Hi everybody,
After being inspired by the work of Rol Klingberg on YouTube building his new ultralight glider, I decided to test out and idea that I had for making the D tube skin using the shear web and ribs as a male mold instead of hot wiring a dedicated male mold. This is of course a trade between build time and the quality of the resulting surface, and I don't claim to have worked out the best way to implement this process yet, so I welcome suggestions for improvements and input from those more experienced than I about how this process might scale up when you try to do an entire wing with it.
As you might imagine, the central problem is that once you pull the skin tight over the ribs, the leading edge will sag between them due to the tensile compliance of the foam core, and the leading edge radius will balloon out between the ribs due to the bending stiffness of the foam. The key idea I had is to sandwich the layup on both sides with a material that has high tensile stiffness and high bending compliance in order to control those undesirable tendencies. For this, I used some medium weight poster board from Staples, which is a bit thicker than cardstock.
I started by making ribs out of some random foam and a shear web with composite flanges in the klingberg style. The rib spacing is 200mm on one side, 400mm on the other, and the sections correspond to the first 25% of a 750mm chord wing. (I botched the molding of this part pretty good, I know)
Taping posterboard over the ribs, 1/8in divinycell is formed as close to finished shape as possible with a heat gun to minimize the foam's spring-back.
Then one layer of vacuum bag goes over the ribs, then the posterboard, then breather, peel ply, layup, peel ply, breather, posterboard, outer vacuum bag layer. The vacuum bag layers are sealed together, the vacuum is pulled, and then the whole assembly is taped down tightly to the ribs and shear web. (full disclosure, I botched this vacuum bag pretty bad and didn't get good compression)
Much of the D tube surface appears quite nice. That kink near the aft edge is due to my attempt to make an integral spar cap in the sandwich core, which is a story for another thread.
Around the leading edge, there is a very slight bump on top of where the center rib was, but it's hard to spot. Both edges of the D tube are also turned up slightly, so on a full wing an inch or two of extra span might need to be built and then trimmed off. Across the longer rib spacing section, I laid a straight edge over the leading edge and measured the sag to be around 1mm.
After being inspired by the work of Rol Klingberg on YouTube building his new ultralight glider, I decided to test out and idea that I had for making the D tube skin using the shear web and ribs as a male mold instead of hot wiring a dedicated male mold. This is of course a trade between build time and the quality of the resulting surface, and I don't claim to have worked out the best way to implement this process yet, so I welcome suggestions for improvements and input from those more experienced than I about how this process might scale up when you try to do an entire wing with it.
As you might imagine, the central problem is that once you pull the skin tight over the ribs, the leading edge will sag between them due to the tensile compliance of the foam core, and the leading edge radius will balloon out between the ribs due to the bending stiffness of the foam. The key idea I had is to sandwich the layup on both sides with a material that has high tensile stiffness and high bending compliance in order to control those undesirable tendencies. For this, I used some medium weight poster board from Staples, which is a bit thicker than cardstock.
I started by making ribs out of some random foam and a shear web with composite flanges in the klingberg style. The rib spacing is 200mm on one side, 400mm on the other, and the sections correspond to the first 25% of a 750mm chord wing. (I botched the molding of this part pretty good, I know)
Taping posterboard over the ribs, 1/8in divinycell is formed as close to finished shape as possible with a heat gun to minimize the foam's spring-back.
Then one layer of vacuum bag goes over the ribs, then the posterboard, then breather, peel ply, layup, peel ply, breather, posterboard, outer vacuum bag layer. The vacuum bag layers are sealed together, the vacuum is pulled, and then the whole assembly is taped down tightly to the ribs and shear web. (full disclosure, I botched this vacuum bag pretty bad and didn't get good compression)
Much of the D tube surface appears quite nice. That kink near the aft edge is due to my attempt to make an integral spar cap in the sandwich core, which is a story for another thread.
Around the leading edge, there is a very slight bump on top of where the center rib was, but it's hard to spot. Both edges of the D tube are also turned up slightly, so on a full wing an inch or two of extra span might need to be built and then trimmed off. Across the longer rib spacing section, I laid a straight edge over the leading edge and measured the sag to be around 1mm.
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