There are many
which influence the amount of aerodynamic
which a body generates. Drag depends
on the shape,
inclination, of the object,
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 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
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.
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.
For a wing, the total
drag coefficient, Cd
is equal to the base drag coefficient at zero lift Cdo
plus the induced drag coefficient Cdi.
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
You can further investigate the effect of induced drag and the other
factors affecting drag by using the
FoilSim III Java Applet.
You can also
your own copy of FoilSim to play with
- Beginner's Guide Home Page