VG placement

[WurlyBird]

I have a Model 3 that is about to return to service after a VERY slow engine swap, all that’s for another discussion. I have searched the threads and read everything relating to VGs here but didn’t find a good answer. So, I will be mounting VGs in the near future and I am trying to find a location that works well. I have the STOLspeed kits and have gone to their website that suggests 7% but then references a video which uses 3%. I have seen the 3% (1.25”) used a few times, have found reference to 8-10%, 3.75”, 5”, 8”. Most of the reports here that have solid performance numbers posted don’t go into detail about location, and those that mention specific placement don’t seem to give solid numbers. I may have missed it if it’s there, but can anyone provide a little info with both location and results?

[Shadowrider]

I have not heard of anyone putting them too close, but have heard of many putting them to far back. On the kitfox if you use stolspeed recommendation you will probably not see any improvement. Use 3% and you will be good to go.

[Denalifox]

Consider the angle of attack of the wing when it stalls. my plan is to stall my wing and note the AOA. I then will place my VG's parallel to the ground at the wings stalling AOA.

[Maverick]

I have a series 5 wing. I watched several videos on YouTube (https://www.youtube.com/watch?v=IYHuXs2F3ds) about it and finally settled on the recommendation of Alex Di Sessa of 35mm back of the leading edge. Not sure it the series 3 wing is similar to a 5. I used StolSpeed VGs also.
I used the template for locating the leading edge and measured back 35mm from the forward most part of the leading edge and marked it. I then used it to stretch a string the length of the wind. I then used the spacing templates from StolSpeed to get the angles right on the VGs. It made a big difference in stall speeds with power on but only a little with power off. I think that the issue is that the angle of attack on landings isn't steep enough that it affects your landing speed much unless you are using a lot of flap. I don't use flaps.so I can only really tell a difference when I'm practicing stalls. Otherwise, not so much.
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[Rodney]

Would recommend 3% of chord. The testing I have seen on those that put them 8-10% back claim no change to stall speed.
The reason is the VG's are too far back and have no opportunity to affect the flow over the wing.

Regards
Rodney

[WurlyBird]

Thanks for the responses. I guess I’m just having trouble wrapping my head around 3% and why the Kitfox seems so different. In theory (used loosely) the VG should be where it will accelerate the most energized air into the boundary layer, which should be the highest point in the undisturbed airstream at stall speed. My plane would stall just about in a 3 point attitude, if lucky, but never a higher AOA. So why wouldn’t the VGs be at or near the “top” the wing in a 3 point attitude? 6.5” (or 15%) in my case. My Maule already had VGs installed when I got it, but those are a little in front of the “top” of the wing in a good landing attitude (way tail low). The VGs span from 7% to 9%. 1.3” from the leading edge just seems so far forward…

[Shadowrider]

Not much laminar flow and the airflow separates early on the kitfox. I dropped almost 10 mph on stall speed, while others claim no speed drop when put farther back.

[Rodney]

Shadowrider is correct. If you place the VG's too far back, the airflow, more correctly the laminar airflow has all ready separated itself from the wing and the
VG's are in the disturbed airflow with no chance to do their thing. Go to a STOL competition and look at where the VG's are placed on their wings.

Regards
Rodney

[WurlyBird]

Just to be clear, I’m not discounting any of your information. I’m just trying to get it to make more sense to me. I’m an A&P in Alaska, so almost every plane has VGs, but they’re all certificated so they do what the STC says. I have not been around many STOL specific experimentals so I don’t have that experience to draw from. At least these should be easy enough to move if the results aren't good enough. Thanks again.

[buckchop]

I installed VG’s on my KF1 wing #0038 here a month ago, had 7hrs TT, now has 9.3hrs TT, not much time yet of course, but HUGE difference in stability at landing speeds, did mine 1.5” back from the wingtip leading edge, and spaced 3.25” apart and 15 degrees to windflow, Also installed them on the bottom of the stabilizer about 1” forward of the hinge and at same spacing as wing. GOOD CHOICE, but i got my info on placement from Leni and Randy on AvidFoxFlyers. I have pic there also.
I also removed the droop wingtips, made wingtip fences and mid wing fences, but when i built my wings i installed another wing rib for mounting the fence to next to one that was in place, and installed a backer on wingtip rib so fence was easy removal.
Id post a pic here but i stopped doing them here cause it a PITA to do it here. 👍