Leading Edge Skin Installation - Some tips

[jmorrical]

I am ready to install the fourth leading edge skin on the first wing. It has nut plates installed. On page #139 of the Extended Fuel Wing Manual it says:

THE MOST INBOARD SKIN SHOULD
OVERHANG THE #1 RIB BY ABOUT 1/4”

Do I overhang the single rib that's riveted to the spars OR is it the whole assembly (glued doubled rib) that I should overhang 1/4"?

Any help?

IMG_5188.jpg

[TroyBranch]

I just centered the bent flange of that skin in the tank bay, cap strip were on when I did it. So centered between edge of cap strips. That made the most sense to me. The 1/4" per the manual made the skin quite offset. It ended up being about 3/8 over hang to the single rib.

Be very accurate on drilling the two holes for the nut plates on each end. They have to hit the center of the rib below or the nut plate will not work. You need cap strips on first though before drilling that hole.

[jmorrical]

I have talked to another builder about this, but would like to hear from others.

So far I can not get the last leading edge skin to fit to my satisfaction.

When the overlapping edge of the skin is square to the back side of the forward spar I can't get the skin to false rib radius to be fully in contact. The most outboard and inboard ribs have rib to skin contact that's great, but the middle three ribs end up with about 3/32", maybe 1/8", gap of about 1 inch distance at the 10 o'clock position in the radius.

To get good contact thru the radius of the rib I have to pull the skin well forward over the longitudinal axis of the spar. This puts the overhanging edge grossly out of square.

Should I square the back side of the overhanging skin and take the skin to rib gap for what it is???

It seems to me the longitudinal bends at nut plate edge have as a result shortened the skin by a fraction causing the skin and rib radius to be out of alignment when the skin is fitted properly to the back side of the spar.

What's your experience?

IMG_5191.jpg


IMG_5186.jpg

This distance off the mark allows better rib/skin radius contact but it's not square with the back edge of spar.



IMG_5188.jpg

The mark shows the position of the skin for square overhang of the spar. Here I don't get an acceptable fit over the rib radius.



IMG_5192.jpg


Jim Morrical

[ceslaw]

Check the nose ribs to make sure they are tightly seated to the the spar. A slight gap top or bottom could cause your problem

I had one rib that was out less than an eighth inch. Drove me nuts trying to get the skins to fit. Discovered the problem after a lot of head scratching, drilled out the offending rivet and then the skins went right into place.

That last skin should fit properly. Something is interfering.

Chuck

[jmorrical]

Here's what I've learned about my dissatisfaction with the fit of #4 leading edge skin. The first three skins are .016 thickness and the forth skin is .020 thickness. I had no idea. I guess that beefs up that section as it includes the fuel tank bay and secures the tank bay cover at the nut plates. The thickness difference could explain why I've found it more difficult to get a fit over the leading edge radius similar to what I've seen at the first three skins.

Has anybody else had this same concern or am I thinking to deeply about these things???

Well, my method up to this point was to position the skin and place the wooden clamps then drill/rivet. I did the top rivet then the bottom rivet. Lastly, the middle rivet was match drilled from the back side of skin at the hole drilled in the rib from a snap line before skins are placed as some have done.

After talking to Mitch I will square the overhanging edge of the skin to the back of the spar as the manual says to do. I won't place wood clamps until the the top and middle rivets are in place. The top or first rivet will be set 1.75" fwd of the inboard edge of the skin for the three false ribs and two main ribs. The manual says the factory does not drill top rivets in false ribs any longer, but it wasn't very clear about this area. There was a statement that first rivets should be 1.75" fwd of inboard edge on nutplated skins. Mitch says on this skin top rivets are to be installed in ALL the ribs under the #4 skin...false and main ribs.

So, I will "keep calm and rivet on!"

Three days later an update: I got a perfect install of the fourth skin by not using the wood clamps until after the top two rivets were installed. Using this technique kept the back overhanging edge square and allowed the radius and bottom to pull right into position. Overall the #4 skin is better fitted than the first three that where clamped first then riveted. On the next wing I may just rivet the top two rivets then use wood clamps and finish with the bottom rivets.

Jim Morrical

[TroyBranch]

Squaring up the skin to the spar is key for good looks of the fit. I would assume trying to get a good fit in that area would be a challenge with the clamps in place and no holes. I used the strap method and installed all upper rivets including the rivets to the spar before pulling the skins. I was able to get good alignment. Pulling the skins with the straps however it is not much fun. My guess is the clamps are much better. I made a clamp to make the final pull at each rib before drilling the final hole. No matter how hard I pulled I still have a slight gap in the skin between the rib and the skin for about 1.5" on the bottom just after the tightest radius. Just enough to feel it if you push at the rib. Mitch says that you cannot get it tight with the straps. The amount of tension is so great on that rivet, does the clamps actually eliminate the small gap? 1/32 or so. I just think the hole leading edge would need more rivets to keep it tight? How did that location turn out with the clamps? Push in about 2" aft of the nose on the bottom at the rib.

[jmorrical]

Troy,

I hope other builders will speak up about their experience with the skins.

I used the wooden clamps builders share. They have worked well. I would like to see others builder or factory skins to see what the expected normal looks like.

I see the same area where skin and rib don't quite make contact at the bottom of most ribs in the area of 1"-2" aft of the radius of the rib. The location is just as you describe on the bottom side of the skin and I estimate the gap is 1/32' to 3/64" on many ribs. I think I pulled the clamps a firm as I should. If you go to the beginning of this thread to Paul's entry where he gives his tips you will see this:

5) Take a look at the gap between the wood cutout and the skin. You’ll see a very slight gap at the very bottom of the leading edge….an area about 2” back from the bottom edge. Don’t worry about this if you can’t get it out, just so long as you are consistent right across the whole wing, both wings.

Jim Morrical

[ceslaw]

Three observations

First, like others there is a slight gap between the bottom of the rib and the skin. It is slight, consistent, and not an issue.

Second, when the skins were fully installed one could see where the leading edge of the ribs met the skins all the way along the wing. So I know the skins were really tight. Run your hand along the leading edge and you can feel the ribs. I would not want to clamp it any tighter to close a tiny gap for fear of crushing something

Third, when the fabric is applied and shrunk it has a way of pulling it all together so that any "ripples" on the bottom of the skins just disappear.

Chuck

[ceslaw]

Not sure this post over at SuperCub.org ever made it here.

http://www.supercub.org/forum/showth...l=1#post600350

[TroyBranch]

Each rib is clearly visible through the skins. It is amazing how much pressure they can take when pulling the skin around. Scary actually, just waiting for a rib to buckle. I drilled the bottom holes with a #40 bit, removed all the clamps, drilled the skins with a #30 bit in the hole center and drilled the ribs with the hole slightly offset aft to help tighten the skins more. Not so much that I would not have a round hole. I would then debur everything, suck it all in tight again with the straps and rivet. Not sure how it could get any tighter. Gap is still there, glad everyone has the same gap.

One thing that is new in my kit is a doubler for the door latch. It goes between the LE skin and the rib in the second bay from the inboard. If you already riveted the tank skin, the rivets will have to come out to fit this in. This is not in the manual.