Aircraft burn fuel to produce thrust to overcome drag. It is easy to imagine the drag produced by the surface area of the total airframe. What you can't see is drag created because the wing is producing lift. When a wing is generating lift there is an area of high pressure under the wing and an area of low pressure above the wing. The imbalance between these two pressures creates the lift force that keeps the aircraft flying. However, there is also a flow of air from low pressure to high pressure around the wingtip which creates a swirl of air known as a vortex. The main vortex on a wing is shed from the wingtip, and you can sometimes see vortices when an aircraft is landing on a moist day.
When an aircraft is flying at speeds approaching the speed of sound the vortex drag represents up to 50% of the drag in climb and cruise. If we can reduce this drag we save fuel!
There are a few options open to the designer who wants to reduce vortex drag:
You will notice that the older A320’s and the A380 have a small wingtip fence. This modifies the lift distribution and is particularly useful when you cannot increase wingspan (eg for parking at airport gates).The Sharklet uses the second option, increasing the effective span for a small increase in wingspan and for a small increase in weight. Since we must burn fuel to carry weight, the savings generated by the Sharklet must exceed the increased fuel burn due to their weight. This is all a delicate balance, but after a few months of operation we can see that on an Auckland to Queenstown sector an aircraft fitted with Sharklets will burn about 120kg less fuel than one of our older A320s, a saving of almost 3%.