Wing lift Holger Babinsky
Вставка
- Опубліковано 12 бер 2014
- A 1-minute video released by the University of Cambridge sets the record straight on a much misunderstood concept - how wings lift. I start by giving the wrong explanation and asking who has heard it and every time 95% of the audience puts their hand up. Only a handful will know that it is wrong.Professor Holger BabinskyIt's one of the most tenacious myths in physics and it frustrates aerodynamicists the world over. Now, the Department's Professor Holger Babinsky has created a 1-minute video that he hopes will finally lay to rest a commonly used yet misleading explanation of how wings lift. "A wing lifts when the air pressure above it is lowered. It's often said that this happens because the airflow moving over the top, curved surface has a longer distance to travel and needs to go faster to have the same transit time as the air travelling along the lower, flat surface. But this is wrong," he explained. "I don't know when the explanation first surfaced but it's been around for decades. You find it taught in textbooks, explained on television and even described in aircraft manuals for pilots. In the worst case, it can lead to a fundamental misunderstanding of some of the most important principles of aerodynamics." To show that this common explanation is wrong, Holger filmed pulses of smoke flowing around an aerofoil (the shape of a wing in cross-section). When the video is paused, it's clear that the transit times above and below the wing are not equal: the air moves faster over the top surface and has already gone past the end of the wing by the time the flow below the aerofoil reaches the end of the lower surface. "What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes - lower pressure on the upper surface and higher pressure on the lower surface," clarified Holger. "This is why a flat surface like a sail is able to cause lift - here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it's the curvature that creates lift, not the distance." Holger is quick to stress that he is far from the only aerodynamicist who is frustrated by the perpetuation of the myth: colleagues have in the past expressed their concerns in print and online. Where he hopes his video will help debunk the myth once and for all is by providing a quick and visual demonstration to show that the most commonly used explanation cannot possibly be correct. The original video, created by Holger a few years ago using a wind tunnel, has now been re-edited in high quality with a voice-over in which he explains the phenomenon as it happens. Holger's research focuses on the fundamental aspects of aerodynamics as they relate to aircraft wings, Formula I racing cars, articulated lorries and wind turbines. One of his visions is to design a wing that will enable aircraft to fly faster and more efficiently. Using a massive wind tunnel within the Department of Engineering, Holger and his team have been modelling the shockwaves that are created on aircraft wings and that restrict the plane's top speed. This video supported lectures Holger gave as part of a series of University of Cambridge Subject Masterclasses aimed at Year 12 school children: "It's important to put out this video because when I give this lecture to school kids I start by giving the wrong explanation and asking who has heard it and every time 95% of the audience puts their hand up. Only a handful will know that it is wrong." - See more at: www.eng.cam.ac.uk/news/how-win...
- Наука та технологія
Ahh 40 years later and I thought so!Thanks!
It would be interesting to see the same experiment with a perfectly flat wing at the same AoA.
same
i always said that the wing generates lift because it trows air down at the end ; the rounded surface at the top is only there to prevent airflow separation as it changes direction gradually
Thank you Dr. Babinsky for a brilliant explanation of how streamlines influence pressure changes resulting in lift on airfoils. This helped explained my research on standard vs KF airfoils which gained first place at a local science fair...Currently, in year 9 and hopeful Cambridge candidate in the future!
didn't understand whether viscosity of air was taken into consideration in above demonstration as it seems different streams of air with smoke touches the wing.
Still not a simple sentence. Can we say that the aerofoil is A) sucked in a direction by reduced air pressure over the long curve, or pushed in a direction by the cushion it creates as its AoA simply must push air down? And....whether against or with gravity (up or down) is dependent on the tail plane doing a separate job?
Perfect!
It would be useful to see the same streams unobstructed (without the aerofoil), to act as a control, to demonstrate whether the actual speed of the airstreams over and under the aerofoil increases and decreases respectively, or if it's simply relative to each other.
I'd like to see the video at a less extreme angle of attack. An aircraft flying (or _trying_ to fly) at this high AOA would certainly be at the incipient stall stage. Maybew the buffeting we feel pre-stall is something to do with the differing arrival times at the trailing edge? show me a wing at a normal angle of attack and I'd be more convinced.
fatcharlieuk, I agree. Can we see another video with a wing that is not entering a stall? That wing may still be generating a bit of lift but it doesn't appear to be at a very good AOA.
But it isn't stalled, and the high angle of attack increases the difference in speeds, making the point more clearly.
Motion (sails)/lift (wings) is mainly due to wind deflection to the stern (sails) / to down (wings).
This ua-cam.com/video/jhem8Z9ujPE/v-deo.html
shows that close-hauled motion is possible with NO wind on leeward side (thus no effect on that side).
Cool !!!
The authors have two wrong scientific approaches: researching the creation of Lift force and Low pressure at upper side of the wing, relative to the ground surface and Earth. I explain the aerodynamic cavitation and existence of Lee side aerocavern, and creation of Aerodynamic force.
How to make smokes like that ??
It's in a high speed wind stream filmed using a high speed camera.
Most likely method would be to use something akin to a fog machine. Obscurant is pumped onto a heating element to aerosolize. Theatre fog is similar density to air and won't move up/down rapidly.
from edupoint
bernoulli was incorrek (sudden gasp) REEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE!
0:42 I don't see that it "speeds up as it approaches the airfoil". If anything, it appears to lag behind the smoke in the undisturbed airflow at the top of the screen. If the air doesn't accelerate over the top of the wing, how can it generate the well established low pressure which generates the lift from the top surface?
If wings could actually create a net acceleration of air, they'd create thrust.
@@user-do5zk6jh1k a) Babinsky says they do and b) I don't think the video shows that, so how does your reply relate to my comment?
@@RationalDiscourse Let me tell you straight. You don't understand what you're looking at, and you forgot what you wrote 5 months ago. You shouldn't be surprised that the freestream airflow is outpacing the upper surface flow. If it were the other way around, the upper surface would be creating thrust.
What is important ; visualize the smoke and it does not deflect down. A propeller , a rotor blade , a fan all move air at 90 degrees from travel of blade this is a known event and supports Newton law of force ; however, the pressure difference is also measured as a dynamic event and thus can not be discounted . Air is sticky when in laminar format and a wing is always in line with flight vector of the incidents of cord of wing. This would support pressure difference of top to bottom of wing as primary event in lift with Newton Law of Reaction as a measurement of the pressure. Like gravity, it is not behaving as anticipated,,,,.
This demo doesn't create vacuum like a plane would.
If you do this experiment in liquid helium, you will see that the equal transit time theory is correct. You will also see that there is no net lift.
Repeat the experiment in liquid sodium. You will find that equal transit time theory is wrong, and net lift is produced. The moving hydrofoil produces a relative displacement of the fluid above and below it. If you follow this back to the place where the hydrofoil started, then you will find a starting vortex whose presence avoids an apparent issue with the conservation of mass.
There is no such starting vortex in liquid helium, which is how I know that equal transit time will be correct.
Can you post a link or reference to those experiments? My supplies of liquid helium and liquid sodium are running a bit low.
@@alans172 I'm making predictions. In liquid helium vorticity is quantised. A very slow hydrofoil will be below the quantum level and no starting vortex is produced. No starting vortex means equal transit time has to be true, but there is no net lift.
@@david_porthouse The essence of a thought experiment (c.f. Einstein's gedankenexperiment) is that it can be imagined.
@@alans172 What can also be imagined in two dimensions is a quantised vortex in Brownian motion with the quantum and the kinematic viscosity having the same value. That's an entity which is arguably both a wave and a particle.
@@david_porthouse Ha ha! And I thought for a moment you were trying to be serious. Nice one!
Bernoulli argument also debunked? Here's what I mean: ua-cam.com/video/rHidaQgBb-Y/v-deo.html
Is it possible that Prof. Babinsky has also debunked the Bernoulli argument as well? The argument that the low pressure above the wing is generated by the increased airspeed over the wing. From the frames at 0:42 onwards, it is quite clear that the air flowing over the upper surface of the wing has not speeded up. In all those frames, the wind over the top surface of the wing is moving at the same speed as the ambient air at the top and bottom of the screen.
Is this a show stopper?
The air stream closest to the upper surface of the wing has traveled a longer distance than the stream at the top of your screen. Yes they reach the trailing edge at the same time, however the stream just above the wing had to curve around the wing a travel farther in the same amount of time. Imagine those lines are strings and if you were to pull the string closest to the surface of the wing tight, it would extend farther than the rest of the strings.
@@rileybullen4961 The smoke over the top of the wing arrives at the trailing edge AFTER the smoke the top of the screen. Pause the video at 0:47 where the smoke above the wing is nearly at the trailing edge. The smoke at the top of the screen has already passed the trailing edge. Take another cool hard look at that frame. They do not arrive simultaneously!
The airflow that is squeezed between the foil and atmospheric pressure , as it flows over the leading edge , is definitely accelerated by dynamic pressure. I'm talking about the flow that is closest to the upper convex surface i.e. the flow that hugs the upper shape. The flow above this region does not change in velocity.
It's all available to see in the video , I'm not making this up , just slow the footage down. Flow just under the foil is slowed down so an increase in static pressure arises , hence thicker smoke flow in that region. Further below that velocity does not change.
To recap , flow immediately above the foil is accelerated and flow immediately below foil is deaccelerated. Air flows that fall out of this domain are unaffected.
From observation there is acceleration of fluid immediately above and fluid deacceleration immediately below , so there is a speed differential in fact. However this does not explain lift.
Atmospheric pressure pushes down on the flow over the upper surface so it follows the shape of the foil. This flow , due to inertia(Newton's 1st law) , resists atmospheric pressure and so must be deducted from the total pressure above the foil. To further clarify , the resistance is due to the flow not wanting to change direction as it follows the rounded shape , just like centrifugal forces.
This is how a pressure differential is achieved, which generates lift, not by Bernoulli but by Newton. Bernoulli accounts for the speed differential that occurs above and below the foil, Newton accounts for the pressure differential that occurs above and below the foil.
@@singh2702 Albeit for entirely different reasons, we agree that the Bernoulli Principle has nothing to do with the aerodynamic force (lift/drag).
But can you cite a reference providing the mechanics behind the "Newton's Law" explanation?
Using mass, momentum, density and velocity, can you show how a light C172 at 100 kts can generate the 10,000 Newtons to balance its 1,000 kg weight, or a B747 at 300 knots can generate the 5,000,000 Newtons of force to balance its 500,000 kg weight?
@alans172 Take the difference in pressure per square inch, which is very small BTW, and multiply it by how many square inches make up the surface area of the wings.
Common sense. That's why low-speed aircraft have wings with large surface areas.