wingtip-vortex

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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Wingtip vortices are the result of high-pressure air from beneath a wing sneaking around the end of the wing to the low-pressure area on top. They trail for long distances behind aircraft, and are, most of the time, an invisible hazard for other aircraft. If you’ve ever sat in a line of airplanes waiting to take off and wondered why there is so much time between subsequent take-offs, wingtip vortices are the answer. The larger a plane, the stronger its vortices are and the greater their effect on a smaller craft. Much of the time between planes taking off (or landing) is to allow the vortices to dissipate so that subsequent aircraft don’t encounter the wake turbulence of their predecessor. Crossing the wake of another plane can cause an unexpected roll that pilots may not be able to safely correct, a factor that’s contributed to major crashes in the past. (Image credits: flugsnug, source video; submitted by entropy-perturbation)

Smoke released from the end of a test blade shows the helical pattern of a tip vortex from a horizontal-axis wind turbine. Like airplane wings, wind turbine blades generate a vortex in their wake, and the vortices from each blade can interact downstream as seen in this video. These intricate wakes complicate wind turbine placement for wind farms. A turbine located downstream of one of its fellows not only has a decreased power output but also has higher fatigue loads than the upstream neighbor. In other words, the downstream turbine produces less power and will wear out sooner. Researchers visualize, measure, and simulate turbine wakes and their interactions to find ways of maximizing the wind power generated. (Photo credit: National Renewable Energy Laboratory)

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)