15-03-2013, 09:59 AM
Finishing the cores:
Freshly wire cut EPS (Expanded PolyStyrene) cores have this strange stringy finish that has to be removed, (see the photo below) If you try to sand it off then it seems to roll around under the sanding paper and slightly mar the surface underneath. I found the nicest way to remove this stringy stuff is to lightly rub it off with a “scotch bright” pad. If there are any ridges sticking up after hot wire cutting I wouldn’t throw away the core. It can be easily saved by carefully sanding back to match the correct profile using 60 or 120 grit emery glued to a block.
After the stringy stuff has been removed, the cores get a light sand with 600 or 800 grit emery, just enough to make it smooth to the touch, we don’t want to modify the profile.
Then it’s on to making out where the spars will go. I guess that these are not really spars, but more correctly sheer webs. Their main job is to be strong in compression and stop the foam from crushing and causing the loaded side of the glass skin to kink in. All the tensional stress on the unloaded side of the wing will be taken locally by the glass skin. The plywood spar in this wing is used quite differently to how a carbon rod or tube would be used. It will add little to the span wise stiffness of the wing it’s there mainly to hold the skins apart when the wing comes under load.
Another thing I’ve done is to end these plywood pieces at different points along the wing. In any structure we never want the strengthening to end abruptly all at one point. Because it does, then a localised weakness is created, a place where it will always fail. By gradually tapering off any reinforcement the load is spread across a larger area and the whole structure becomes inherently stronger. When the skin is applied it will extend out span-wise past the ply continuing this principle.
I’ve used the off cut to support it while I cut the slots and the slots go through the bottom off cut as well. The best thing to do is find a saw with the same kerf as the thickness as the ply. Two hacksaw blades is about the right thickness. It’s better to be slightly looser than too tight.
I should have mentioned earlier that the off cuts from the wire cutting are used to support the cores for all of the sanding, cutting and glueing work that will be done to them.
Freshly wire cut EPS (Expanded PolyStyrene) cores have this strange stringy finish that has to be removed, (see the photo below) If you try to sand it off then it seems to roll around under the sanding paper and slightly mar the surface underneath. I found the nicest way to remove this stringy stuff is to lightly rub it off with a “scotch bright” pad. If there are any ridges sticking up after hot wire cutting I wouldn’t throw away the core. It can be easily saved by carefully sanding back to match the correct profile using 60 or 120 grit emery glued to a block.
After the stringy stuff has been removed, the cores get a light sand with 600 or 800 grit emery, just enough to make it smooth to the touch, we don’t want to modify the profile.
Then it’s on to making out where the spars will go. I guess that these are not really spars, but more correctly sheer webs. Their main job is to be strong in compression and stop the foam from crushing and causing the loaded side of the glass skin to kink in. All the tensional stress on the unloaded side of the wing will be taken locally by the glass skin. The plywood spar in this wing is used quite differently to how a carbon rod or tube would be used. It will add little to the span wise stiffness of the wing it’s there mainly to hold the skins apart when the wing comes under load.
Another thing I’ve done is to end these plywood pieces at different points along the wing. In any structure we never want the strengthening to end abruptly all at one point. Because it does, then a localised weakness is created, a place where it will always fail. By gradually tapering off any reinforcement the load is spread across a larger area and the whole structure becomes inherently stronger. When the skin is applied it will extend out span-wise past the ply continuing this principle.
I’ve used the off cut to support it while I cut the slots and the slots go through the bottom off cut as well. The best thing to do is find a saw with the same kerf as the thickness as the ply. Two hacksaw blades is about the right thickness. It’s better to be slightly looser than too tight.
I should have mentioned earlier that the off cuts from the wire cutting are used to support the cores for all of the sanding, cutting and glueing work that will be done to them.
“The knack of flying is learning how to throw your machine at the ground and miss.”
"When once you have tasted flight, you will forever walk the earth with your thoughts turned skyward, for there you have been, and there you will always long to return." ~Leonardo Da Vinci