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Leading Edge Sheeting

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mstull

R.I.P.
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
One thing I've always struggled with in ultralight design is leading edge sheeting. The more you use, the less the fabric dips between the ribs. But the sheeting adds precious weight. Then there's the question of what to make the sheeting out of: thin aircraft plywood; aluminum; carbon; or fiberglass? Then there's how to keep the aft edge of it from creating a corner... especially where the fabric dips between the ribs.

I've tried various solutions for this problem, and usually ended up with a reasonable result. But I've always felt it was a compromise. More sheeting and/or more ribs makes a better airfoil, but add weight. More ribs can usually allow thinner sheeting. But the weight you save with the thinner sheeting is lost with the added ribs. Then there's false (partial) ribs, if your structure provides a way to support them. I use heat shrunk Dacron fabric (Stits) that tries real hard to dip between ribs.

I'm posting this to hopefully get some new ideas. I have seen a few designs that don't use any sheeting at all. They usually use more ribs and/or false ribs, and sometimes carry those ribs over the leading edge tubing. But I don't like the way it looks, with the fabric dipping tremendously between ribs. I'm sure it's as reasonable a compromise as other ideas though. And I've looked at all the other common solutions/compromises I could find.

One solution would be if you could make the fabric have much more tension span wise than it does chord wise. That would keep it from dipping between ribs. I've thought about trying to accomplish that by how I glue the fabric on the frame. But it just doesn't seem to work out in the gluing process.

Another idea would be to use a special fabric that shrinks a whole lot more one way than the other. The warp could be made of ordinary Dacron, and the fill could be made of a different kind of thread. I don't know anything about fabric weaving. But it would seem that it should be possible. I don't know how to find companies that weave aircraft grade fabrics... and whether they'd be willing to make a small batch at a reasonable price. And I don't know what kind of thread to ask for in the fill, other than maybe cotton. A small batch to a fabric weaver might be hundreds of yards.

Is it possible to weave the fabric with the fill not pulled tight, and still end up with a reasonably smooth fabric? That way it could use the same Dacron for the warp and fill. And when you shrink it, it (hopefully) wouldn't pull as tight in the warp direction. Again, I would need a fabric weaver to make a special batch. And it would be totally experimental about how lose the fill would need to be, in terms that a fabric weaver uses.

What about temporarily laying a 2 by 4 (or something) span wise on the frame, so when you glue the fabric on the perimeter (over the 2 by 4), there'll be a certain amount of slack when you remove the 2 by 4. But how do you deal with that slack when you glue the fabric at the tip and root ribs? It would seem that you'd have to glue it with tiny pleats every inch or something. And that might be impossible to do evenly, if at all.

What if you heat shrunk the fabric, just along the tip and root glue lines, before gluing the tips and roots, to avoid this problem, still using the 2 by 4 idea? You could glue the leading and trailing edges. Then remove the 2 by 4. Then heat shrink the fabric only where the tip and root glue lines will be. And then glue those.

Removing the 2 by 4 without snagging the fabric could be a problem. So It would be easier to use short segments, or something other than wood. A better material might be those long, brightly colored, tubular shaped, foam floats used as swimming toys. They're several feet long by a few inches in diameter. I think this idea might be worth a try. But it might take some experimenting to find the right diameter toy so you end up with just the right tension but no slack after final shrinking.

Do you creative people have any ideas? If we can find a reasonable solution, we can eliminate the leading edge sheeting in a way that is both aerodynamically and weight efficient.
 
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