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Winglets provide a lift boost that would be useful to a plane of this size. But the An-255 does not have any winglets.

Why is it so?

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    $\begingroup$ Winglets are less efficient than a longer wing - if you cannot increase wingspan then you use winglets (or you use winglets if you want your plane to look "cool" - which is part of marketing) $\endgroup$ – slebetman Jul 8 at 0:38
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    $\begingroup$ Because Russian and Ukrainian aerodynamicists prefer pragmatism and don't give a s**t about marketing. $\endgroup$ – Peter Kämpf Jul 8 at 3:57
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Winglets are less effective at producing added lift than an equal-length wingspan extension. They are used on airliners when simply enlarging the wing would put the plane into a larger size class, or outside the 80 m "box". A longer wingspan, which the An-225 has (10% over the limit), is better at producing lift.

The An-225 is a special purpose cargo aircraft for delivering very large loads such as the Buran space shuttle. It doesn't have to care for commercial airport traffic patterns meant for high-throughput operations - servicing thousands of planes separated into fixed size groups.

In its original role the An-225 would operate from airstrips purpose-built for it, and seeing little other traffic. Today, on the rare (or not so rare, but its most public deliveries are to remote destinations) occasion when it has a commercial airport in its itinerary, they can deal with the extra requirements of a somewhat larger plane.

Think of it like a wide load truck: they can go outside the 102"+3" limit at the expense of pre-planning the route. It's always a special delivery with these planes.

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    $\begingroup$ So you're saying winglets aren't to be cooler, they're to fit in airport gates. $\endgroup$ – Harper Jul 7 at 21:53
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    $\begingroup$ The winglet was a way to improve the wing's efficiency without adding span either because of a dimensional limitation as Therac mentioned, or because you didn't want to add to the wing's bending by extending the tip. A pure winglet stands straight up and all of its lift is horizontal - inboard and slightly forward. You get a net thrust force from the forward component of the lift vector and you get an "outwash", a sideways downwash, that opposes the tip circulation. The circulation has to be strong to make it worth it and only really works on airplanes that cruise "slowly" at high altitude. $\endgroup$ – John K Jul 8 at 4:04
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    $\begingroup$ Is it really rare for An-225s to visit commercial airports? I'd have thought they'd do it almost all the time -- where else would one land? Airport terminals care about the exact wingspan of the planes because they have to park next to each other. But cargo planes don't use the passenger terminal. $\endgroup$ – David Richerby Jul 8 at 12:46
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    $\begingroup$ Ultimately there is only one AN-225 and it doesn't fly that much (indeed it's not clear at all if it's flying at the moment, the latest evidence I could find with my google searches was from 2018). So even if all it's landings were at commercial airports they would still be a tiny tiny proportion of total landings at those airports. $\endgroup$ – Peter Green Jul 8 at 15:23
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    $\begingroup$ @Harper Not only gates, also runways, taxiways etc. $\endgroup$ – yo' Jul 9 at 9:33
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Winglets were just beginning flight tests at NASA with a modified KC-135 when the AN-225 was designed. No one at the Antonov Design Bureau could have known at the time the benefits of using them.

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    $\begingroup$ Winglets have been used on corporate aircraft like the Lear 28 and Gulfstream III since the late 1970’s. The AN-225 first flight was 1988 and could have been designed with winglets, but instead has a large wingspan which eliminates any need for winglets. $\endgroup$ – Mike Sowsun Jul 7 at 17:07
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    $\begingroup$ @JuanJimenez The biggest reason to use winglets is to get extra lift or less drag within the same size group. If you don't care how much parking space your plane takes - and if it's the An-225, of which only a few were meant to exist, you don't care - you'll choose more wingspan every time. $\endgroup$ – Therac Jul 8 at 0:49
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    $\begingroup$ I like your assumption that NASA are the only organization in the world which knows about aerodynamics :) $\endgroup$ – alephzero Jul 8 at 11:16
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    $\begingroup$ The Tu-204 flew for the first time less than a month (2-01-1989) after the first flight of the An-225 (21-12-1988), so the knowledge was very much there, don't you think? $\endgroup$ – AEhere Jul 8 at 12:28
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    $\begingroup$ Well, your answer is speculative regarding what Antonov knew or not, so I guess speculation is on topic? The An-225 development ran 1984-1988, around the same years as the Tu-204 (1985-1989) according to the Russian wikipedia, so they are directly comparable: same tube-and-wings config, same country of origin, built by engineers likely educated on the same source material, etc $\endgroup$ – AEhere Jul 8 at 13:18

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