My personal experience with them was that they improved the tracking and corner significantly on a Whitely Shoestring that was a little too heavy.
The airplane flew fairly well and was reasonably easy to get through a pattern with no real problems. It was well trimmed as blessed by RJ W., Bart K., and Me...We all agreed that it flew a little "heavy", but was basically an "honest airplane".
I obtained some of Howards VG's and installed them on the Shoestring...Voila...different airplane!
The heavy feel was gone and the corner was much improved. All that flew it agreed. In fact it was the first airplane to appear here in Tucson with VG's and as a result of this success they began to "bloom" on a lot of other stunters here with very "mixed" results.
When I first put them on the Shoestring and sent a photo to Howard He very kindly informed me that I had installed them in a configuration that was angled in oppisition to normal practice (I think this was Howards nice way of saying I was stupid and put them on the wrong way!)
So after about 20 flights I changed them to what was supposed to be the conventional pattern and surprise, surprise...they didn't seem to work nearly as well! So I put them back on "Wrong" and they worked again...Go figure!
So my opinion at this point is that they work but better on some airplanes than others, and how they are installed does make a difference, but I haven't a clue as to why or how that should be on different wings.
Incidentally they were installed on the Stab also and remain there.
The biggest problem I had with them is keeping them on the airplane!!! When they fall off, the flight characteristics of the airplane are noticeably different! In fact it caused hingeing on the airplane when they came off the top of outboard wing and stayed on the inboard!!!
There's certainly nothing "scientific" about this post but it did simply raise a lot of questions for me!
I'm not at all sure I will use them on my New Shoestring as it's going to be significantly lighter and a litte larger than the one described above. My opinion is simply that they do something and I'm not sure what!!
The configuration shown in the photos is the way they work best..."the wrong way".
Randy Cuberly