OK, I went back and read thru the posts, and have figured out that I disagree with both Ken and banktoturn. [sm=eek.gif] The physical cause of induced drag is the pressure differential at the wing tip. This causes the vortex flow, with both spanwise flow and downwash being the result. Niether is the physical driver or cause of the phenomena.

Now on to the quotes:

I can't see the relevance of this distinction at all. OK, so say the surface layer of the air has "a boundary condition on the wing which precludes spanwise flow". So what? A molecues' distance above this you have spanewise flow, and all your condition might have done is increased the thickness of the boundary layer. That has no real effect on the spanwise flow of the mass of air being influenced by the wing.

Hmm, better let the wind tunnels know. I've seen plenty of real 2d wings built for wind tunnels - that experience real aerodynamic forces. Plenty of lift, no tip vortex circulation, no induced drag.

That's just silly. Obviously you can reduce vortex strength to zero and maintain lift - as in the 2-d wind tunnel wing ( or airfoil to be techinically correct). Wingtip shapes are just an attempt to emulate this on aircraft. That wingtip shape will not necessarily reduce lift, in fact it will increase it if designed well. I have a design report somewhere on my bookshelf including the design of winglets onto a wing. The winglets were designed to both reduce induced drag and improve lift distribution - and thus overall lift of the wing at the same angle of attack. -Computed with CFD and proven in flight. Your statement would have me believe that the downwash and vortex strength are preserved in this case. I do not.

I've had enough. I'll leave further replies to others.