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Vortex generators are mounted on the top side of wings, and are designed to create small vortices as air passes over the wing.
As a result of these small vortices, vortex generators (Let's call them VGs, OK?) are responsible for doing some pretty amazing things. Concerning VGs, Wikipedia says the following:
"Vortex generators are likely to be found on the external surfaces of vehicles where flow separation is a potential problem because vortex generators delay flow separation.  On aircraft they are installed on the front third of a wing in order to maintain steady airflow over the control surfaces at the rear of the wing.boundary layer, and run in spanwise lines near the thickest part of the wing. They can be seen on the wings and vertical tails of many airliners. Vortex generators are positioned in such a way that they have an angle of attack with respect to the local airflow. They are typically rectangular or triangular, about 80% as tall as the boundary layer, and run in spanwise lines near the thickest part of the wing. They can be seen on the wings and vertical tails of many airliners. Vortex generators are positioned in such a way that they have an angle of attack with respect to the local airflow.
A vortex generator creates a tip vortex which draws energetic, rapidly-moving air from outside the slow-moving boundary layer into contact with the aircraft skin. The boundary layer normally thickens as it moves along the aircraft surface, reducing the effectiveness of trailing-edge control surfaces; vortex generators can be used to remedy this problem, among others, by "re-energizing the boundary layer".
The use of VGs on FAR Part 103 ultralight aircraft has been debated at least a little; I've decided to get into the debate by actually giving them a try on our Dragon Superlite. A couple of days ago, I temporarily attached a set of VGs to the Superlite and then flew the plane. Here's what they look like:
How did they work out? I don't really know. The wind was gusty aloft, so it was difficult to check out stall speed, landing characteristics, and I've reached no conclusions so far. But I've decided to leave them on the Superlite as it heads to Oshkosh. Come on by our North display area and take a look.