Why is a wing tip's front edge tapered?
The swept-back wingtip 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 (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. Picture by Makaristos.

Boeing has now introduced this detail as the "raked wingtip" in the 787 and the 747-8. Compared to a straight wing, a swept wingtip offers some 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.
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.


In order to prevent the outer wing from stalling first, you want to give it a lower lift coefficient, which can be achieved by locally adding chord over what is required for an elliptical wing planform and a lower local incidence. However, doing so will still add lift proportional to the local chord when the angle of attack is increased. Now the wing will show the ideal elliptic circulation distribution only for one angle of attack. By adding sweep to the outer wing, its lift curve slope is reduced, so its off-design circulation distribution is closer to the elliptic ideal.