FYFD

Tag: lift

  • Reader Question: Winglets

    Reader Question: Winglets

    Reader tvargo writes:

    First off… love your blog! I know very little about physics, but love reading about it. Could you potentially explain what the little upturned ends of wings do? looking on wikipedia is see this: “There are several types of wingtip devices, and although they function in different manners, the intended effect is always to reduce the aircraft’s drag by partial recovery of the tip vortex energy.” huh?

    Thanks! That’s a great question. Winglets are very common, especially on commercial airliners. To understand what they do, it’s helpful to first think about a winglet-less airplane wing. Each section of the wing produces lift. For a uniform, infinite wing, the lift produced at each spanwise location would be the same. In reality, though, wings are finite and wingtip vortices at their ends distort the flow. The vortices’ upward flow around the ends of the wing reduces the lift produced at the wing’s outermost sections, making the finite wing less efficient (though obviously more practical) than an infinite wing.

    Adding a winglet modifies the end conditions, both by redirecting the wingtip vortices away from the underside of the wing and by reducing the strength of the vortex. Both actions cause the winglet-equipped wing to produce more lift near the outboard ends than a wing without winglets.

    But why, you might ask, does the Wikipedia explanation talk about reducing drag? Since a finite wing produces less lift than an infinite one, finite wings must be flown at a higher angle of attack to produce equivalent lift. Increasing the angle of attack also increases drag on the wing. (If you’ve ever stuck a tilted hand out a car window at speed, then you’re familiar with this effect.) Because the winglet recovers some of the lift that would otherwise be lost, it allows the wing to be flown at a lower angle of attack, thereby reducing the drag. Thus, overall, adding winglets improves a wing’s efficiency. (Photo credit: C. Castro)

  • Hummingbird Hovering

    Hummingbird Hovering

    The hummingbird has long been admired for its ability to hover in flight. The key to this behavior is the bird’s capability to produce lift on both its downstroke and its upstroke. The animation above shows a simulation of hovering hummingbird. The kinematics of the bird’s flapping–the figure-8 motion and the twist of the wings through each cycle–are based on high-speed video of actual hummingbirds. These data were then used to construct a digital model of a hummingbird, about which scientists simulated airflow. About 70% of the lift each cycle is generated by the downstroke, much of it coming from the leading-edge vortex that develops on the wing. The remainder of the lift is creating during the upstroke as the bird pulls its wings back. During this part of the cycle, the flexible hummingbird twists its wings to a very high angle of attack, which is necessary to generate and maintain a leading-edge vortex on the upstroke. The full-scale animation is here. (Image credit: J. Song et al.; via Wired; submitted by averagegrdy)

  • Wingtip Vortices

    Wingtip Vortices

    Newton’s third law says that forces come in equal and opposite pairs. This means that when air exerts lift on an airplane, the airplane also exerts a downward force on the air. This is clear in the image above, which shows a an A380 prototype launched through a wall of smoke. When the model passes, air is pushed downward. The finite size of the wings also generates dramatic wingtip vortices. The high pressure air on the underside of the wings tries to slip around the wingtip to the upper surface, where the local pressure is low. This generates the spiraling vortices, which can be a significant hazard to other nearby aircraft. They are also detrimental to the airplane’s lift because they reduce the downwash of air. Most commercial aircraft today mitigate these effects using winglets which weaken the vortices’ effects. (Image credit: Nat. Geo./BBC2)

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    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)

  • Sochi 2014: Ski Jump, Part 2

    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

    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.

  • Dynamic Stall

    Dynamic Stall

    In nature, birds and other flying animals often use unsteady flow effects to enhance the lift their wings generate. When a wing sits at a high angle of attack, it stalls; the flow separates from the upper surface, and its lift force is suddenly lost. If, on the other hand, that wing is in motion and pitching upward, lift is maintained to a much higher angle of attack. The reason for this is shown in the flow visualization above. This montage shows a rectangular plate pitching upwards. Flow is left to right. Each row represents a specific angle of attack and each column shows a different spanwise location on the plate. As the plate pitches upward, a vortex forms and grows on the leading edge of the plate. Eventually, the leading-edge vortex separates, but not until a much higher angle of attack than the plate could sustain statically. This effect allows birds to maintain lift during perching maneuvers and is also key to helicopter rotor dynamics. (Image credit: K. Granlund et al.)

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    Falcon vs. Raven

    Earth Unplugged has posted some great high-speed footage of a peregrine falcon and a raven in flight. Notice how both birds draw their wings inward and back on the upstroke. By doing so, they decrease their drag and thus the energy necessary for flapping. On the downstroke, they extend their wings fully and increase their angle of attack, creating not only lift but thrust. The falcon boasts an incredibly streamlined shape, not only along its body but also along its wings. In contrast, the raven has broader wings with large primary feathers that fan out near the tips. Splaying these large feathers out decreases the strength of the bird’s wingtip vortices, thereby reducing downwash and increasing lift, much the same way winglets do on planes. That extra lift and control the big primaries provide is important for the raven’s acrobatic skill. (Video credit: Earth Unplugged; via io9)

  • Flow Over a Delta Wing

    Flow Over a Delta Wing

    Fluorescent dye illuminated by laser light shows the formation and structure of vortices on a delta wing. A vortex rolls up along each leading edge, helping to generate lift on the triangular wing. As the vortices leave the wing, their structure becomes even more complicated, full of lacy wisps of vorticity that interact. Note how, by the right side of the photo, the vortices are beginning to draw closer together. This is an early part of the large-wavelength Crow instability. Much further downstream, the two vortices will reconnect and break down into a series of large rings. (Photo credit: G. Miller and C. Williamson)

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    Simulating a Curveball

    Spinning an object in motion through a fluid produces a lift force perpendicular to the spin axis. Known as the Magnus effect, this physics is behind the non-intuitive behavior of football’s corner kick, volleyball’s spike, golf’s slice, and baseball’s curveball. The simulation above shows a curveball during flight, with pressure distributions across the ball’s surface shown with colors. Red corresponds to high pressure and blue to low pressure. Because the ball is spinning forward, pressure forces are unequal between the top and bottom of the ball, with the bottom part of the baseball experiencing lower pressure. As with a wing in flight, this pressure difference between surfaces creates a force – for the curveball, downward. (Video credit: Tetra Research)