Real aerodynamics
There is no magic and don't want to say they are bad. VG's are wonderful "problem solvers" and are all over aircraft of all kinds from STOL aircraft to Large Jets and fighters. However the RV does not have a problem. The RV is already excellent in the low speed department or high-speed department. In other words the RV is PFM, pure freaking magic, in a way. Van hit upon the right combo of airfoil (NACA 23013.5), wing planform-area, aircraft weight, power and attention to detail. Other planes have
more to gain from VG's.
Yes almost any airplane can benefit to a degree, but
there is no free lunch. If you gain low-end performance you will loose some top end. I have talked to 3 of the VG sales guys and some are realistic, while others imply that you should get a lower stall while loosing no top end. Right
The link I noted above is fairly accurate. It is from a RV builder who does sell VG's. Measuring low speed difference of a few MPH is always hard to do. This site is the only one to do actual real documented flight test data, and I can't argue with his method.
I found out from a friend who likes to do dogfights that after he added the VG's to his RV-4, he could turn in-side another RVs without VG's. In a RV to RV dogfight what happens is you get into tighter and tighter turns and spirals until you bleed off your airspeed and need to back off or stall. The guy with lower stall will win in this scenario.
Personally I love to experiment with stuff. I would not put this on as I said, because I am optimizing the top end of my RV-7.
The Guy?s who love this slow speed VG stuff fly planes that top out at 90-110mph, so they don?t care if they loose a MPH or two.
If you are a real bush pilot and want to get in real short, soft, obstacle mountain fields all the time than VG's might help, a little. However the RV has the ability to get into fields almost shorter than it can take off from.
Unless you have the skill to fly on the edge of the envelope, just flying better and light weights will get you shorter landing limits than VG's will get you.
If you make your RV as light as possible (with max thrust to weight ratio) you will get great low speed performance out of the RV.
If you build a fat RV with all the bells and whistles, a few VG's will not make it a STOL plane. Keep it light. (The stall speed between solo and gross is 7MPH!)
VG?s is not a great match to a high-speed plane like the RV, IMO, which already has good low speed performance. Van's advice: Build it per plans; keep it light, keep is simple.
Cheers G, RV-4, RV-7 (building)