I mean that no one has a particularly good theory of the mechanism by which they operate, and how they help, and the proposed mechanisms I have heard have ranged from complete nonsense, to (charitably) hand-wavy.
That doesn't mean they *don't work*, just that no one has a good theory of how they work, so you can't easily tell whether or how much they might help in any particular case, unless you try it.
The mechanism is well-known: they make vortices that bring high-speed air down to the surface of the wing to reenergize the boundary layer. Here's a paper: Pauley, W. R. and Eaton, J. K., Experimental study of the development of longitudinal vortex pairs embedded in a turbulent boundary layer, AIAA Journal, 26 (7), 816 (1988). I recently stumbled on some wind tunnel results that indicate that they can reduce lift hysteresis in an airfoil that increases in angle of attack until it's near stall, then decreases in angle of attack. That sounds useful for stunt. I'd like to do (or better yet, have others do) some more experimenting on where to put them. That's why I asked where Ken put them that helped so much.
Vortex generators were once an issue in a French national election. The party in power backed development of an Airbus airplane, the wing of which the opposition claimed was so poorly designed that it required vortex generators to fix.
Igor uses them. Paul uses them.
Igor doesn't use them, maybe because his airfoil doesn't have lift hysteresis. Paul uses them, but in different amounts on different airplanes, as determined by experiment. I first used them in 1975 on a Jr. Satan, which they didn't help. PJ was the first person I saw using them on a stunt plane.
Here's a piece I wrote on how to stick them on a wing:
I have used two methods to attach them:
1. first putting down a strip of Scotch Multitask Tape (the stuff we use to seal control surface hinge gaps), then gluing the VGs to the tape with cyanoacrylate glue..
2. gluing the VGs directly to the airplane. I have tried various glues. I recommend the following, both of which allow nondestructive VG removal:
Elmer’s Rubber Cement
This works a treat. VGs stay on in flight, come off easily on demand, and leave no residue on either acrylic lacquer or catalyzed polyurethane. The brush in the bottle isn’t much use for VG work, though. Starbucks stirring sticks work well for applying the glue. Q-Tips work well for removing excess glue around the VGs easily without solvent.
UHU Twist & Glue
This holds the VGs on a little better than rubber cement, and it dries faster. VGs still come off when you want them to. They leave some glue on the surface, but it’s easily removed (from catalyzed polyurethane clearcoat) with isopropyl alcohol. I have only used the version of this glue commonly available in Poland, but I presume that the same glue is available elsewhere.
Mind you, this is for my ABS VGs with wide bases that provide some area for glue.
I should mention that there are some substandard VGs coming out of Canada, a foreign country, made out of cheap polystyrene by immigrant labor. They are devoid of color ("colour", as they put it), and have deformed sickle-cell-looking bases. I would avoid these. Some authentic American-made artisanal VGs may be available at the Goyet and team trials if the wind blows in the wrong direction this week at JCT manor. Otherwise, I'll be practicing stunt.