Johng,

Of course an infinite span wing has no induced drag. I'd say we've pretty well covered that. No faulty assumption was being made, since I referred to a lifting, finite span wing. I am not too concerned about a type of wing that cannot be built. When I refer to spanwise flow, I mean at or very near the surface of the wing. I think this is the most common usage of the term. Spanwise flow at the surface of the wing is not required for induced drag to occur. If you can make a compelling case to the contrary, I would like to hear it. Obviously, if a tip vortex exists, there will be spanwise flow somewhere far from the wing. Yes, reducing the tip vortex strength will reduce induced drag. The only problem with that is that you can't reduce the tip vortex strength without reducing lift. Methods that reduce induced drag without reducing lift (significantly) do so by modifying the effect of the tip vortex on the wing, usually by shifting the tip vortices out, or increasing the vortex span. This is why it pains me to hear people attribute induced drag to the tip vortex, rather than the downwash. It leads to this fallacy that tip vortices can be somehow impeded or reduced, which can really only happen by reducing lift.

banktoturn