Lumenier RB2205C-12 2400KV SKITZO Ceramic Bearing Motor
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Old May 07, 2012, 02:39 PM
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Originally Posted by BavarianCharles View Post
Speed is good - wait until you find yourself downwind and have to come home against a breeze, and watch it penetrate. Makes a Blaster look like it's standing still.

Tell us more - CG, control surface throws, etc.

The CG is at 72mm. I checked it before I left to fly yesterday. I'll get back to you on the control surface throws as I revised them while flying and I don't have them measured.
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Old May 07, 2012, 02:41 PM
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I would expect compression failure to be more likely than tension failure as well. Kevlar is strong in tension but rather weak in compression. Also for our relatively thin shell structures, buckling failure is what will lead to the failure. A structure can't buckle on the tension side.

Gerald
Old May 07, 2012, 02:45 PM
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The compression strength is really a function of the resin used, right?
Old May 07, 2012, 03:14 PM
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Somewhat. If the resin has enough stiffness and strength to prevent the fibers from losing their alignment and buckling, then the fibers (glass and Carbon at least; less so for Kevlar) take the majority of the load. This is a function of alignment of the fibes of course. Edge-on, a fiber will have its greatest compressive strength and stiffness, provided it is adequately supported by the surrounding matrix of epoxy and fibers.

To get an idea, look up the compressive stiffness and strength for epoxy of your choice, and compare to compressive stiffness and strength ratings of various fiber composites in epoxy.

Gerald
Old May 07, 2012, 04:06 PM
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Experience. Gavin and I have each failed one of these fuses, and both failures were on the throwing peg side of the fuse, starting as a little discoloration, probably due to cracking in the resin and ultimately letting go on a later throw. They both let go between the LE and the canopy opening, near the canopy opening. These were earlier layups with fewer layers but the mode was the same in both. With the current layup I doubt that small holes will have any effect at all.

Pull strings, absolutetly... you don't want the ballast banging on them when you throw the ship.
Old May 07, 2012, 04:14 PM
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Originally Posted by Mr. Wiz View Post
The compression strength is really a function of the resin used, right?
The compression strength of most resins is around 15,000 psi. Unidirection carbon with a Vf of 60% and aligned with loading axis is around 175,000 psi. Move the orientation to 45* to the loading axis and you can see the comp strength of the same UD layup drop to around 35,000 psi. The resin does matter but nearly as much as the fiber and its orientation.

As Gerald mentioned, our thin laminates typically fail from compression forces but not from a lack of compression strength. Our thin laminated buckle long before the compression strength of the materials in reached. Buckling and compression strength are not the same thing. Wall thickness and the material stiffness play the largest roll in preventing laminate buckling.

Toms fuses are very robust though....and it probably doesn't matter. It would be fun to watch a frontal fuse failure in slo-mo.
Old May 07, 2012, 04:18 PM
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I would tend to agree. I think I could use that fuse to knock a ball right out of the park!
Old May 07, 2012, 04:35 PM
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Originally Posted by wyowindworks View Post
It would be fun to watch a frontal fuse failure in slo-mo.
I have a new goal. Anybody got a high frame-rate camera and an afternoon to kill? I would gladly sacrifice a beater for a good video clip
Old May 07, 2012, 04:43 PM
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I have a new goal. Anybody got a high frame-rate camera and an afternoon to kill? I would gladly sacrifice a beater for a good video clip
The best I can do is 60 fps.....which isn't enough. I've tried many time to capture video of destruction testing. I'm pretty sure it takes something in the thousands of frames per second.
Old May 07, 2012, 04:55 PM
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Or a strobe.

Gerald
Old May 07, 2012, 05:15 PM
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Originally Posted by tom43004 View Post
...both failures were on the throwing peg side of the fuse, starting as a little discoloration, probably due to cracking in the resin and ultimately letting go on a later throw. They both let go between the LE and the canopy opening, near the canopy opening.
That's where the compression zone is for the front part of the fuselage. Imagine applying an instantaneous 2lbs force to the vertical tail towards the peg. Now where will the nose bend? The nose will bend towards the peg. So the failure was indeed in the compression zone, makes good sense.
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Old May 07, 2012, 05:22 PM
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Ah, so you're saying that the force that broke the fuse isn't the pulling of the nose away from the thrower, but the recoil of the tail after release?

That makes far more sense.
Old May 07, 2012, 06:37 PM
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Here's a few picts of my install. The 285's are snug. I put them on a plywood/balsa mounting plate that I hold in the fuse with a couple servo screws. They lay in the fuse at a slight angle.

For the wing mounts I found these aluminum threaded blind rivets. I set them in a piece of plywood and then glue in the fuse with epoxy/micro balloons.

Hope this helps.

RdJay
Old May 07, 2012, 06:52 PM
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Thanks Rick. I knew you and a couple of other guys had used 285s so I was hoping you'd post.

That heating element is still running strong in my hotbox. I expect it to outlast me
Old May 07, 2012, 07:04 PM
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Happy to share. Flew all three of my scratch built Zone V2 wings on these fuse's last weekend and they are working great. If anyone needs more or closeup pictures let me know.


RdJay


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