In 1970s, when the price of avition fuel was soring high, airlines and aircraft manufacturers have searched numerous ways to improve the operating efficiency of the aircrafts. Winglets have become the most feasable and affordable inventions to be adopted at that point of time.
In one liner we can say winglets help in reducing induced drag at the wingtips of the wings
Basics of Lift
Aircraft wings are so shaped that it creates negative pressure at the upper surface of the wings and positive pressure at the lower end of the wings, meaning the total pressure at the bottom surface of the wing is more providing lift to the aircraft. It is governed by the formula: P1*V1 = P2*V2, where P1, V1= pressure, velocity of air at the upper surface of the wings and P2, V2= pressure, velocity of air at the lower surface of the wings.
The real Problem
As we all know that fluid always flow from a high pressure area to a low pressure area and this is what which increases the induced drag at the wing tips.
The high presssure air tries to move upwards from the lower surface of the wings to the upper surface of the wings through the wing tip area in the normal wings, according to the above statement. Due to this, the high pressure wind acts in opposite direction at the wing tip by proving a downward force on the upper surface, at the wing tips by gradually moving up and ultimately forming a Vortex of air. Formation of these vortices causes induced drag.
Solution = Winglets!
These winglets are actually the extended part of the wingtips in a converging manner. Now when the Vortices are formed when air moves from high pressure area (lower surface) to the low pressure area (upper surface), these winglets help it to get raised a bit and changes the direction of the force applied by the air. With the help of this the force now applied by the vortices formed is in horizontal direction (perpendicular to the winglets), hence the induced drag is reduced and overall aerodyanamic efficiency increase.
The Truth !
The shapes and sizes of winglets, and the angles at which they are mounted with respect to the main wings, differ between the many types and sizes of aircraft produced but they all represent improved efficiency. Throughout the aviation industry, winglets are responsible for increased mileage rates of as much as 7%.
In a Nutshell
Dr. Richard Whitcomb in 1970s discovered this concept by observing the wings of birds which have a curl at the tips. By reducing wingtip drag or induced drag, fuel consumption goes down and range is extended. Aircraft manufacturers and makers of add-on winglets have also reported improved cruising speeds, time-to-climb rates, and higher operating altitudes.