Maple tree seeds flutter and spin as they descend. The above video, which shows flow visualization of a freely falling seed, demonstrates that the so-called helicopter seed’s autorotation creates a vortex along the leading edge. Watch as the seed’s “wing” sweeps through and you will notice the vortex along the upper surface. This leading edge vortex generates high lift on the maple seed, allowing it to stay in the air more effectively than other seeds, thereby increasing the maple’s reproductive range. (Video credit: D. Lentink et al.; see also Supplemental Materials)
Tag: lift
Stalling a Wing
At small angles of attack, air flows smoothly around an airfoil, providing lifting force through the difference in pressure across the top and bottom of the airfoil. As the angle of attack increases, the lift produced by the airfoil increases as well but only to a point. Increasing the angle of attack also increases the adverse pressure gradient on the latter half of the top surface, visible here as an increasingly thick bright area. Over this part of the surface, the pressure is increasing from low to high–the opposite of the direction a fluid prefers to flow. Eventually, this pressure gradient grows strong enough that the flow separates from the airfoil, creating a recirculating bubble of air along much of the top surface. When this happens, the lift produced by the airfoil drops dramatically; this is known as stall.
Micro Air Vehicle Flow Viz
A smoke wire shows the deformation of streamlines around a swept-winged micro air vehicle (MAV). These crafts typically feature wingspans smaller than one foot and, thus, never develop the type of flow fields associated with larger fixed-wing airplanes. This complicates theoretical predictions of lift and drag for MAVs as well as making them difficult to control. MAVs have numerous commercial and military applications, including search and rescue operations. (Photo credit: Tom Omer)
Vortices on an Airliner
Wingtip vortices form on airplanes due to the finite length of their wings. In general, lift on the wings results from low-pressure, high-velocity air moving over the top of the wing and high-pressure, low-velocity air moving below the wing. Near the wingtips, the high-pressure air is able to slip around the edge to the top of the wing, generating a vortex that then trails behind the airplane. The same thing is occurring in the video above, except the edges of the wing’s control surfaces are serving as the tip of the wing. Similar vortices also exist at the wingtips, but they are not made visible by condensation as the aileron vortices are.
Wind Tunneling Testing for BASE Jumpers
While we usually think of wind tunnel testing airplane models, the truth is that wind tunnels today test a much wider array of subjects. From oil rigs and skyscrapers to athletes and police sirens, if you can imagine it, it’s probably been stuck in a wind tunnel. This video shows some wind tunnel testing of a tracking suit used for BASE jumping. The primary focus seems to be on lift and drag at angle of attack–which can be used to determine glide ratios for the pilot–but there is also some study of localized turbulence generation, as evidenced by the use of smoke generators and the streamers attached to the suit’s arms and legs. (submitted by Jason C)
Circulation Around an Airfoil
As a followup to yesterday’s question about ways to explain lift on an airfoil, here’s a video that explains where the circulation around the airfoil comes from and why the velocity over the top of the wing is greater than the velocity around the bottom. Kelvin’s theorem says that the circulation within a material contour remains constant for all time for an inviscid fluid. Before the airplane moves, the circulation around the wing is zero because nothing is moving. As shown in the video, as soon as the plane moves forward, a starting vortex is shed off the airfoil. As the plane flies, our material contour must still contain the starting position and thus the starting vortex. However, in order to keep the overall circulation in the contour zero, the airfoil carries a vortex that rotates counter to the starting vortex. This is the mechanism that accelerates the air over the top of the wing and slows the air around the bottom. Now we can apply Bernoulli’s principle and say that the faster moving air over the top of the airfoil has a lower pressure than the slower moving air along the bottom, thus generating an upward force on the airfoil. (submitted by jessecaps)
Reader Question: How Airfoils Produce Lift
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.
Dove in Flight
This spectacular high-speed video shows a dove in flight. Note how its wings flex through its stroke and the way the wings rotate over the course of the downstroke and reversal. There is incredible beauty and complexity in this motion. The change in wing shape and angle of attack is what allows the bird to maximize the lift it generates. Note also how the outer feathers flare during the downstroke. This promotes turbulence in the air moving near the wing, which prevents separated flow that would cause the dove to stall. (See also: how owls stay silent. Video credit: W. Hoebink and X. van der Sar, Vliegkunstenaars project)
Skydiving Indoors
Vertical wind tunnels like this one simulate the experience of skydiving with air speeds up to 270 km/h (168 mph). Here expert freefallers perform a routine similar to synchronized skydiving. By changing the angle and shape of their body with respect to the air flow, they are able to control their lift and drag to produce complex motion in three dimensions.
Stall-Sensing Hairs
Bats use tiny hairs on their wings to sense the direction and speed of air flow. Researchers found that removing these hairs caused bats to fly faster and make wider turns, likely because the bat believed it was on the verge of stalling and losing lift. Engineers are considering whether artificial versions made of flexible polymers that respond to strain could provide improved stall sensing on fixed-wing aircraft. # (Photo credit: justynk)
