the wing, causing lift. The video proved that. When the plane is upside down...the bottom is now the top. But the video states that the common misconception is that lift is cause by air having to travel a greater distance over the top of the wing. Anyone who knows just the simple bit about aerodynamics is that lift is caused by air moving faster, which the video proves. Odd video really.
All this proves is that the air above the wing increases in speed more than we thought.
Oh, Stick and Rudder (written in 1944) also gives a good explanation of how a wing works (turning a fluid)
Together, both theories fully explain how a wing works and we've known all about it for over 70 years !!!!
But, by all means, keep doing the experiments so you can tell us stuff we already know :/
IgIgIggl, It's called elevator authority.
AT LAST THE TRUE EXPLANATION that ejects many false theories and rumors about airplanes. Thanks professor !
@lglglggl Angle of attack!
Who is the self confident yahoo that analyzed and voiced this video? Clearly does not have an understanding of aerodynamics and flow properties. I'd state all the errors in OP's deductions, but there are plenty of comments correctly correcting him already.
but thats over simplifying. the prof would do a much better (and less condescending) job of explaining
As a student in Aerospace Engineering, is you for serious?
@lglglggl the long answer, get a book on aerodynamics and read it.
or the very dumbed down answer... err... some aerofoils are symmetric, and others are asymmetric. passenger aircraft have asymmetric aerofoils, they dont fly well upside down, if they can at all. fighter/acrobatic aircraft have symmetric (or close to) aerofoils; the wing is the same shape top and bottom. the angle of attack is what produces the lift