ORIGINAL: KenLitko
Banktoturn,
We're saying the same thing in two different ways. I'm saying that spanwise flow causes vortices, vortices cause downwash, downwash results in (effectively) a backward-tilted lift vector. The upward component is the actual lift, the backward component is the induced drag.
I'm going to the root cause of induced drag, your saying that it strictly a product of downwash.
The fact is that by modifying the spanwise flow on the wing (with engineered wingtips), you can modify this downwash field. In so doing, you can modify the induced drag (hopefully lowering it, and not getting more parasite drag in echange due to the size/wetted-area of the engineered tip).
KenLitko,
I think it is useful to go to the root cause, but I don't think we are saying the same thing. Spanwise flow is not the root cause of induced drag. Lift is the root cause of induced drag (the term induced drag is short for lift induced drag). If you could build a wing with no spanwise flow, it would still suffer from induced drag whenever generating lift. One reason that it is useful to go to the root cause of a phenomenon is so that one can rationally determine how to design around it. In particular, it is not valid to choose a wing tip with the idea of making it harder for flow to spill from the bottom surface to the top, at least not for the purpose of reducing induced drag. If you could show, or reason, that modifying the spanwise flow near the tip could favorably modify the downwash distribution, that would be a valid approach, but it is not the same thing as trying to impede the spill from the bottom to the top. This rationale seems to stem from the notion that the tip vortex causes drag, so if you can prevent the tip vortex, you can prevent the drag. This notion is invalid, much like the commonly heard explanation that entropy is disorder. Unfortunately, it seems to be almost as widely believed.
banktoturn