There are many factors which influence the amount of aerodynamic drag which a body generates. Drag depends on the shape, size, and inclination, of the object, and on flow conditions of the air passing the object. For a three dimensional wing, there is an additional component of drag, called induced drag, or drag due to lift. On modern airliners, the wing tips are often bent up to form winglets. Winglets were wind tunnel tested and computer analyzed by Richard Whitcomb of the NASA Langley Research Center in the mid 1970's.
Induced drag is a three dimensional effect related to the wing tips; induced drag is a wing tip effect. So if the wing tip represents only a small fraction of the total wing area, the induced drag will be small. Long thin wings have low induced drag. Wings with an elliptical planform also have lower induced drag than rectangular wings. For many years, wing designers have attempted to reduce the induced drag component by special shaping of the wing tips. The Wright Brothers used curved trailing edges on their rectangular wings based on wind tunnel results. The outstanding aerodynamic performance of the British Spitfire of World War II is partially attributable to its elliptic shaped wing which gave the aircraft a very low amount of induced drag.
The idea behind the winglet is to reduce the strength of the tip vortex and therefore cause the flow across the wing to be more two-dimensional. Flight tests at the NASA Dryden Flight Research Center have found a 6.5% reduction in the fuel use of a Boeing 707 type airliner when using winglets. Winglets must be carefully integrated into the total wing design, which explains why many different winglet designs appear on various airliners.
Cd = Cdo + Cdi
The drag coefficient in this equation uses the wing area for the reference area. Otherwise, we could not add it to the square of the lift coefficient, which is also based on the wing area
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