On Friday, March 4, 2016 at 1:14:43 PM UTC-8, Robert M wrote:

...if you are not flying absolutely in trim they will shed a vortex...

Robert raises some good points. The one thing that I'd add is that fences will probably shed a vortex even if you _are_ absolutely in trim. At issue is that airflow on the bottom surface will tend to have a slight outboard component, and that on the top a slight inboard component. These spanwise components will tend to be stronger nearer the tip.

Thanks, Bob K.