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Old Oct 02, 2013, 06:00 AM
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Bradford West Yorkshire, UK
Joined Apr 2010
1,533 Posts
Quote:
He had just about enough time to look out the window to see what the tearing sound was before the aircraft spiraled in.
Bit of a difference between a toy aeroplane and full size.

Quick test, support a handkerchief by the corners and hit centre (center) with a hammer. Do same with a same sized sheet of glass.

Been using Silks and Polyesters for years without a problem. High Stress? Use Carbon Fibre.

Regards Ian.
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Old Oct 02, 2013, 09:52 AM
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USA, IL, Wheaton
Joined Oct 2010
463 Posts
Thanks for the reminders on how to get a smooth glass/epoxy joint.

I think that model weight and stress at the joint have a lot to do with whether to use that sort of joint reinforcement. For planes of 2 pounds or heavier, especially if they are launched with good acceleration or do maneuvers, the glass/epoxy joint makes a lot of sense. Of course, carbon fiber mat can make a good substitute for the glass, if it is bi-axial or oriented the right way.

At the other end of the scale, around one ounce or less, a strip of silkspan and PVA glue would be more than enough.

I've built a 6 pound R/C trainer, a Midwest Aerostar 40, according to the instructions with a fiberglass/epoxy dihedral joint reinforcement, and been satisfied with the joint. I got some carbon fiber uniaxial tow from Steelhead Products a couple years back and found lots of uses for it reinforcing spars. In my Push Panther build, I made a pretty strong foam core wing for a 60 inch motor glider using both materials.

http://www.rcgroups.com/forums/showp...&postcount=493

For a rubber band powered P-30 weighing 2 ounces, a couple strips of the carbon fiber tow along the spars across the joint should do the job.

Only way to find out is to try it. The dilemma is that adding strength can add enough weight to increase stress and lower performance.
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Old Oct 02, 2013, 06:38 PM
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United States, MA, Waltham
Joined Dec 2001
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Don't expect much from carbon fiber mat. Basically, that lets you apply a thick layer of epoxy with a few curved carbon fibers of random orientation in it. I've tested it, and it's much less stiff than glass cloth and epoxy. Considering how much stiffer carbon is than glass, that tells you something about the structural efficiency of carbon mat. If you want to use it to build up a finish, that's another story.
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Old Oct 11, 2013, 02:41 AM
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USA, CA, Selma
Joined Sep 2004
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What about using thin CA? If you just lay a strip of 1/2-3/4 oz. cloth across the joint and put a drop of CA on it, it will spread and pull the cloth down against the wood. Just keep dropping across the cloth and it will pull it down nicely (Just make sure the cloth isn't wrinkled and where you want it, trying to smooth it before you drop on the ca will cause snags that won't pull down flat). This works just like when you put complicated tissue lettering down by laying the letter on the partially doped (dry) wing and dropping thinner on it. The thinner pulls the tissue down without wrinkles.

John
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Old Oct 11, 2013, 06:34 PM
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United States, MA, Waltham
Joined Dec 2001
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Suggest holding the cloth down with smooth (i.e. no wrinkles) plastic pulled over it. Should drop the glue to fiber ratio a bit and leave a smooth surface as well. I don't know how CA compares to epoxy in strength, but I suspect it's usually inferior. May not matter if it's strong enough for that application.
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Old Oct 12, 2013, 03:36 PM
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United States, OR, Corvallis
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My rank amature 2-cents.......

For smaller planes with lower strain, I really, really like the simplicity of good old prewetted (and patted with a towel to "damp") cotton or other cloth and yellow alphatic resin glue. I have never have had a joint fail yet with that, I've also reinforced noses of fusalages for small gliders this way that have totally augered in with lawn dart landing and had no damage.

That said - I use epoxy and fiberglass or whatever on anything bigger than "small".

Adios - Paul
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Old Oct 12, 2013, 04:55 PM
Balsadustus Producerus
Escondido, CA USA
Joined Jan 2001
903 Posts
I bought a VHS video years ago from Satellite City, the makers of Hot Stuff brand CA glues, where they used 3M 77 spray contact cement to secure the glass.

Essentially, they recommended cutting the glass to size/shape, between layers of wax paper for stability while cutting to that shape if need be, then laying the glass down on a surface you don't mind getting sticky and lightly spraying the 77 on the back side of the glass. Use very little as it goes a looong way for what you are doing here. Wait a few minutes until the adhesive is tacky but does not transfer and stick to your finger when you touch it. About like blue masking tape for tack.

Lay the glass part, sticky side down, on the airplane and smooth it out as needed, lifting and re-arranging until you have no wrinkles and it's the way you want it. Notice how the 77 helps prevent the edges from unraveling. You have a few minutes to do this before the glue dries out. If you wait too long, a light-light second coat is OK.

Pick up the now-glassed part, tip or tilt it up near vertical and add your epoxy or CA glue, starting from the top and letting it run downhill slowly. If using epoxy, a heat gun or hairdryer can be used to warm up the epoxy without letting it get so hot it bubbles, and push it down to the bottom edge of the glass with the air blast from the heat gun, but not too close. Watch the glue's progress and you'll know when to stop adding glue and let the process go on it's own. Obviously, you can't use a 5-minute epoxy for this as the heat will set it off immediately, ruining the job. I've used HobbyPoxy Formula II, no longer available, and now use WEST Systems epoxy #105 and hardener #206 (slow) with good results. I've also used this system on full-size airplanes with WEST and the German-made MGS epoxies with good results; they're still flying I don't see a reason why you could not use Nitrate or Butyrate dope here as well.

CA is similar except you don't heat it. In both cases, done properly, there will be a uniform coating of adhesive without lumps or dry spots, which can be dealt with as required if there are any. Lightweight Spackle or similar will fill any glass weave and heatshrink films will stick better to that surface. Feather the edges of the glass into the non-glassed areas at that time and you're done.
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