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)
Nectar-drinking species of hummingbirds and bats are both excellent at hovering – one of the toughest aerodynamic feats – but they each have their own ways of doing it. Hummingbirds (bottom) use a nearly horizontal stroke pattern that’s quite symmetric on both the up- and downstroke. To keep generating lift in the upstroke, they twist their wings strongly midway through the stroke. So although hummingbirds get most of their lift from the downstroke, they get quite a bit from the upstroke as well.
Bats, on the other hand, use an asymmetric wingbeat pattern when hovering. Bats flap in a diagonal stroke pattern, using a high angle of attack in the downstroke and an even higher one during the upstroke. They also retract their wings partially during the upstroke. This flapping pattern gives them weak lift during the upstroke, which they compensate for with a stronger downstroke. Compared to non-hovering bat species, nectar-drinking bats do get more lift during the upstroke, but they’re nowhere near as good as the hummingbirds. The bats compensate by having much larger wings compared to their body size. Bigger wings mean more lift.
In the end, the two types of hovering cost roughly the same amount of power per gram of body weight. That’s great news for engineers designing the next generation of flapping robots because it suggests two very different, but equally power-efficient methods for hovering. (Image credit: Lentink Lab/Science News, source; research credit: R. Ingersoll et al.; via Science News; submitted by Kam Yung-Soh)
Spend a summer afternoon floating in a kayak and chances are you’ll see some impressive aerial acrobatics from dragonflies. One of the dragonfly’s superpowers is its ability to fly backwards, which helps it evade predators and take-off from almost any orientation. To do this, the dragonfly rotates its body so that it is nearly vertical, thereby changing the direction it generates lift. In engineering terms, this is “force-vectoring,” similar to the techniques used by helicopters and vertical-take-off jets.
Scientists found that backwards-flying dragonflies could generate forces two to three times their body weight, in part due to the strong leading-edge vortices (bottom image) formed on the forewings. They also found that the hind wings are timed so that their lift is enhanced by catching the trailing vortex of the first pair of wings. Engineers hope to use what they’re learning from insect flight to build more capable flying robots. (Image and research credit: A. Bode-Oke et al., source; via Science)
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)
Mosquitoes are unusual fliers. Their wings are long and skinny, and they beat at around 700 strokes a second – incredibly quickly for their size. Examining how they move has uncovered some interesting mechanics. Despite their short stroke length, the mosquito generates a lot of lift on both its upstroke (when the wing is moving backward) and its downstroke (when the wing moves forward). Some features of the mosquito’s flight are highlighted in the images above. In the animation, blue indicates areas of low pressure and red indicates high pressure.
Like most flapping fliers, the mosquito generates a leading-edge vortex during its downstroke (and its upstroke). This vortex helps concentrate low pressure on the upward-facing wing surface, thereby creating lift. One of the things that makes the mosquito unique, however, is that it also creates trailing-edge vortices on both half-strokes. To do this, the mosquito rotates its wings precisely to catch the wake of its previous half-stroke. The flow gets trapped near the trailing edge of the wing and forms a vortex and low-pressure region. Like the leading-edge vortex, this low-pressure area on the upward-facing wing surface creates lift. For more secrets of mosquito flight, check out this video from Science or the original paper. (Image credit: R. Bomphrey et al., source)
My question involves “fenestrated rudders”, a Chinese invention that
involved cutting diamond-shaped holes in the rudders of ancient Chinese
sailing ships (known as Junks). According to several articles (on the
internet, ha ha), it reduces the amount of effort required to steer the
ship at higher speeds with “no loss of function”. All I can find is
anecdotal evidence and I’d like to know if these claims hold water or if
they’re just steering us in the wrong direction.
First off:
Now, I’m no expert on ships or sailing, but let’s talk rudders. Ships use rudders for steering. The rudder is completely submerged and turning it deflects water and creates a side force that helps steer a boat. In essence, it’s an underwater wing that generates lift in the side-to-side direction. Modern rudders even have the same shape as airfoils. That’s clearly not the case with the rudders of Chinese junks, but flat plates are a lot easier to make.
There’s another key feature of the junk’s rudder, and that’s the way it’s mounted. The junk’s rudder attaches to the ship such that it rotates about its leading edge. This makes it an unbalanced rudder. More modern rudders are typically mounted so that they rotate around an axis that’s partway back on the rudder. This is called a balanced rudder; I’ve illustrated both below.
The advantage of the balanced rudder is that it’s easier to turn. You can see this for yourself without adding water into the equation. Imagine holding a big rectangular sheet. If you hold it by one edge and try to rotate it, you can do it, but it’s kind of difficult. If you instead hold it about a third of the way across, you’ll find rotating it easier. Once you have a fluid moving past, it will only magnify how hard it is to turn the rudder.
So the Chinese junks had rudders that were harder to handle (by later ship-building standards) to begin with. By putting holes in the rudder, they equalized the pressure on either face of the rudder. That does make it easier to steer, since the helmsman is no longer fighting pressure differences across the rudder, but it would also reduce steering efficiency. It’s likely, however, given the slow speed of the junks, large rudder area, and their low hydrodynamic efficiency to begin with, that any drop in efficiency was negligible compared to the reduction in force necessary to steer.
Since modern designs rely on foil shapes to generate pressure differences (and therefore side force) across the rudder, adding holes to them would be a bad idea. But back in the Song dynasty, the fenestrated rudder was major advance in nautical engineering!
(Image credits: Chinese junk ship model – Premier Ship Models; Joffrey applauding – HBO; Rudder diagram – N. Sharp)
Both synchronized swimming and water polo require competitors to hold themselves stable above the water’s surface without touching the pool’s bottom. One of the basic techniques for doing so in both sports is known as the eggbeater kick, shown above. The eggbeater kick is very similar to the motion for the breaststroke’s kick, but it’s performed upright and with alternating leg motions, sweeping a clockwise circle with the left leg and a counterclockwise one with the right.
A swimmer typically stays afloat due to a buoyant force equal to the weight of the volume of water the swimmer displaces. Rising further out the water means reducing the buoyant force, so the swimmer must generate other forces to counter their weight. The eggbeater kick does this two ways. First, as the swimmer sweeps their foot around, it acts like a hydrofoil, generating lift that holds the swimmer up. Second, other parts of the kick cycle force water downward, which, by Newton’s third law, pushes the swimmer up.
Keeping a wide stance and sweeping the legs alternately allows the athlete to balance the horizontal forces their motions create while keeping the upward forces generated relatively constant. This gives them a stable, arms-free platform that’s a foundation for everything else their sport requires. (Image credits: GettyImages; The Studio WLV, source)
Like many sharks, the great hammerhead shark is negatively buoyant, meaning that, absent other forces, it would sink in water. To compensate, sharks generate lift with their pectoral (side) fins to offset their weight. Their dorsal (top) fin is used to generate the horizontal forces needed for control and turning. However, both captive and wild great hammerhead sharks tend to swim rolled partway onto their sides. The reason for this unusual behavior is hydrodynamic – it is more efficient for the shark. Unlike other species, the great hammerhead has a dorsal fin that is longer than its pectoral fins. By tipping sideways, the shark effectively creates a larger lifting span and is able to induce less drag than when it swims upright. Models show that swimming on their sides requires ~8% less energy than swimming upright! (Image credit: N. Payne et al., source)
In flight, airplane wings produce dramatic wingtip vortices. These vortices reduce the amount of lift a 3D wing produces relative to a 2D one. How much they influence the lift depends on both the strength and proximity of the vortex. The stronger and closer it is, the more detrimental its effect. One way airplane designers reduce the effects of wingtip vortices is by adding an extra section, called a winglet, to the end of the wing. Among other effects, the winglet moves the wingtip vortex further away from the main wing, which reduces its influence and allows the airplane to regain some of the lift that would otherwise be lost. (Image credits: A. Wielandt et al., source)
Evolution often requires compromise between competing effects. Large-eared bats, for example, rely on the size of their ears to aid their echolocation, but such large ears can hurt them aerodynamically, thus limiting their flight. Results from a recent experiment, however, suggest that large ears are not a total loss aerodynamically speaking. Researchers used particle image velocimetry to study the wakes behind free-flying, large-eared bats and found significant downward flow behind the bats’ bodies. This indicates that the bats generate some lift with their ears, body, and/or tail. The position and tilt of the ears in flight is similar to forward swept wings, which the authors suggest could help contract the wake behind the ears and reduce its negative influence on flow over the wings. Although the evidence is not yet conclusive, the study does suggest that large ears may be more aerodynamically beneficial than they appear. (Image credit: L. Johansson et al./Lund University, source; via Jalopnik)
The next FYFD webcast will be this Saturday, May 21st at 1pm EDT. My guests will be Professor Jean Hertzberg of the University of Colorado at Boulder and Professor Kate Goodman of the University of Colorado at Denver. Dr. Hertzberg is the creator of the course Flow Visualization, an interdisciplinary course combining engineering, art, and fluid dynamics. It’s a class (and website) that’s been an inspiration for me and FYFD since the early days! Dr. Goodman, an expert in engineering education, earned her PhD studying the Flow Viz course and its impact. This will be wide-ranging discussion – with everything from experimental fluid dynamics and engineering education to art, photography, and hopefully even cardiac fluid dynamics!
