ORIGINAL: KenLitko
The physical cause of induced drag is the pressure differential at the wing tip.
That's what I'm saying. Not just at the tip, but the pressure differential between the bottom and the top of the wing, which causes fluid it to flow around the tip. High pressure on the bottom moves fluid outboard, low pressure on the top moves fluid inboard... which is spanwise flow. This spanwise flow "rolls up" into the tip vortex.
Ken,
I just visited a web page, compliments of Irumd, which contains the following interesting text:
* Winglets, etc.
It is a common misconception that the wingtip vortices are somehow associated with unnecessary spanwise flow, and that they can be eliminated using fences, winglets, et cetera. The reality is that the vortices are completely necessary; you cannot produce lift without producing vortices. By fiddling with the shape of the wing the designer can control where along the span the vortices are shed, but there is no way to get rid of the vorticity without getting rid of the lift.
The URL is: [link]http://www.av8n.com/how/htm/airfoils.html#sec-flow-intro[/link]
banktoturn