Why do some airplanes have the wing tips folded up at about 80 degrees and some planes do not?
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These are winglets and they reduce drag for a given wingspan. The most efficient wing is very long like a glider’s but at some point on commercial jets that becomes a problem especially for fitting into airport gates. The winglet mimics a longer wing by reducing the strength of the wingtip vortex, and so reduces drag.
A wing provides lift because it creates low pressure on top of the wing, and higher pressure below. As a side effect this causes the air to swirl around the tip of the wing in an effort to equalize the pressure as air on the high pressure side tries to reach the low pressure side. This is undesirable because it screws up the airflow around the end of the wing, and also creates dangerous vortexes that trail behind the aircraft. One way of minimizing this problem is by using “winglets” that serve as a fence to block the air under the wing from being able to curl around the end. This makes the wing work more efficiently and reduces those vortexes.
Wing tips create a vortex in the air which increases drag reducing fuel efficiency. By bending the wing tips up it reduces the vortexing, which improves efficiency.
A new plane Boeing 777x the tips fold up which helps reduce wing span on the ground allowing it to operate at smaller airports.
Sooo, why some planes still don’t have their wing tips folded?
Someone I know had then installed on his rather fancy private jet about 4 years ago. They have already more than paid for themselves in fuel savings.
The air flow around the wing tip of an airplane is extremely complex.
The original vertical winglets designed in the late 80’s and early 90’s weren’t very good. They saved fuel but they added weight. They barely paid for themselves. I would say these winglets were more for cosmetics and made the planes look more cool.
The newer vertical winglets worked much better and do save fuel.
However, more recently, Boeing has decided that they have a better solution than a vertical wing tip. They use a rake wing wing tip. Take a look at the 787 and 777-9. They believe that these are more fuel efficient.
[https://www.flickr.com/photos/27889738@N07/6714985457](https://www.flickr.com/photos/27889738@N07/6714985457)
[https://d3lcr32v2pp4l1.cloudfront.net/Pictures/780xany/0/9/3/67093_777xv77wcomparison_250019.jpg](https://d3lcr32v2pp4l1.cloudfront.net/Pictures/780xany/0/9/3/67093_777xv77wcomparison_250019.jpg)
Basically, the air under the wing wants to escape toward the low pressure air above the wing. The problem gets bigger when your wings are swept back as the air has more time to do it.
A simple solution is to make the wing longer in the initial design.
However, if the plane already exists and you want to make it better, you can’t simply strap one meter of extra wing. The wing is already designed and any extra load you put on the top will act as a lever and stress the wing too much. On some planes the added dimension is also a problem for parking.
You can retrofit a vertical obstacle to the escaping air. It doesn’t extend the wing width so it doesn’t it add “lever” stress. In theory, you want it to face downward, but this could touch the ground in certain difficult landings. So there is the option to make it go up. If it goes up you need a bigger addition to compensate the reduced effect.
Another solution is to make a small addition down, using all the clearance you have toward the ground, and add a second one on top to improve the effect.
Last, to reduce drag you better reduce sharp intersections to the minimum. So some are made with a wide radius curve at base, and then go straight later. (Surfaces meeting at 90degrees do make vortexes, that’s also why you see the wing root intersecting the fuselage with a smooth flare instead of going straight into the fuselage)
This cover more or less all the shapes you see.
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