How Winglets Cut Drag at the Wingtip
Winglets are upturned wingtips that weaken the swirling vortex at the tip of a wing, cutting induced drag. Here is how they work, where they came from and what savings are claimed.
In this article
A winglet is a small, near-vertical extension at the end of a wing. Its job is to weaken the whirling air that forms at the wingtip, which reduces a kind of drag called induced drag. Less drag means less fuel burned for the same flight, which is why many airliners now have them.
Where induced drag comes from
A wing makes lift because the air pressure under it is higher than above it. At the tip, the high-pressure air tries to curl around to the low-pressure side. This creates a spiralling vortex trailing behind each wingtip. According to Wikipedia's article on wingtip devices, the vortex forms as high-pressure air beneath the wing flows around the tip.
NASA Glenn's winglets page explains that flow near the tips produces these tip vortices, and that they strongly affect how much induced drag the wing makes. It gives the induced drag coefficient as Cdi = Cl² / (π × AR × e). Here AR is the aspect ratio, how long and thin the wing is, and e is an efficiency factor. A higher aspect ratio lowers induced drag; an elliptical wing has e = 1.0 and a rectangular one about 0.7.
Why not just make the wing longer?
A longer wing would indeed cut induced drag. But Wikipedia notes it would also add drag from the extra surface and need a stronger, heavier wing, so the benefit eventually disappears. Airport gate widths can also limit wingspan. A winglet raises the effective height of the wing without adding much span.
NASA Glenn puts the idea simply: the idea behind the winglet is to reduce the strength of the tip vortex.
The history
Early flat end-plates did not help overall, because the extra drag from their surface outweighed what they saved, according to Wikipedia. The breakthrough came in the 1970s. NASA Langley engineer Richard Whitcomb tested and analysed winglets in the mid-1970s using wind tunnels and computer methods, per NASA Glenn. Wikipedia links this work to the 1973 oil crisis and says carefully designed near-vertical winglets gave a net drag reduction. Whitcomb's own biography on Wikipedia says he reported improvements of about 5 percent and notes it took nearly three decades for his proposals to become commonplace.
Flight tests followed. Wikipedia says NASA and the US Air Force tested the designs on a KC-135 in 1979 and 1980. NASA Glenn says NASA flight tests found a 6.5 percent reduction in fuel use for a Boeing 707-type airliner fitted with winglets. Learjet fitted the first production winglets in 1977 according to Wikipedia, which reports a range increase of about 6.5 percent, so the dates and figures in different accounts do not all line up.
Kinds of winglet
- Canted winglets angle upward from the wing, as on the Boeing 747-400, which Wikipedia says gained about 3.5 percent in range.
- Blended winglets join the wing in a smooth curve to reduce interference drag. Wikipedia credits the Boeing 737 version with about 4 percent fuel savings on long flights.
- Sharklets are Airbus's blended winglets for the A320 family, which Wikipedia says are claimed to cut fuel burn by up to 4 percent on longer routes.
- Raked wingtips sweep back at the tip rather than turning up. Wikipedia reports tests of up to 5.5 percent drag reduction, but notes they can worsen flutter.
Trade-offs
Winglets add weight and cost. Wikipedia says Boeing's 747-400D left them off on short-haul routes where the benefit is small. NASA Glenn also cautions that winglets must be integrated into the whole wing design, which is why so many shapes exist. In its student wind tunnel tests on a scale Cessna 172 model, some winglets had little effect and some even increased drag because of their added surface area.
Wikipedia adds two side benefits: reduced take-off noise and weaker wake vortices behind the aircraft. These depend on the design.
The short version
Winglets do not add lift. They reduce the energy wasted in the tip vortex. The same job could be done by a longer wing, but a winglet does it within a fixed span. The result, a few percent less fuel on suitable routes, adds up over thousands of flights, which explains why the idea moved from NASA test aircraft to everyday airliners. For specific aircraft, the manufacturer's own figures are the best guide.
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