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Permalink Reply by John Austin on April 30, 2023 at 5:13pm

Mike, 

I assume you're installing StolSpeed VG's.  I wrote up a post "Slats and VG's" on this topic quite a while back and within that discussion you'll find my detailed instructions about installing them.  Here's an excerpt with the important info:

I have had a request for some pics of the VG's, so I thought I'd append them to this thread, too. The Stolspeed VG's are Lexan - pretty much indestructible except for avgas - so you need to be very careful when refueling. They are paintable, but the clear Lexan makes them seem to blend with the wing and they're sometimes almost invisible. Supposedly, they contain an "extra dose" of UV inhibitor and Russell told me his have held up well so far. I haven't had any come off - they now come with a laser-cut sheet of 3M adhesive - you just peel the paper off (already laser cut in the shape of the VG's base), stick the VG on the adhesive, then peel the adhesive off the backing and stick it on the wing, guided by a template and string-line. Per Stolspeed's instructions, I put the tip of the VG at 7% chord so as to put the top of the "sail" of the VG at about 8-9% chord. One thing mentioned but not emphasized in the instructions: chord is measured from leading edge to the trailing edge of the flaperon. On the 750's wing, the 7% chord line for aligning the tips of the VG's worked out to measuring 155mm directly on the skin (use a flexible ruler) forward of the main spar rivet line. If you make a mistake and need to reposition the VG, the adhesive will come loose with steady pressure and doesn't harm the paint - you just rub the adhesive and it balls up. I haven't had one come loose otherwise yet!

All that being said, I really don't think it is worthwhile to install the VG's with the slats installed.  I've flown hundreds of hours with no slats/naked wing, no slats+VG's, and slats+VG's.  VG's definitely help a wing without slats, but don't help a slatted wing.  OTOH, I don't think VG's produce any negative effects with slats - I left mine on when I reinstalled the slats after a few hundred hours without them and didn't want to remove them in case I ever decide to take the slats back off.

Hope this helps!

John

N750A

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Permalink Reply by Michael A Housewert on May 1, 2023 at 9:57am

John . . . I sent a reply to you in a different thread . . . . not sure if you received it.   Thank you for your response and I have read all your comments regarding VG's with and without flaps.  I am going to remove the slats for sure and add on the VG's.

A few questions:  

1.  The manufacturer of the VG's has changed the recommended location of the VG's to be 5% chord instead of 7%.  Any thoughts on this?

2.  Regarding the VG's on the tail, do you have a recommendation for the location?

Thank you so much.  You are appreciated!!  Mike N750ZT

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Permalink Reply by John Austin on May 1, 2023 at 10:32am

Mike,

The StolSpeed website's general recommendation is 5%, but then it says there are specific recommendations for some Zenith models on the main menu, but I can't find that.  There are testimonials and under the 750, the 7% was used.  All I can say is that my VG tips were at 7% and it worked phenomenally well with a 5 kt decrease in stall on an un-slatted wing!  Perhaps you could contact them and clarify, but I think you'd be fine with the 7%.

I used the Zenith VG's on the elevator and way back when I was building, talked to Caleb, the engineer at Zenith, and he said no one had come up with a better location than that given in the Zenith VG plans.  I installed them during the build so I have no before-and-after experience.  However, I also highly modified the tail with HT fences, elevator tip extensions, and fuselage/HT fairings and these made a dramatic difference.  I had Zenith look at these mods and they saw no problems with them.  Several others have copied these mods and seem to universally agree that they also saw dramatic improvements in pitch authority at low speeds/landing.  Of course, I must add my CYA statement that I am not advocating you do this - you're on your own! - and simply relaying mine and others' experience.

John 

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Permalink Reply by Michael A Housewert on May 1, 2023 at 12:30pm

John . . . OK . . . 7% it is. Thank you for that information.

1.  Being a novice at this, would it work out better for me to measure the forward of the "...main spar rivet line 155mm directly on the skin (using a flexible ruler)"?  Would this then be equivalent to the 7%?

2.  Do you have location measurements for the VG's for the tail?

3.  Where are you located?  I am at C77.

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Permalink Reply by John Austin on May 1, 2023 at 1:33pm

Mike,

1. Yes - Here's how I did it:  locate the main spar rivet line and draw a centerline between rivets on the inboard and outboard sections of the wing.  Lay a flexible ruler on the centerline and press it down against the curved skin and mark at 155mm (this measurement is allowing for the curvature of the wing skin).  Securely tape a string over the 155mm marks (I hung a weight off the wing tip with one end of the string to keep a constant tension on it.)  I also recommend you tape the string down to the wing at several points to keep from inadvertently deflecting it while sticking down the VG's.  When you load the template approach the string from the aft side and line up the tips of the VG's to where they are barely touching the string - this is 7% at the tip of the VG and the main portion of the "sail" of the VG will be at about 8-9%.

2.  Just to be clear, this is for the optional Zenith VG's, which are quite different from the Stolspeed VG's and go on the underside of the elevator.  They are hidden by the spar in cruise and exposed with upward deflection of the elevator:

3.I'm in East Tennessee: TN66 about 12nm south of KCSV/Crossville, Tennessee.

John

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Permalink Reply by Michael A Housewert on May 2, 2023 at 9:06am

John,

You are truly appreciated for taking the time to respond to my request. 

As I progress through the process I will send you a few pics.   

We passed through Tennessee a few weeks ago on our trip from Florida to Illinois on route 24, not too far from your location.

Thank you again,

Mike

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Permalink Reply by Michael A Housewert on May 4, 2023 at 1:51pm

John .... Then approximately how from the wing tip before transitioning from 30 to 60 mm spacing for the VG's?

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Permalink Reply by John Austin on May 4, 2023 at 2:43pm

If you have 30/60 spacing templates, you have the wrong kit for the main wing - I think that kit is for a horizontal stabilizer.  The main wing kit/120 VG kit has 60/90 spacing.  As per the Stolspeed Instructions, start the VG's with 60mm template spacing about 2" inboard from the wingtip and after 15 VG's, transition to the 90mm spacing template for the remainder of the inboard wing.

John

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Permalink Reply by Michael A Housewert on May 4, 2023 at 4:20pm

Thank you for the clarification. The templates have the 30, 60, and 90 mm spacing slots. As per the I instructions I will start with the 60 mm for 15 VG's and then change to the 90 mm spacing.
If you look at this picture again, the top template shows the 90 mm spacing but it is upside-down in the picture. I should be fine.

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