Montague,

I'm not armed with wind tunnel data here, but I don't think that a rounded wing tip is especially high drag, and certainly not because of the wingtip vortex. First, this would imply that induced drag is a big component of the total drag, which is usually not the case. Second, it would imply that the wingtip shape has a big effect on induced drag, which is also not usually the case. Profile drag is likely to be a more important source of drag, and rounding the wingtip will help a little bit with that. If induced drag really is an issue, it would be silly to worry about the wingtip shape until one has increased the aspect ratio as much as is practical, and made sure the the lift distribution is approximately elliptical. These two measures are much more important for induced drag than anything done at the wingtip.

There is a widely held view that induced drag is caused by the wingtip vortex. This is not really true. Induced drag is caused by the downwash, which is due to the fact that the wing is generating lift. While the downwash can be viewed as a vortex that has its centerline near the wingtip, the air that spills over the wingtip is pretty much incidental. The reduction in induced drag that some well designed wingtips can give is due to their effect on the distribution of downwash along the span of the wing. In particular, you cannot assume that any wingtip that discourages the spilling of air around the wingtip is reducing induced drag.

banktoturn