|Jan 08, 2014, 11:05 AM|
Elevator Hardpoint Materials
I had the chance recently to examine the elevators on two molded stabs. I was intrigued with how the hardpoint mounts were incorporated and formed smoothly with the skins.
In the past I have either embedded a vertical grained balsa plug and sanded the plug flush before bagging or hollowed out a section of the foam after bagging and injected a mixture of epoxy and cabosil fibers to form the plug.
This is a minor question but what product filler do molders use to mix with epoxy for the hardpoint and how do you smooth the epoxy as it cures to confirm to the stab core prior to adding the skins? Does one make a fairly dry mix of filler and epoxy that is added by spatula and smoothed over with it? Does one a brass tube as a cookie-cutter of sorts to remove the hardpoint foam section?
Thanks in Advance.
|Jan 08, 2014, 12:23 PM|
United States, MD, Lusby
Joined Nov 2003
I can answer on hollow wings: I use a mixture of chopped fibers, graphite and microballons with Resin Research 2050 laminating epoxy, syringed into a depron ring front and back. There might be a tiny bit of foaming agent left over, but I'm mostly only using that in the TE and for joining the foam shear web to the upper and lower skins. I had a rear hardpoint let go due to low surface area and a void in the epoxy as a result of the foaming agent. The molds themselves squeeze the epoxy to the correct shape, excess ends up inside the wing or possible squeezed through the two mold halves, so go light! I'm also adding carbon patches around the hardpoints now, which is why my wing weights haven't come down but the overall strength has gone up and wing weight has remained constant (128g).
Looking at the hardpoints in the molded tails I have, they don't use foaming agent, just cabosil+microballons+epoxy. Again, excess ends up in the skin. I suspect the hardpoints are drilled when the foam core is milled, or maybe done on the drill press before the layup. Some tails might not have epoxy hardpoints in them, relying on upper and lower carbon only, but I'm not 100% on that.
|Jan 08, 2014, 12:48 PM|
United States, CA, Tehachapi
Joined Jun 2011
I've tried using a patch of 6oz carbon top and bottom instead of a hard point in a stab. The foam still compressed under the pressure from the screw, so I ended up adding a hard point afterward anyway. On all of my verticals I've been using for testing I've added an epoxy/microballoons mixture to a pre-cut hole in the core during layup. So far it's worked very well. I cut out a hole in the core where the hard point will go and then when it comes time to close the mylars I lay the core on one mylar, spoon enough of the mixture into the hole to fill it, and then lay the other mylar on top. As long as I get enough to fill the hole I don't get a dip or anything from being under vacuum. It's just a smooth continuation of the skin.
|Jan 08, 2014, 02:23 PM|
Thanks. I hadn't thought about adding the mix during the bagging or molding step. Now I understand how the stabs I saw had the CF spar embedded in the hardpoint as nicely. The mix just flowed around it. I was trying to avoid microballoons because they can "break" if crushed, and not using the cabosil fibers I have because it might not flow as well. I know Wyoming Wind Works uses some sort of filler for the white stab mounts he made. The filler did not appear to be balloons or fiber.
I think I tend to avoid 6oz CF cloth. I have wetted some CF cloth out and then rolled it between 2 pieces of plastic to spread out the tow and make it thinner. I did notice that the Uni on the stabs was much thinner, and when I saw the 38 g uni from R&G, I understood why it was so thin. Most of my uni is at least twice as thick.
The one thing I know from my building is that I tend to "overkill" whatever spot I try to reinforce. I have never had a stab fail, so I just need to think and work lighter.
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