This replica of the Wright brothers’ 1902 glider demonstrates one of the important innovations the brothers contributed toward powered heavier-than-air flight. To control an aircraft in roll, the Wright brothers developed the idea of wing-warping. The pilot would lie in the cradle (center of image) and shift his body to one side. A system of wires and pulleys would then twist the wings from their rear edge, pulling one side down and the other up. This deflection is akin to changing the wing’s angle of attack. Deflecting the right wingtip downward increased the lift on the right side of the glider, while simultaneously the upward deflection on the left decreased the lift on that side. This causes the glider to bank, or roll, with the right wing up, thereby generating a leftward turn. The lift differential also caused a drag differential, though, with increased drag on the lifted (right, in this case) wing. That extra drag tended to pull the aircraft’s nose rightward, a condition known as adverse yaw. To counter it, the Wright brothers installed a steerable rudder and linked it to the wing-warping mechanism, allowing them to turn with much less effort than other conventional craft. Although wing-warping has been replaced with ailerons, the control principles remain the same. For more, watch this demo of the wing warping mechanism on a 1903 Wright Flyer replica. (Image credit: C. Devers)
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Brazuca
Since 2006, Adidas has unveiled a new football design for each FIFA World Cup. This year’s ball, the Brazuca, is the first 6-panel ball and features glued panels instead of stitched ones. It also has a grippy surface covered in tiny nubs. Wind tunnel tests indicate the Brazuca experiences less drag than other recent low-panel-number footballs as well as less drag than a conventional 32-panel ball. Its stability and trajectory in flight are also more similar to a conventional ball than other recent World Cup balls, particularly the infamous Jabulani of the 2010 World Cup. The Brazuca’s similar flight performance relative to a conventional ball is likely due to its rough surface. Like the many stitched seams of a conventional football, the nubs on the Brazuca help trip flow around the ball to turbulence, much like dimples on a golf ball. Because the roughness is uniformly distributed, this transition is likely to happen simultaneously on all sides of the ball. Contrast this with a smooth, 8-panel football like the Jabulani; with fewer seams to trip flow on the ball, transition is uneven, causing a pressure imbalance across the ball that makes it change its trajectory. For more, be sure to check out the Brazuca articles at National Geographic and Popular Mechanics, as well as the original research article. (Photo credit: D. Karmann; research credit: S. Hong and T. Asai)
Hawk in Flight
For a little more than century, mankind has taken flight in fixed-wing aircraft. But other species have flown for much longer using flapping techniques, the details of which humans are still unraveling. To really appreciate flapping flight, it helps to have high-speed video, like this beautiful footage of a goshawk attacking a water balloon. The motion of the hawk’s wings is far more complex than the simple up and down flapping we imitate as children. On the downstroke, the wings and tail stretch to their fullest, providing as large an area as possible for lift. During steady flight, the bird flaps while almost horizontal for minimal drag, but as it approaches its target, it rears back, allowing the downstroke to both lift and slow the bird. In the upstroke, the bird needs to avoid generating negative lift by pushing air upward. To do this, it pulls its wings in and simultaneously rotates them back and up. Its tail feathers are also pulled in but to a lesser extent. Leaving them partially spread probably maintains some positive lift and provides stability. At the end of the upstroke, the hawk’s wings are ready to stretch again, and so the cycle continues. (Video credit: Earth Unplugged/BBC; h/t to io9)
When Turbulence Is Desirable
One of the common themes in aerodynamics, especially in sports applications, is that tripping the flow to turbulence can decrease drag compared to maintaining laminar flow. This seems counterintuitive, but only because part of the story is missing. When a fluid flows around a complex shape, there are actually three options: laminar, turbulent, or separated flow. An object’s shape creates pressure forces on the surrounding fluid flow, in some cases causing an increasing, or unfavorable, pressure gradient. When this occurs, fluid, especially the slower-moving fluid near a surface, can struggle to continue flowing in the streamwise flow direction. Consider the video above, in which the flow moves from left to right. Near the surface a turbulent boundary layer is visible, where fluid motion is significantly slower and more random. Occasionally the flow even reverses direction and billows up off the surface. This is separation. Unlike laminar boundary layers, turbulent boundary layers can better resist and recover from flow separation. This is ultimately what makes them preferable when dealing with the aerodynamics of complex objects. (Video credit: A. Hoque)
Sochi 2014: Speed Skiing
As FYFD wraps up coverage of #Sochi2014, let’s take a look at a winter sport not currently contested at the Olympics. This year’s Winter Games featured 12 new events. Speed skiing was not among them, though it was a demonstration sport in the 1992 Olympics. Like many of the sports in Sochi, speed skiing is gravity-driven, and friction and drag serve only to slow competitors. Speed skiing is about getting from the top of the course to the bottom, in a straight line, as fast as possible. Athletes reach velocities as high as 250 kph (155 mph), and aerodynamics are of the utmost concern. The skiers’ rubberized speed suits include airfoil-shaped fairings behind their calves that mold the airflow, and athletes wear giant aerodynamic helmets to smooth flow over their heads and shoulders. They spend their entire descent in an aerodynamic tuck, arms extended ahead of them like a cyclist in a time trial. It looks a pretty crazy ride. Would you like to see it added to the Olympics? (Video credit: R. Sill/University of Cambridge)
FYFD is celebrating #Sochi2014 with a look at fluid dynamics in winter sports. Check out the previous poss on why ice is slippery, the aerodynamics of speedskating, and how lugers slide fast.
Sochi 2014: Bobsledding
Today bobsledding is an sport rife with modern technology and design techniques. In recent years, companies better known for their expertise in automobiles and Formula 1 racing have become players with BMW designing American sleds, McLaren making the UK sleds, and Ferrari providing for the Italian team. Like many winter gravity sports, contenders can be separated by as little as hundredths of a second. This makes aerodynamics a serious concern, but the variability of the sled’s position and orientation over a run makes realistically simulating the aerodynamics, either in a wind tunnel or computationally, extremely difficult. Additionally, the sport’s governing body restricts a sled’s dimensions, weight, shape, and other details; for example, bobsleds are not allowed to use vortex generators that would help maintain attached flow and reduce drag. Instead, designers try to shave drag elsewhere, in the shaping of the sled’s nose or by tweaking the back end of the sled to reduce the drag-inducing wake. Even the shape of the driver’s helmet is aerodynamically significant. (Image credits: Exa Corp, Getty Images, BMW)
FYFD is celebrating #Sochi2014 by looking at fluid dynamics in winter sports. Check out our previous posts on how skiers glide, the US speedskating suit controversy, and why ice is slippery.
Sochi 2014: Downhill Skiing
Like the athletes who compete on ice, skiers rely on a film of liquid beneath their skis to provide the low friction necessary to glide. The moisture results from the friction of the ski’s base and edges cutting into the snow, and, depending on the conditions of the snow, different surface treatments are recommended for the skis to help control and direct this lubricating film. Similarly, skiers uses various waxes on their skis to lower surface tension and provide additional lubrication. Fluid dynamics can also play a role in tactics for various ski-based events. In endurance events like cross-country skiing, drafting behind other skiers can help an athlete avoid drag and save energy. When drafting, cross-country skiers have lower heart rates. Drag and aerodynamics can also play a significant roles in alpine skiing, especially in speed events like the downhill or super G. In these events solo skiers reach speeds of 125 kph, where drag is a major factor in slowing their descent. Between turns smart skiers will tuck, decreasing their frontal area and reducing drag’s effects. Athletes use wind tunnel testing to dial in their tuck position for maximum effect, and, like speedskaters, skiers may also wear special aerodynamic suits. (Photo credits: F. Cofferini/AFP/Getty Images, C. Onerati; h/t to @YvesDubief)
Sochi 2014: Speedskating Redux
Since we wrote about the US team’s speedskating suits last week, they have become the subject of major controversy. After six events, the US team had not placed higher than seventh despite strong World Cup results during the autumn. The Wall Street Journal reported that three people familiar with the team suggested a design flaw:
Vents on back of the suit, designed to allow heat to escape, are also allowing air to enter and create drag that keeps skaters from staying in the low position they need to achieve maximum speed, these people said. One skater said team members felt they were fighting the suit to maintain correct form. #
To address this, some members had seamstresses sew fabric over the vent. The upper left image shows the original suit and the one on the right shows a team member in a modified suit. The change made no apparent impact on the skaters’ finish. The US team has no gone so far as to get a special dispensation to switch back to their older suits but still the podium eluded skaters in Saturday’s events.
Now, to be clear, I have not seen any data on the development of Under Armour’s suits beyond the public coverage, and I have no connections to any of the parties involved. However, given the extensive nature of the wind tunnel development that went into these suits, I would be exceptionally surprised if there was a design flaw capable of slowing skaters down by nearly 1 second over 1000 meters. It would require a major flaw in the testing design and methodology to overlook such a substantial drag effect.
At the same time, there are other factors that may be affecting the US team adversely. Sochi’s races are taking place at low altitudes, where the air is denser and drag is greater. This does affect all competitors, but it is worth noting that many of the US speedskaters train at altitude in Salt Lake City and that the entire team had their training camp at high altitude in Italy prior to Sochi. Another factor is the ice conditions. Salt Lake has what is considered fast ice that permits longer glides between each step, whereas Sochi has soft ice, which requires a faster tempo and does not glide as easily. (Image credits: Under Armour, Getty Images, P. Semansky/AP)
Sochi 2014: Ski Jump, Part 2
Yesterday we talked about the technique ski jumpers use to fly farther. Generating lift without too much drag is the key to a good jump. But jumpers are subject to ever-changing wind conditions, and those can help or hurt them. Unlike most sports, in ski jumping a headwind is desirable. This is because the added relative air velocity increases the jumper’s lift and helps them fly farther. A tailwind, on the other hand, saps their speed. Since 2009, ski jumping competitions have included a wind compensation factor that tries to account for these effects. Wind velocity is measured at five points along the jumper’s flight path and the tangential (i.e. head- or tailwind) components are weighted and averaged. The weighting factors seem to be individual to each hill – not all hills are built with the same profile. This average tangential wind speed is then a linear variable in an equation for wind factor. The goal of the wind factor is as much to make the competition run smoothly as it is to increase fairness. The trouble is that the wind speed effect is non-linear; in other words, a headwind does not help a jumper as much as a tailwind can hurt them. In one simulation study, researchers found a 3 m/s headwind carried jumpers 17.4 m further while a tailwind of the same magnitude shortened the jump by 29.1 m. The wind differences in competition may not be as drastic, but truly evening the playing field may require a more complicated compensation system. (Photo credit: B. Martin/Sports Illustrated)
FYFD is celebrating the Games with a look at fluid dynamics in the Winter Olympics. Check out our previous posts on the aerodynamics of speed skating, why ice is slippery and how lugers slide so fast.
Sochi 2014: Ski Jump
Great ski jumpers are masters of aerodynamics. There are four main parts to a jump: the in-run, take-off, flight, and landing. An athlete’s aerodynamics are most vital in the in-run and, naturally, the flight. During the in-run, the athlete is trying to gain as much speed as possible, so she tucks down and pulls her arms behind her back to streamline her body and keep her frontal area as small as possible. This limits her drag so that she can maximize her speed at take-off. Once in the air, though, the jumpers act like gliders. In flight, there are three forces acting on the the jumper: gravity, lift, and drag. Gravity pulls the jumper down, and drag tends to push her backwards up the hill, but lift, by counteracting gravity, helps keep jumpers aloft for a greater distance. To maximize lift, a jumper angles her skis outward in a V and holds her arms out from her sides. This configuration turns the jumper’s body and skis into a wing. The best jumpers will tweak their positions with training jumps and wind tunnel time to maximize their lift while minimizing their drag in flight and on the in-run. Technique is critical in ski jumping, but conditions play a significant role as well. Tomorrow’s post will discuss why and how judges account for changing conditions. (Photo credits: L. Baron/Bongarts/Getty Images; D. Lovetsky/AP; E. Bolte/USA Today)
FYFD is celebrating the Games with a look at fluid dynamics in the Winter Olympics. Check out our previous posts on the aerodynamics of speed skating, why ice is slippery and how lugers slide so fast.