F-799 Wing Root Fairing Too Tight

Hi All,
I'm in the process of finally installing my F-799 wing root fairings and have run into a fit problem that I can't find addressed elsewhere. When I riveted my tank I used the fay sealing method where you assemble the entire tank with clecoes then come back the next day and rivet it. I also wet riveted all the lower fuselage skins on my aircraft and noticed that if you set the rivets when the fay seal is still wet a lot more sealant squeezes out. Thus I suspect that the increased thickness in the tank has increased the radius slightly, which is making my fairing fit really tightly. As you can see in the pic below, this is pulling it in substantially at the root. To fit the fairing (since instructions are almost non existent) I started by drilling and clecoing the most aft hole on the bottom side then drilling and dimpling for the existing tank screw right near this for a bit more purchase, then pulled the fairing up and around the leading edge and clecoing the 3/32 holes along the wing walk doubler. My plan at this stage is to relieve the 3/32" holes in the fairing at the wing walk doubled to an "out of round" state until the fairing becomes square again, then drill the leading edge holes along the line 1/4" in from the edge I have already marked, then finally open the wing walk holes to full size and hope this works. Has anyone run into this problem before and can they provide any advice so that I don't have to rely on hope and blind luck?
Tom.

Solved.

So relieving the holes on the fairing along the section that covers the wing walk solved the issue. 3/64" aft relief on each hole worked perfectly and allowed them to line up with the holes in the wing underneath, whilst addressing the downward bowing at the leading edge. Disaster averted. In hindsight however, I think it would have been best that I hadn't drilled and dimpled the tank attach screw underneath, then I would have only had two holes to relieve underneath. I guess that's why they put two wings on these things, so that we get a second chance to do things better.
T.

If you want, you can also call Vans and see if they would supply you with a 799 that is un-punched. That way you can drill the holes where you wish.

Seems to me that you need to tape the fairing in place square to the wing, with the fairing only touching the fuselage at its widest point. Then you measure the gap at the front and back of the fairing and build a gauge to mark where to trim the fairing to fit.

For instance, if the fairing is tight against the fuselage at its widest point, and you have a 1/8" gap at the trailing edge of the gap and a 1/16" gap at the leading edge of the fairing, you're gonna need to trim the fairing 1/8" where it contacts the fuselage and less elsewhere.

So you make a 1/8" follower gauge to slide along the fuselage while you mark the entire length of the fairing (still taped in place). Your mark should give you a trim line that tells you not to trim anything at the trailing edge, gradually increasing to 1/8" at the fuselage's widest point, then down to 1/16" at the leading edge. Trim to that line and it'll fit like a glove.

Line

Kyle Boatright said:

Seems to me that you need to tape the fairing in place square to the wing, with the fairing only touching the fuselage at its widest point. Then you measure the gap at the front and back of the fairing and build a gauge to mark where to trim the fairing to fit.

For instance, if the fairing is tight against the fuselage at its widest point, and you have a 1/8" gap at the trailing edge of the gap and a 1/16" gap at the leading edge of the fairing, you're gonna need to trim the fairing 1/8" where it contacts the fuselage and less elsewhere.

So you make a 1/8" follower gauge to slide along the fuselage while you mark the entire length of the fairing (still taped in place). You're mark should give you a trim line that tells you not to trim anything at the trailing edge, gradually increasing to 1/8" at the fuselage's widest point, then down to 1/16" at the leading edge. Trim to that line and it'll fit like a glove.

Click to expand...

Yep. Find a washer with the right diameter ring. Lay it down, place the marker in the washer and roll it along the fuse while drawing the cut line.

Hi Tom,

How much 'taper' is there in the fairing where you're holding the straightedge? In the pic, it looks like it drops close to a half inch from the rib to the fuselage. If it's really that extreme, are you sure the nose area of the fairing is shaped to match the tank? Have you laid a long straightedge along the nose of the wing/tank/fairing?

If the tank is really causing the problem, it does point out a reason I could never get comfortable with the method of waiting until the proseal cures to rivet. It would say that the skin is standing off the rib flanges quite a bit, which means all the rivets are now loaded in bending instead of shear. Obviously a lot of guys are getting away with it, but it's certainly counter to best practices that are recommended for assembling multiple layers in a load bearing structure.

Charlie

Hi Charlie, yes I would say the taper was somewhere between 1/2 and 3/4" at the worst position. It was incredible that just relieving the holes by 3/64" resuolved such a big dip, but it did. I am more convinced than ever that this is due to the marginally thicker faying gap with the 100% cleco method. I'm not saying it is a bad way of sealing the tanks and would still do it that way, but the secondary impact on this fairing is just something to keep in mind.
Tom.

tgmillso said:

Hi Charlie, yes I would say the taper was somewhere between 1/2 and 3/4" at the worst position. It was incredible that just relieving the holes by 3/64" resuolved such a big dip, but it did. I am more convinced than ever that this is due to the marginally thicker faying gap with the 100% cleco method. I'm not saying it is a bad way of sealing the tanks and would still do it that way, but the secondary impact on this fairing is just something to keep in mind.
Tom.

Click to expand...

Hmmmm...I used that method, and the fairing fit perfectly (after trimming using the washer method as given above). Not convinced the cleco/proseal method could add that much to cause such a misalignment.

FWIW, when we did my buddy's -8 before my build (I have him to blame for getting me into this airplane-building stuff ), same results...perfect fit.

So I'd look elsewhere for the misfit...but since you've got it fixed now, build on!

I fit the left wing root fairing this past Sunday it mine fit perfectly too. I started by clecoing the top aft strip, then pulled it back up and drilled/countersunk the top screw at the tank corner. Next I wrapped it around the wing with assistance from another and drilled/countersunk the bottom aft screw. From there I drilled/clecoed the rest of the holes.

The fit is so nice (less than 1/8") that I plan to skip the trimming and rubber seal. It will receive a nice clean proseal bead instead.

You do realize, that fairing will likely come off/on several (many?) times over the life of the plane, right? (Tank leak, front tank mount bolt check, etc.) Proseal there seems like a lot of future work.

Charlie

rv7charlie said:

You do realize, that fairing will likely come off/on several (many?) times over the life of the plane, right? (Tank leak, front tank mount bolt check, etc.) Proseal there seems like a lot of future work.

Charlie

Click to expand...

If it's once or twice a year, I'm ok with the hassle. I can always change my mind later, trim and install the ugly rubber strip. I love the smell of proseal