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Are there any positive advantages or catastrophic effects on adding winglets to an aircraft not designed with winglets originally?

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  • $\begingroup$ kind of related: aviation.stackexchange.com/q/31551/1467 aviation.stackexchange.com/q/8556/1467 $\endgroup$ – Federico May 15 '17 at 12:59
  • $\begingroup$ All B-737's coming off the production line now have winglets, although for the longest time, the B-737 didn't have winglets. So there are plenty of those jets flying around now that had them retrofitted, which is exactly what you're asking as far as "adding winglets to an aircraft not designed with winglets originally." The advantage to adding the winglets was in reduced fuel burn; there were (obviously) no catastrophic effects in adding them, and some fairly minor engineering issues that were addressed. $\endgroup$ – Ralph J May 15 '17 at 19:34
  • $\begingroup$ I heard a story about the A320, that because of production tolerances (and possibly the uncertainty of the calculations) they made the original wing spar much stronger than necessary. As such, there is quite some reserve in the strength of the spar, and they could add the winglets without replacing the wing. In essence, this means that the strength to support the winglets was provided by having smaller strength margins. $\endgroup$ – ROIMaison May 16 '17 at 8:13
  • $\begingroup$ @RalphJ That looks like most of a decent answer. $\endgroup$ – KorvinStarmast May 16 '17 at 20:21
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Aircraft wings are designed for specific weight and cruise speed. When later the manufacturer wants to create a new variant with higher maximum weight, it should get longer wings for optimal efficiency. However, redesigning the wing is a lot of work, because extending the wing means the root bending moment increases, which requires stronger spar, which means heavier and all the calculations have to be redone.

Here is where winglets get useful. They increase the effective wingspan, but while they are somewhat less efficient than extending the wing, they also cause lower increase of the root bending moment. Often, this smaller increase fits in the original extra margin, so winglets can be simply bolted on without having to redesign the wing, making the process much cheaper.

That's why we often see winglets on next generation of anything and rarely on the first generation. On first generation, they are designing the wing from scratch anyway, so they just make it appropriate length for minimum drag.

The main reason to use winglets from the start is if the emerging design just slightly exceeds some size category for airport gates. Then it makes sense to reduce the span to fit and add winglets so the aircraft can be used at smaller—and therefore cheaper and more common—gates.

You can read how winglets work in Is a winglet better than an equal span extension?.

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  • $\begingroup$ Isn't the "winglets not on the first generation" thing more the result of those first generations being designed before the benefits of winglets were appreciated (NASA research around 1980), and the increasing cost of fuel made retrofitting them cost-effective? $\endgroup$ – jamesqf May 22 '17 at 4:12
  • $\begingroup$ @jamesqf, no, because designs created after 1980 either don't have them (B777, B787), or have only small ones (A330, A380). $\endgroup$ – Jan Hudec May 22 '17 at 6:04
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I helped install winglets on 4 B757's a few years ago, I cannot remember the exact figures, but by fitting the winglets we improved the fuel burn for flights over 3000 miles by about 8%, over less than 2000 miles the fuel burn actually increased and made the aircraft less efficient by approximately 4%. So it can sometimes be a trade off.

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