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The wing tips are very different on these airplanes. The wingtip on a Boeing 787 is a sharp triangle, while the wingtip on a Boeing 737-300 is flat.

British Airways Boeing 787-8 G-ZBJC wingtip | by StephenG88

British Airways Boeing 787-8 G-ZBJC wingtip | by StephenG88 (source)

Why is the wingtip on the Boeing 787 different from that of the Boeing 737?

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  • $\begingroup$ I promise you guys a picture of what I mean but my laptop has not been working correctly $\endgroup$
    – Ethan
    Aug 19, 2015 at 10:04
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    $\begingroup$ possible duplicate of What do winglets do to increase aircraft performance? $\endgroup$
    – Simon
    Aug 19, 2015 at 10:14
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    $\begingroup$ @Simon: No, Ethan refers to what is called a raked wingtip. Works better than a winglet, too. $\endgroup$
    – Peter Kämpf
    Aug 19, 2015 at 11:23
  • $\begingroup$ @Simon I have to agree with Peter Kämpf, raked wings work differently than winglets (even though they do similar things), so this is not a duplicate of that particular question. And none of the answers really dive into how a raked wings works so... Yeah, not a duplicate. $\endgroup$
    – Jay Carr
    Aug 19, 2015 at 13:16
  • $\begingroup$ @PeterKämpf At the time of my comment, the picture was not in the post and there was only mention of a "triangle" which I took to mean a sharklet. Got it now so removed my vote to close. $\endgroup$
    – Simon
    Aug 19, 2015 at 14:53

2 Answers 2

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The raked wingtip of the Boeing 787 is now "le denier cri" in wingtip design, but has been successfully employed by water fowl for many millennia. The use of composite structures has made the three-dimensional shaping of wingtips much easier, and the first airplanes to use increased sweep at the wingtip were indeed composite gliders such as the Schempp-Hirth Discus.

Seagull (left) Schempp-Hirth Discus 2

Seagull (left) Schempp-Hirth Discus 2 (right, source)

In the times of metal wings, Dornier introduced their TNT (Tragflügel Neuer Technologie, in English: Wing of new technology) on aircraft like the Do 228, Do 328 and the Dornier ATT flying boat. It used a triangular wingtip with no change in trailing edge sweep, which is easier to manufacture in aluminium.

An Air Alps Dornier 328-110 landing at Fiumicino Airport, Rome, Italy, viewed from Ostia Antica

An Air Alps Dornier 328-110 landing at Fiumicino Airport, Rome, Italy, viewed from Ostia Antica. Picture by Makaristos.

You can think of the raked wingtip as of a winglet which has been folded down. Increasing the wingspan will involve a bigger mass of air into the creation of lift, and the induced drag is reduced. This explains already most of the effect, but the increased sweep adds some more benefits.

  • Higher sweep lowers the lift curve slope. This helps to reduce the lift contribution (and, consequently, the bending moment!) of the wingtip at high angles of attack. Therefore, raking the wingtip will result in a lower maximum bending moment.
  • The sweep also shifts the center of pressure of the wingtip backwards, adding a negative torsion moment. This moment lowers the angle of attack over the whole outer wing in gusts and at high load factors, again helping to lower the maximum bending moment at the root.
  • Especially in gliders, it allows to combine a near-elliptical circulation distribution over a wide range of angles of attack with good stall characteristics. The lower lift curve slope of the wingtip lets it stall at a higher angle of attack and reduces its lift contribution relative to its area at increasing angle of attack.

This aeroelastic tailoring, together with the low fuel consumption of modern high-bypass-ratio engines, allows the Boeing 787 to use a wing of an aspect ratio of 10 and the A350 one of up to 11.8. Previously, - apart from gliders - only special designs for extreme flying time and range like the B-24 Liberator or the Breguet Atlantique sported such high aspect ratios. Older airliner designs had to use lower aspect ratios (between 6 and 8.5) to have enough wing volume for fuel.

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The purpose of the raked wingtip is same as that of the winglet- to reduce drag by reducing the effect of the wingtip vortices.

Boeing 767-600 Raked Wingtip

Source: www.boeing.com

Most aircrafts use winglets to improve performance. Basically, they reduce the wingtip vortex by [deflecting the air](and the associated drag by deflecting the air that wants to escape over the tip back down) that wants to escape over the tip back down. This reduces the induced drag.

In case of raked wingtips, the tips of the wings have more sweep compared to the rest of the wing.

Boeing 787 with raked wingtips "B787-800v1.0" by Julien.scavini - Own work. Licensed under CC BY-SA 3.0 via Commons.

The raked wingtips also reduce the wingtip vortex- by redirecting the wingtip vortices further outboard and aft of the wing.

The raked wingtips are more efficient compared to the winglets in cruise and are used in ultra long range aircrafts.

Comparison of drag reduction due to wingtip devices

Source: www.boeing.com

One problem with the raked wingtips is that they increase the span. This increases the bending moment. Another reason we are not seeing a number of aircrafts with raked wingtips is due to the fact that they can't be retrofitted (Though there was a proposal to retrofit KC135 with raked wingtips).

Note: The photo is not very clear. I'd simply answered about the raked wingtip.

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    $\begingroup$ I am afraid I will never understand how a vortex behind the wing can cause drag. $\endgroup$
    – Peter Kämpf
    Aug 19, 2015 at 12:19
  • $\begingroup$ This is due to the three dimensional nature of the flow; if the flow was in two dimensions (i.e. around an airfoil), there would be no wingtip vortices and no induced drag. $\endgroup$
    – aeroalias
    Aug 19, 2015 at 12:36
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    $\begingroup$ @aeroalias, no, Peter is right. The vortices can't cause drag. They have the same cause as induced drag, but they are not the cause themselves. $\endgroup$
    – Jan Hudec
    Aug 19, 2015 at 12:56
  • $\begingroup$ The wingtip vortices cause the downwash, which reduces the effective angle of attack of the wing. This effectively tilts the lift vector backwards, causing a downstream facing force (drag) on the wing. This drag is called the induced drag as it has been induced by the action of tip vortices. $\endgroup$
    – aeroalias
    Aug 19, 2015 at 13:09
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    $\begingroup$ Yes, and wet streets cause rain. Everybody knows this -it's obvious! $\endgroup$
    – Peter Kämpf
    Aug 22, 2015 at 4:16

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