Is wingtip design mature?

Question

There are many wingtip design, all made to reduce vortex related drag, and neither boeing nor airbus seems to prefer one over another. They move from one design to another at each new aircraft, always claiming the new choice is more efficient than the previous one. In detailed, all generation mixed, it looks like (non exhaustive list):

winglet: B737NG, A330, B747-400, A320NEO, A350

wingtip fence: A320, A380, B737MAX

raked wingtip: B787

EDIT: The other elements of aircraft design are almost the same for all new aircraft (low wing, engines under the wing, classical landing gear, APU in the tail, ...), so I assume it has reached a great level of maturity, nothing has to be improved and introduced on a new aircraft generation. This is obviously not the case for wingtip.

Answer 1

The wingtips are a unique part of the aircraft in that they really have one job; minimize drag. Their design is easy to see, and they are a fairly recent addition to transport aircraft. Developments continue to be made in the study of aerodynamics, with modern computing power and methods allowing much better modeling of flow patterns to evaluate designs, and improvements in materials allowing more complex designs to be built.

The key is that the winglet must be properly designed. As wing designs change, the flow over the wings changes, and new winglet designs are invented.

Since it is heavily dependent on the specific application, it's more relevant to look at the evolution of the designs on different aircraft. The design chosen is dependent on the technology available at the time, both for evaluating the design and building it, the design of the wing, intended use of the aircraft, and the previous experience of the manufacturer.

The 737 is an old design that has evolved over time: Original/Classic: none, NG: winglet, P-8: raked tip, MAX: split scimitar

The A330 is a fairly new design: Current: small winglet, neo: winglet

The 747 has also evolved over time, and the latest version shares technology with the 787: Original: none, -400: small winglet, -8: raked tip

The A320 has also evolved over time: Original: none, Current: tip fence, neo: winglet

The A380 is a new design: tip fence

The A350 is a new design: winglet

The 787 is a new design: raked tip

The 777 is a fairly new design: Basic: none, -200LR and -300ER: raked tips

You will notice a trend from smaller designs to larger ones. On larger aircraft though, the weight of a large winglet and the loads it produces can get too large. New designs like the 777, A380, 787, and 747-8 all use either raked tips or small winglets.

As for other parts of aircraft, engines are changing more than you think. The jet engines on the early jet transports look very different from modern high-bypass turbofans. The GEnx engines on the 787 and 747-8 have chevrons at the back to reduce noise. All you see on the outside is the nacelle that minimizes drag; most of the changes go on inside. Also, if you look closely at the front, fan blades on newer engines have curves in them instead of having straight edges.

Most other parts of the aircraft have more complex jobs, not primarily external. A fuselage should be efficient at carrying things inside but also keep drag low. If you look closely you will notice that new planes like the 787 and A350 have a lot in common with previous fuselages, but they are shaped differently as well. It's harder to see the other drastic changes like the use of composites in place of traditional aluminum, electric systems instead of hydraulic or bleed air, fly by wire, etc.

Control surfaces must provide sufficient control while minimizing drag. The size and shape has also changed over time, though the general design has remained.

Answer 2

I'm not an aeronautical engineer, so this answer may contain errors.

The design of an aircraft pretty much is the design of a wing. The fact that wingtip designs change is only due to the fact that wing design changes with every aircraft type as well.

Fuselage design is mature in the sense that circular is better than square. Engine design is mature in the sense that engines are interchangeable; as long as you've got the correct hardpoints on the wing for fuel, bleed air, and so on, it doesn't matter whether you choose to install the Wilbur 400 turbofan or the Orville 602N turbofan.

But the wing is integral to the aircraft. For each airliner you design, you design a wing to go with it. Center of gravity, center of pressure, critical mach, all of these things and lots more are taken into account and each are different for each aircraft, so they have to be addressed differently. The fact that wingtips differ from model to model only means that wings differ from model to model.

Answer 3

It really doesn't matter that much.

Look at birds: They use two different wingtip designs. I guess we will both agree that those designs are mature after millions of years of evolution. And still there are two distinct wing tip shapes: One is the "fingered" wingtip with large, spread-out feathers, and one is the pointed tip you find from seagulls to albatrosses, and also in swallows. Why is there not one, mature design?

This has to do with the environment they are used in. Seabirds and swallows fly in an unobstructed environment with steady winds and need to stay aloft for longer duration. All other birds, also lake-dwellers like Pelicans, have to cope with trees obstructing a straight flight path and gusts from hills, or those trees. They need to maneuver quickly and cope with gusty winds. This is helped by a reduced span and the possibility to fold the outer wing in or fan it out in an instance. Hence the fingered wingtip.

For most aircraft, the seabird solution should work best. But sometimes the environment adds new demands, like span restrictions in the competition classes of gliders. Then it makes sense to add a winglet.

Another "restriction" is the high cost of developing a new wing, not to speak of the tooling and certification effort. Then it helps to add a winglet so the outer wing can contribute more lift if the aircraft mass is increased by stretching beyond the capabilities of the original wing. But this will drive up wing bending moments disproportionally, so the overall benefit is small. It will always be better for performance to design a new wing with a straight wingtip.

If you see all kinds of fancy winglets on modern airliners, this has to do more with marketing than with aerodynamics. After all, no bird has winglets. Not a single one.

But now picture the Airbus board (full of bean counters and lawyers, and void of aerodynamicists) after Boeing has hyped up their new wingleted wing: The immediate reaction is: We need this wonder device, too! But it must look different! Sadly, this has led to the quasi-religious following of the winglet cult, where winglets are made to be the cure-all of aerodynamic problems.

Now one can argue that nature never came around to try the winglet modification. Not in more than 100 million years, not in one of several independent developments of wings (pterodactyls, insects, birds and bats. You can even add fish to that list, their flippers have a similar function). This could be, but now the question comes down to Occam's Razor: Which is more likely?

• Nature never developed winglets, despite their advantages? And please don't call the elastic deformation of tip feathers winglets. This is plain silly.
• Or Nasa and Boeing marketing have hyped up a device which does improve aerodynamics in some conditions, but is not as good as a straight wing extension if the conditions allow for that extension?

Judge for yourself.