What does a winglet do, besides make an airplane look cool? They're known to increase performance, increasing range and decreasing fuel burn, but why?
Winglets oppose the drag wingtip vortices create by harnessing the vortices' airflow. NASA engineer Richard Whitcomb pioneered the technology in the 1970s, and they've become a fixture on almost every modern jet.
So how do they work?
First, you need to understand how wingtip vortices form and why they create drag. Then, you'll understand how winglets counter that drag with lift.
Wingtip Vortices: Spinning Air And Adding Drag
What are wingtip vortices? They're swirling tunnels of air that form on your wingtips. High-pressure air from the bottom of your wing escapes around the wingtip, moving up towards the lower pressure area on the top of the wing. This movement creates a vortex or tunnel of air, rotating inwards behind the wing.
They're strongest when the air pressure difference between the top and the bottom of the wing is the greatest - which happens when you're generating the most induced lift. This occurs when you're at high angles of attack.
During takeoff and landing, you're slow - so you're at a high angle of attack and generating strong wingtip vortices.
When you're cruising at high altitudes, like a jet in the flight levels, the air is thin. So, you need a high angle of attack to generate enough lift to stay level, even though you're moving fast. Your wingtip vortices are stronger here, too.
Why Do Wingtip Vortices Generate Drag?
Why do wingtip vortices generate drag? They actually angle your wings' lift backward, turning some of your lift into drag.
A wing generates lift perpendicular to the relative wind. If you didn't have wingtip vortices, lift would point nearly straight up.
However, the wingtip vortices curve up and around the wingtips, pushing the air flowing over the wing downward. That angles your relative wind downward and tilts the lift vector backward.
This causes two problems. First, some of your lift is now pointing backward, adding to drag. And second, you don't have as much lift pointing upward, countering weight. So, to maintain level flight, you need to increase your angle-of-attack and generate more lift. And generating more lift means you're generating more induced drag. This extra angle of attack you need is called the induced angle of attack.
How do winglets help?
Winglets Are Wings That Generate Forward Lift
Winglets are actually little wings that generate lift. And, just like any other wing, they generate lift perpendicular to the relative wind. If you didn't have wingtip vortices, the winglet would generate lift inward, which isn't very helpful.
But, wingtip vortices change the direction of the relative wind at the wingtip. Because the vortices move up and over the wing, they add a component of wind that flows toward the fuselage - bending the relative wind inward. Now, when you draw the lift vector from the winglet, the lift vector points forward a little.
It might not look like much, but that little bit of forward lift helps - it opposes the drag produced by the vortices when they're strong. So, when you're at a high angle of attack - like high altitude cruise or during takeoff and landing - winglets reduce your drag.
Blending Winglets Reduce Even More Drag
Older winglets attach to the wing at nearly a 90-degree angle, which generates interference drag.
Interference drag shows up anywhere you have tight angles. Airflow at these angles mixes and becomes turbulent, creating drag.
With composites and new manufacturing technology, you can now blend winglets into the wing, eliminating interference drag and making the winglets even more efficient.
And finally, here's a great video to help visualize wingtip vortices.
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