Vortex generators

[jeffbock]

Have the VG's for my Kitfox 7 and would welcome any members input on install. Assume from the instructions that I should use 3.34 in. between to get 50 per wing. Or... Should they go as far as outboard of flaperon?
Looks like I can fabricate my own template on paper and then cover with packing tape to stiffen.
A friend has to fuel with 5 gal. cans and doesn't like having the VG's around the fuel fillers. I don't have that issue as I have a tank with hose in my hangar.
Hope this isn't too convoluted and will get some sage advice. Jeff in N. Idaho (500hrs on my bird, running great)

[Shadowrider]

On the STI wing they made a huge difference. Lowered stall speed 8mph. Breaks harder when stalling, but stalls much slower. I would not go off what the vg instructions say. I went with stol speed vgs. I went 2 inches back from leading edge.

[JoeRuscito]

Which wing do you have? And do you have the laker leading edge installed? I ask because I have the standard wing and no laker leading edge. I put the vgs as far forward as possible with this setup and while I did see a significant improvement in no flaps and one notch of flaps (4-5 mph). I saw little to no improvement in a full flaps config. Maybe 1 mph if I’m being generous. The laker leading edge I believe would allow a more forward placement and more improvement at the highest angles of attack. In my opinion they are not worth the pia of cleaning and the less predictable stall (more aggressive break).

That all said I do think the horizontal vgs made a notable improvement.


Sent from my iPhone using Tapatalk

[mooreaa]

Jeff if you haven't already, check these two videos out:

I think these two cover the subject pretty well and provide a lot of insight into where to place them, spacing, etc.

About2Stall -
https://www.youtube.com/watch?v=IRNPM7A54Yk

Bryan Bowen / Project Kitfox -
https://www.youtube.com/watch?v=vGzHZKRZi7s

[N14ND]

Here's a couple of other videos on Kitfox VG installation.


https://www.youtube.com/watch?v=XPC8egjwjY0

https://www.youtube.com/watch?v=0tBYH1c2ZhQ

[Okent]

Great video's!
I think I'll pass on the vg's.
Never really thought about the fact that it would take more correction to break the stall.

[jeffbock]

After 500 hours on my "7" am getting fairly comfortable on short fields. At my age, gravel bars and waterskiing are out so see no need to rough up my wing with VGs. One knot difference at full flaps is of no consequence, and makes its more difficult to wash. My two cents and thanks to the others helping me make the decision. Jeff, North Idaho

[N14ND]

So here’s some chum for the water. For the record, I am not saying that VG’s don’t lower your stall speed and increase the AOA that your wing will fly at. There is a safety factor of a wider AOA and airspeed envelope while “flying”. There is also the argument that VG’s energize the air over the wing and may allow the wing to produce more lift at the normal 3-point landing or takeoff. However, the best way to utilize that increased AOA that VG’s offer and lowered airspeed at touch down is by getting the wing to that higher AOA when the wheels contact the ground. Which means that to takeoff or land at a slower airspeed you need to also increase your “sitting” 3-point attitude. (Or lighten the load that the wings have to lift). My Kitfox sits at roughly 15° AOA. Airframe mods you can do to raise AOA is mount larger tires and/or taller gear. If you’re landing you would need to have the tailwheel lower than the mains at touchdown, which isn’t an unusual occurrence as long as it doesn’t exceed the structural strength of the airframe. Repeated landings like this can cause fatigue failures. I figured it out once for my Kitfox and I think it was one degree increase in AOA for every 3” I raised the bottom of the fuselage with the tailwheel on the ground.
Keeping the stock wing and current build weight you’d need bush gear, tundra tires 26” or larger to raise sitting AOA to really slow your landing speed. At this angle I think VG’s are probably worth it. I have watched many videos and seen their results on lowered stall speed, but their deck angle is much higher than any landing attitude you want to try. If you were in an emergency and slowing to land in a tree canopy the higher deck angle and slower airspeed would be an advantage. Theoretically, to really test landing speed would be to fly over a lake bed a couple of inches off the ground and slowly decrease speed until all three wheels touched down. Then see what that speed was.

All that to say that for a normal Kitfox, if you are not going to change your gear set up, VG’s aren’t going to lower your landing speed. You can get slow flight bragging rights, but you’re not going to effectively lower your speed at touchdown. You may feel more confident in your aircrafts performance, which in itself maybe worth the install.

[jmodguy]

I went with the “About to Stall” configuration from day 1 so I do not have a no VG baseline. I based my decision on their before and after testing and some research on the shape of airfoils between the ribs. My no flap stall is 45mph with a mild break, and full flap stall is 45mph that just mushes.
I have the standard wing with no Laker LE. Empty wt is 1013.

[desertdave]

My no flap stall is 45mph with a mild break, and full flap stall is 45mph that just mushes.
So you have zero difference in performance between clean and full flaps? Your numbers are much greater than my standard wing with no VGs. Something doesn't jive here.

[jmodguy]

Clean I get a definite break at 45mph. Full flaps it just sinks at 45mph. Didn’t really try any slower. I’m spinning an 80 in prop at 680 rpm at idle. At that time I had about 20 deg pitch in the prop. This prop throws a LOT of air, even at idle!

[jrevens]

... Your numbers are much greater than my standard wing with no VGs. Something doesn't jive here.

Is that probably because his airplane is considerably heavier than yours?

[desertdave]

Is that probably because his airplane is considerably heavier than yours?
Who knows. I can't say I have ever seen an airplane that the flaps didn't lower the stall speed.

[JoeRuscito]

I think I have an idea why this happens, similar on mine where I saw a large decrease in stall speed clean and one notch but very little with full flaps. My thinking is that if the VGs aren't far enough forward, the airflow separates before reaching them at the highest angle of attack (full flaps), but they are very effective at the lower angles where they are in front of the separation point. On the standard wing, as I said above you can only put them so far forward.

[PapuaPilot]

I'm guessing you have a very forward CG. Are you running out of elevator? Adding flaps definitely gives a down pitching moment and makes it harder to hold your nose up.

Do you have the elevator trim assist spring? If so it might be good to take it out temporarily so you can feel the actual pitch forces.

FYI I have a forward CG too, but my plane is not as heavy. I am typically at 1200 to 1250 lbs flying weight. Each notch of flaps on mine decreases my stall speed by 3mph. At GW mine stalls at 49/46/43 mph. No VGs.

[atosrider]

the effect of the Laker leading edge should be showing up in how one measures the chordline for VG's, that doesn't seem to be taken into account in these random performance numbers so how can we compare? does the factory spec sheet help out there with some kind of baseline? (pardon me but I'm back with model IV wings)

and I would like to bring emphasis back to what was said earlier in this thread, VG's bring an extra factor of safety in the flight speed envelope ... to take that one step further, other pilots have shown that turning base to final while dropping airspeed has been shown to produce stall spin situations ...

and my take-away from this thread? nice to hear VG's on the horizontal being praised, a timely reminder for where I am in the rebuild ... this winter will be ideal for installing those ... and that would be going with the idea that it would provide the best effort / benefit ratio for flight control feedback / improvement ...