The Winglets is a curved aerofoil part on the leading edge of the wing, which are turned in an up direction. The angle of incidence of the wing tip is higher than wing root at the leading edge. The Winglets are used to increase the span of the wing, which effectively increases the Aspect Ratio of the wing
(Aspect Ratio = Span ÷ Chord).
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It decrease wing tip vortices caused by a normal wingtip and reduces higher drag caused by them.
Wing Tip Vortices forms as follows-
At the top of the wing the airflow of high velocity and low pressure moves inwards
At the bottom of the wing the airflow of comparatively low velocity and high pressure move outwards.
The airflow of high pressure at the bottom of the wing moves outward and gets concentrated at the wingtips.
The airflow of low pressure and high velocity at the top of the wing moves outwards and meets the high pressure airflow from the bottom of the wing, at the wingtip.
These two types of airflow when meet at the wingtip they create a rotating motion of air called vortices or Wing Tip Vortices.
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Further strength to these vortices are given by the Sweepback Wing design A/c as follows-
The airflow on the Sweepback wing gets divided into two components one flows in the Span wise direction and the other flows in the Chord wise direction. This airflow in Span wise direction moves outward to meet the vortices and further strengthen the vortices.
These vortices are stronger at slow speed flights.
They can be said to be Directly Proportional to the angle of the Attack Or Inversely Proportional to the airspeed of the A/c.
The Winglet increases the Aspect Ratio of the wing, making it enable for slow speed flight handling, reduces the drag, therefore decrease fuel consumption and increases the range of the A/c.
The Winglets are very crucial at speeds below Minimum Drag Speed.