Need a little refresher on how airplanes fly? The middle school students of The Nueva School have you covered with their latest science rap parody. They take a look at the four main forces on a flying airplane and even dig a little bit into the principles behind lift generation. Check it out! (Video and image credit: Science With Tom/Science Rap Academy)
Gorgeous new research highlights some of the differences between fixed-wing flight and birds. Researchers trained a barn owl, tawny owl, and goshawk to glide through a cloud of helium-filled bubbles illuminated by a light sheet. By tracking bubbles’ movement after the birds’ passage, researchers could reconstruct the wake of these flyers.
As you can see in the animations above and the video below, the birds shed distinctive wingtip vortices similar to those seen behind aircraft. But if you look closely, you’ll see a second set of vortices, shed from the birds’ tails. This is decidedly different from aircraft, which actually generate negative lift with their tails in order to stabilize themselves.
Instead, gliding birds generate extra lift with their maneuverable tails, using them more like a pilot uses wing flaps during approach and landing. Unlike airplanes, though, birds rely on this mechanism for more than avoiding stall. It seems their tails actually help reduce their overall drag! (Image and research credit: J. Usherwood et al.; video credit: Nature News; submitted by Jorn C. and Kam-Yung Soh)
One of the most vexing topics for fluid dynamicists and their audiences is the subject of how wings generate lift. As discussed in the video above, there are a number of common but flawed explanations for this. Perhaps the most common one argues that the shape of the wing requires air moving over the top to move farther in the same amount of time, therefore moving faster. The flaw here, as my advisor used to say, is that there is no Conservation of Who-You-Were-Sitting-Next-To-When-You-Started. Nothing requires that air moving over the top and bottom of a wing meet up again. In fact, the air moving over the top of the wing outpaces air moving underneath it.
In the Sixty Symbols video, the conclusion presented is that any complete explanation requires use of three conservation principles: mass, momentum, and energy. In essence, though, this is like saying that airplanes fly because the Navier-Stokes equations say they do. It’s not a terribly satisfying answer to someone uninterested in the mathematics.
Part of the reason that so many explanations exist – here’s one the video didn’t touch on using circulation – is that no one has presented a simple, intuitive, and complete explanation. This is not to say that we don’t understand lift on fixed wings – we do! It’s just tough to simplify without oversimplifying.
Here’s the bottom line, though: the shape of the wing forces air moving around it to change direction and move downward. By Newton’s 3rd law (equal and opposite reactions), that means the air pushes the wing up, thereby creating lift. (Video credit: Sixty Symbols)
everyonelikespotatissallad asks:
so, how is lift actually generated? i’ve been going through Anderson’s Introduction to Flight (6th Ed.) and while it offers the derivation of various equations very thoroughly, it barely touches on why lift is generated, or how camber contributes to the increase of C(L)
This is a really good question to ask. There are a lot of different explanations for lift out there (and some of the common ones are incorrect). The main thing to know is that a difference in pressure across the wing–low pressure over the top and higher pressure below–creates the net upward force we call lift. It’s when you ask why there’s a pressure difference across the wing that explanations tend to start diverging. To be clear, aerodynamicists don’t disagree about what produces lift – we just tend to argue about which physical explanation (as opposed to just doing the math) makes the most sense. So here are a couple of options:
Newton’s third law states that for every action there is an equal and opposite reaction. If you look at flow over an airfoil, air approaching the airfoil is angled upward, and the air leaving the aifoil is angled downward. In order to change the direction of the air’s flow, the airfoil must have exerted a downward force on the air. By Newton’s third law, this means the air also exerted an upward force–lift–on the airfoil.
The downward force a wing exerts on the air becomes especially obvious when you actually watch the air after a plane passes:
This one can be harder to understand. Circulation is a quantity related to vorticity, and it has to do with how the direction of velocity changes around a closed curve. Circulation creates lift (which I discuss in some more detail here.) How does an airfoil create circulation, though? When an airfoil starts at rest, there is no vorticity and no circulation. As you see in the video above, as soon as the airfoil moves, it generates a starting vortex. In order for the total circulation to remain zero, this means that the airfoil must carry with it a second, oppositely rotating vortex. For an airfoil moving right to left, that carried vortex will spin clockwise, imparting a larger velocity to air flowing over the top of the wing and slowing down the air that moves under the wing. From Bernoulli’s principle, we know that faster moving air has a lower pressure, so this explains why the air pressure is lower over the top of the wing.
Asymmetric Flow and Bernoulli’s Principle
There are two basic types of airfoils – symmetric ones (like the one in the first picture above) and asymmetric, or cambered, airfoils (like the one in the image immediately above this). Symmetric airfoils only generate lift when at an angle of attack. Otherwise, the flow around them is symmetric and there’s no pressure difference and no lift. Cambered airfoils, by virtue of their asymmetry, can generate lift at zero angle of attack. Their variations in curvature cause air flowing around them to experience different forces, which in turn causes differing pressures along the top and the bottom of the airfoil surface. A fluid particle that travels over the upper surface encounters a large radius of curvature, which strongly accelerates the fluid and creates fast, low-pressure flow. Air moving across the bottom surface experiences a lesser curvature, does not accelerate as much, and, therefore, remains slower and at a higher pressure compared to the upper surface.
(Image credit: M. Belisle/Wikimedia; National Geographic/BBC2; O. Cleynen/Wikimedia; video credit: J. Capecelatro et al.)
At very small scales, the interaction of solids and liquids is governed by molecular forces. Here researchers demonstrate how carbon nanowires of only a few nanometers in diameter draw liquid up in a film or bead when inserted in a pool. Capillary action is the name we give this gravity-defying force generated between the liquid and solid molecules. Although this behavior was predicted theoretically, it had not been previously observed at this scale due to the need for electron microscopy. Such microscopes require a vacuum, which boils off almost any liquid instantaneously. Researchers used a special fluid that remained in a liquid state even under near-vacuum pressures in order to make these observations. (Video credit: J. Li et al/MIT News; submitted by 20percentvitaminc)
In this still image from a student experiment, smoke visualization shows the formation of a vortex over the wing of a paper airplane during a wind tunnel test. This wing vortex is mirrored on the opposite wing, though there is no smoke to show it. At high angle of attack, the delta-wing shape of the traditional paper air plane creates these vortices on the upper surface, which helps generate the lift necessary to keep the plane aloft. (Photo credit: A. Lindholdt, R. Frausing, C. Rechter, and S. Rytman)
doughboy3-deactivated20120305 asks:
I’m a Undergrad Aeronautical Engineering student. I’m curious as to your opinion as to how airfoils produce lift. I know the usual theory told in this situation. However my aerodynamics professor says that there are many things going on during the flow around an airfoil. I’m hoping to get a better idea of the different mechanisms responsible for lift.
There’s a common misconception of Bernoulli’s principle that’s often used to explain how an airfoil creates lift (which I assume is the “usual theory” to which you refer), and while there are many correct (or, perhaps, more correct) ways of explaining lift on an airfoil, I think the only opinions involved are as to which explanation is best. After all, opinions don’t keep a plane in the air, physics does!
I tackled the air-travels-farther-over-the-top misconception and presented one of my preferred ways of looking at the situation in a previous post; in short, the airfoil’s shape causes a downward deflection of the flow, which, by Newton’s 3rd law, indicates that the air has exerted an upward force on the airfoil. There’s a similar useful video from Cambridge on the topic here.
Another explanation I have heard used concerns circulation and its ability to produce lift (see the Kutta-Joukowski theorem for the math). In this case, it’s almost easier to think about lift on a cylinder instead of lift on a more complicated shape like an airfoil. If you spin a cylinder, you’ll find that the circulation around that object results in a force perpendicular to the flow direction. This is called the Magnus effect and, in addition to explaining why soccer balls sometimes curve strangely when kicked, has been used to steer rotor ships. One of my undergrad aero professors used to do a demonstration where he’d wrap a string around a long cardboard cylinder and demonstrate how, by pulling the string, the cylinder’s spinning produced lift, making the cylinder fly up off the lectern and attack the unsuspecting students.
An airfoil doesn’t spin, but its shape produces the same type of circulation in the flow field. Without delving into the mathematics, it’s actually possible through conformal mapping and the Joukowski transform to show that the potential flow field around a spinning cylinder is identical to that around a simple airfoil shape! Although that mathematical technique is not all that useful in a world where we can calculate the inviscid flow around complicated airfoils exactly, it’s still pretty stunning that we can analytically solve potential flow around (and thus estimate lift for) a host of airfoil shapes on the back of an envelope.
In short, your aerodynamics professor is right in saying that there are many things going on during the flow around an airfoil. If you get a roomful of aerodynamicists together and ask them to explain how airfoils generate lift, you would be faced with a lively discussion with about as many competing explanations as there are participants. As you learn more in your classes, you’ll gain a better intuitive feel for how it works and you’ll learn more of the nuances, which will help you understand why there is no one simple-to-understand explanation that we use!**
** Lest I confuse someone into thinking that aerodynamicists don’t know how airfoils produce lift, let me add that the argument here is over how best to explain the production of lift, not over how the lift is produced. We have the equations to describe the flow and we can solve them. We know that lift is there and why. We simply like to argue over how to explain it to people without all the math.
xkcd identifies a very common misconception about how airfoils work! (via Vinnchan and jasonas14) #
