Best way to measure for VG placement?

[dholly 718]

I'm attaching a pic of the contraption I cobbled up to help measure back from the leading edge and mark the desired % of wing chord. I know others have simply laid the long leg of an 'L' square on a lower capstrip, with the short leg up and against the LE. Any comments or consensus on what the best way to do this is?

[BryceKat 125]

Doug,

That's how I did mine. However, I did it twice at 5% & 7% w/ no improvement, ended up removing them. They work great under the horizontal stab. Our wings have great lift, the tail is lacking in its ability to create enough down force to hold up the nose. Probably from to short of lever/fuselage. Bryce

[akflyer 1427]

Doug,

That's how I did mine. However, I did it twice at 5% & 7% w/ no improvement, ended up removing them. They work great under the horizontal stab. Our wings have great lift, the tail is lacking in its ability to create enough down force to hold up the nose. Probably from to short of lever/fuselage. Bryce

Make sure you only figure the wing cord into it. Some said to use the over all width including flaperons, but that is not good info. ONLY the wing cord comes into play for placement.

[EDMO 1078]

Make sure you only figure the wing cord into it. Some said to use the over all width including flaperons, but that is not good info. ONLY the wing cord comes into play for placement.

Are the flapperons included by Avid/KF in the wing square footage?

Just nosey - could figure this out, but lazy. Nothing to do with VG statements.

I know little to nothing about placing VGs, but wonder if the rib between the VGs could negate the vortex you want to form?

ED in MO

Edited 19 Jun 2012 by Ed In Missouri

[akflyer 1427]

Are the flapperons included by Avid/KF in the wing square footage?

Just nosey - could figure this out, but lazy. Nothing to do with VG statements.

I know little to nothing about placing VGs, but wonder if the rib between the VGs could negate the vortex you want to form?

ED in MO

Wont negate it any more than it does in a cub or any other fabric wing aircraft.

The biggest thing I noticed with the VG's was the rediculously high deck angle I have to get it to now before the stall happens. I can really drive it around with the nose way high in the air before it tries to fall out fo the sky.

[dholly 718]

Bryce- I think you're right about the Avid HH and KF 1-3 under cambered airfoils, but these vg's are going on my KF 4. Bottom curve is entirely different and many have reported that they really do make a noticeable difference on this wing. Based on almost unanimous positive feedback, I am also doing the underside of the HS.

Leni- You make an interesting comment about not including flaperon in the chord %. I have seen it argued both ways and was confused enough to go the extra mile and confirm with the vg maker. You are correct.

Now, as for where to place them: the KF 4 wing chord from the POH specs is 42" wing only (51" wing with flaperon). Conventional wisdom and 'rule of thumb' both say 10% of wing chord is best. My math says that would be ~4-1/4" behind the LE. According to the FAQ's HERE, 8%-12% of wing chord is fine. That would be ~3-3/8"-5.0" back. However, when you dig into posts regarding vg placement on the KF 4-7 wing specifically, you find most experienced users who have taken the time to actually test multiple locations felt the 'sweet spot' is at 7%-8%. That is from 2-15/16"-3-3/8" behind the LE. Also must keep in mind that these measures are the line to which the tips of the vg's are aligned. Therefore, the true vg placement as a % of wing chord is actually a tad lower (that is measured at the highest point on the vg).

Also, as I understand, more forward placement improves stall speed characteristics at the expense of increased drag. Too far forward will cause a more abrupt break and tendency to drop a wing, though recovery on the KF is generally accomplished well before a spin starts. More rearward placement, particularly as you get into the thicker boundary layer of air, reduces the vortexes which, in turn, greatly reduce the effectiveness of the vg's in general. So, it would appear the acceptable vg placement range for my application is between ~2-3/4"-3-1/4" behind the LE. This is the vg itself as % of wing chord, not the line to which the front of the vg's are aligned along.

With all this in mind plus the knowledge gained from my conversation with the product designer, I think I'll try my vg's at 3-3/16" behind the LE. Again, this is the vg itself as % of wing chord, not the line to which the front of the vg's are aligned along. This translates into vg's at ~7.5% of wing chord, right in the middle of the KF owners experience range and no terribly aggressive to the forward side. If you're still with me and so inclined, please double check my math and feel free to add your thoughts. This is some serious looking glue and placement is final!

Thanks

[akflyer 1427]

If you're still with me and so inclined, please double check my math and feel free to add your thoughts. This is some serious looking glue and placement is final!

Thanks

I glued mine on and have lived to regret it! I got a roll of 3M take that the guy from STOL SPEED sends out with his VG's. If I were to do it again, I would use this on the wings. At some point, you will get a lil gas slop on your VG's. You can kiss those good bye in a matter of hours. The double sided 3M tape works AWESOME and you can get them off without screwing up your paint. I got a sheet of the tape off ebay for around 15 buckes delivered. I then stuck the new VG's on the tape n cut around them with an exacto knife. It took less than 2 beers to get all 100 of them done I put these on the under side of my stab and they have held up great.

One of these days soon, I need to pull the ones off my wing, and that will require me to do alot of paint work cause it take it down to bare fabric when I pop them off

Those will be replaced with new ones using the 3M tape.

[tcj 150]

Ed, yes the flaperons are included in the wing square footage published by kitfox.

[dholly 718]

tcj-

Yes, KF does list the wing chord in the M4-1200 POH both with and without flaperons, 42" and 51" respectively. I never had an Avid POH with my Mk-IV and don't recall seeing those specs in the construction manual, but I do have the following attachment in my files.

Avid Specs.short.pdf

Edited 20 Jun 2012 by dholly