forward skin to rib contact

[wingnut99]

We installed the forward skin on the right wing last weekend and everything went smoothly until we flipped it over and started attaching it to the other side. Once you bend it over the nose it gets a little belligerent about wanting to press down against the ribs, especially the front 2 rows of rivet holes. The front row has almost a quarter inch gap between the underside of the skin and the rib looking from the open end of wingtip. My mentor says that's normal and I'll need to make small aluminum washers and stack enough of them to fill the gap on each rib so that the skin is tight against each rib. Did any of you have this issue and the ratchet straps can only get the 2nd side that close or is there a method to get that skin down tight against the ribs all the way to the tip of the ribs?

[thomas]

For what it's worth, I'm currently building my wings and spent quite a bit of time over this issue. I thought about the aluminum washer approach but decided not to go that route because the profile of the ribs are what define the airfoil for the skins. Washers will likely alter that profile. How much can you deviate from the airfoil without appreciable performance problems? I didn't want to find out so I took a step back and used the vacuum (re)bending method to tighten up the nose radius. SonexFlight has a great podcast on this method.

A lot can go wrong with extra handling of the skins so I don't think it's a decision for everyone. I had to buy a replacement right skin after oil-canning my first one. I killed nearly 20 hours in the shop with setup, handling, and bending, and at least as much time head scratching. However, the forward skins now fit reasonably well and I have less pronounced ballooning of the skins between leading edge ribs with the tighter radius.

[peter anson]

Many builders re-bend the skins using either the vacuum bag method or the Tony Spicer bend brake method. You may also need to file round notches at the front of the nose ribs to avoid getting little bumps in the skin. There's bound to be someone with a picture of this but unfortunately not me. You should be able to get the skins to fit reasonably well without using lots of force. If you do have to use a lot of force to fit the skins you'll end up with large dimples around all the rivets - looks terrible and probably isn't great aerodynamically.

Peter

[Bryan Cotton]

I riveted the tops first, so they would have the best surface. The last two rows on the bottom we did as a two man operation. It was not 1/4" for me. I pressed the skin down hard and Adam pulled the rivet.

[Rynoth]

You may also need to file round notches at the front of the nose ribs to avoid getting little bumps in the skin.
Peter

I did a little bit of this when it was clear that the front edges of the nose ribs were forcing the leading edge radius slightly open in certain places. The ribs come pre-bent, but that still doesn't mean they're all exactly the same, and even a slight variation of how the flanges were bent can change the geometry of the leading edge radius a little bit and affect the whole skin.

[wingnut99]

Thanks for all the comments. My thought was why isn't the leading edge bent to the correct profile to start with? None of these solutions would be necessary if it just slid over the ribs with the correct bend. If I shove spacers in the gap then the wing profile is going to be affected and that has to change the performance from what Sonex designed the wing to do, maybe not substantially but I will also have to alter the other wing the exact same amount or that will result in unequal lift. That can't be good. I'll check out the vacuum bagging technique and maybe just increasing the bend somehow. It just needs a tighter bend in the radius. If that is being intentionally under bent as part of the 51% rule, they need to rethink that one. How many builder's have access to a 11' brake?

[Area 51%]

I had a terrible time trying to get the skins to lay down. If I put the top of the skin down first, the bottom would dimple. Put the bottom down first......you get the idea. I saw the "brake" they use at Sonex. It's made of some of the finest Home Depot pine stock available. It has it's best days behind it, and is apparently unable to make a consistent bend. Check your LE skins and you'll see pressure marks on the ends but not the middle, and no where along it's length does the angle match what's showing on the print. I know...I know...there isn't an actual degree measurement on the plans, but it definitely shows an acute angle (about 80deg). Look at the Sonex tips and tricks for leading edge install, and you'll notice their skin has a noticeable less-than-90 bend. Out of the box, our skins were 115deg.

I vacuum bagged. I didn't listen to the pod-cast, so this may have been addressed. DON'T PUT THE AFT EDGES EVEN WITH EACH OTHER!! The top of the skin is about 3/4in longer than the bottom from the tangent line, and if you don't account for it, it WILL relocate the radius. Ask me how I know.

[Corby202]

I also had to reform the LE using a vacuum. I taped an extra strip of aluminum along the bottom skin so both edges would meet and be taped evenly. Cant remember the size of pipe I used 38mm perhaps. One of the problems I think is because the nose ribs supplied with the kit do not match the plans hence the smaller radius required to get the skins to sit down properly.

[peter anson]

My thought was why isn't the leading edge bent to the correct profile to start with?

Indeed, why not?
Area 51 had a caution about vacuum bagging and I have another. I have not heard of anyone else having this problem but it will give you an idea of how critical this is. I scratch built so had to bend my own LE skins so decided to try the vacuum bag method. I had the skins water-jet cut but made a small mistake on my drawing and had the forward-most pilot holes 0.2" further forward than shown on the Sonex drawing. There was still plenty of room for riveting so no problem, but it meant that those holes were a little closer to the location of the bend. They formed a slight weak line and that is where the skin bent, so scratch $150 worth of material and $100 worth of water cutting. OK, my error caused the problem but it shows how close you are to producing scrap using this method.
If you go for the Tony Spicer method, one of the things I found was that a large wooden beam with the bending pipe attached is not stiff enough to bend the skin evenly. I got around this by "pre-bending" the tube/beam assembly to allow for the deflection. A friend who is building a Xenos, which has a smaller nose radius so smaller more flexible tube, made an arrangement to apply force to the middle of the tube/beam assembly as well as the ends. His end result was an almost perfect fit with almost no dimpling around the flush rivets. It really was worth the extra effort.
Peter

[wingnut99]

I watched a YouTube video on vacuum bagging and it looked pretty successful. The builder marked the centerline of the radius and clamped the pipe down in the exact location. Since mine are pre-bent I don't think it would take that much suckage, I just made that up, to get the additional tightening I need to get the contact to both sides of the rib portion that everyone struggles with. I think I'm going to get a 1 1/4" gas pipe and some plastic sheet and see if I can tighten it up. If I can't then I'll get some 2x4 and make a trough like Tony Spicer used and use the pipe with a 2x6 or 2x8 spine on top of it and draw the pipe down into the trough with the bend underneath getting pressed into shape. More work than bagging but whatever it takes.