Raked Wingtips

[MK940]

Just wondering how these Raked Wingtips actually work. I have research a few sites but can't seem to come up with anything. I understand how winglets work but can't seem to find any explanations on these Raked Wingtips(B767-400).
Look forward to reading your answers or reference to a particular site.
Thanks in advance. ;)

[Old Aero Guy]

If you undestand how a winglet works, then just think of a raked tip as a winglet laid flat.
They're both doing the same job, i.e. increasing the effective wing span to reduce induced drag.
The raked tip is a bit more effective for a given increase in wetted area and it doesn't carry it's load through an angled joint.
If your airplane is span constrained, then a winglet might be a better choice.

You can read about raked tips at the following Web Site (cuttently offline for an update.)

http://www.clarkson.edu/~visser/research/wingtips/

[MK940]

Thanks for your contribution OAG. I'll check that site out too. What do you mean by wetted area. I have never heard of such an expression. Can you elaborate a bit more?

[Old Aero Guy]

Wetted area is a shorthand way of describing all the surface area of a piece of airplane structure exposed to airflow.
If you think about the hydrodynamic roots of aerodynamics, it would correspond to area of a ship's hull below the water line.

In the case of a winglet or raked tip, the added wetted area would be the sum of the areas of upper and lower surfaces and the tip fairing.
The wetted area includes the curvature of the surface, not just the projected planform area.
As you might suspect, wetted area is directly related to the skin friction drag of the component.