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Old Feb 01, 2015, 10:48 PM
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Help!
CF joiner construction

I need to make a CF joiner rod for an 8m scale Nimbus4 sailplane I'm scratch building. (build posted in Scale Sailplanes). It needs to be a rectangular shape, 3/4" x 1.5" x 40" to allow for wing dihedral. It will consist of 175 or so, 50k cf tows, layed up in a piece of 3/4" x 1.5" aluminum channel. Question: after the tows are wet out and laid out in the channel, if I put a flat, rigid "top" piece over them that just fits inside the channel, allowing it to press down between the channel sides, and bagged it (my vacuum goes to 25 lbs.), will the vacuum push the top piece down, squeezing and compressing the joiner tows adequately, or do I need to squeeze it together with a bunch of c-clamps? Thanks.
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Old Feb 01, 2015, 11:28 PM
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Asher Carmichael's Avatar
Guntersville, AL
Joined Jun 2004
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Bob,
It should push it down but it may not be consistent, yielding a joiner that may not be the same thickness at each end. You might consider adding a plate on top of the "floating" cap that will bridge across the edges of the channel and stop how far the cap will depress into the channel. I question whether it is really necessary to have CF tow at the extreme surface of the joiner.
On another note, you might consider adding a layer of thin tape on each surface of the mold so that you could use a piece of material sized to the mold channel as a spar box in your wing. The tape will give you some clearance. This might make "building" boxes around your joiner unnecessary. Make sure to wax and use PVA.
I'm sure others with more experience will chime in with other options.
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Old Feb 02, 2015, 12:33 AM
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Originally Posted by Asher Carmichael View Post
Bob,
It should push it down but it may not be consistent, yielding a joiner that may not be the same thickness at each end. You might consider adding a plate on top of the "floating" cap that will bridge across the edges of the channel and stop how far the cap will depress into the channel. I question whether it is really necessary to have CF tow at the extreme surface of the joiner.
On another note, you might consider adding a layer of thin tape on each surface of the mold so that you could use a piece of material sized to the mold channel as a spar box in your wing. The tape will give you some clearance. This might make "building" boxes around your joiner unnecessary. Make sure to wax and use PVA.
I'm sure others with more experience will chime in with other options.
Hi Asher, The channel is only 1/16" thick, so I did plan to line it with packing tape, waxed, for the mold, and the channel is going to be used for the 3 boxes, wrapped with cf and Kevlar. Actually, the 1.5"channel is made out of 2 - 3/4" x 3/4" angles, supported on a base and on the sides to maintain size, so I can take them apart to release the joiner. I just didn't know if the vacuum would be adequate if only the top was floating or "exposed". I only have a few clamps, so also could stack weights on it, but it would have to balance. I'll press it when it looks full, and keep adding tows until it is level when pressed. The rigid top piece on the floater sounds good to keep one end from compressing unequally or going too far if the pressure isn't equal . All opinions welcome. Thanks.
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Old Feb 10, 2015, 01:25 PM
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Aluminium channel that thin will distort and bend with any pressure that is applied.
I use heavy walled RHS (rectangular hollow section) for this purpose; much more rigid.
As for opening question, I use clamps to make my joiners in a similar way, only because they are more controllable for the situation.
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Old Feb 12, 2015, 10:50 PM
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.75" x 1.5" x 40" joiner rod

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Originally Posted by Jim.Thompson View Post
Aluminium channel that thin will distort and bend with any pressure that is applied.
I use heavy walled RHS (rectangular hollow section) for this purpose; much more rigid.
As for opening question, I use clamps to make my joiners in a similar way, only because they are more controllable for the situation.
Jim, What I am planning is to use a piece of 3/4" x 14" x 40" MDF board as a base. Then take two 3/4" MDF boards, each 6" x 40", and screw them down to the base, leaving a 1.5" space between them the length of the base. The thin 3/4" x 1.5" x 40" alum. channel, which is actually made of two 3/4" angles, will fit snugly into the space, and will act as a uniform liner. I'll line the alum. with packing tape. The 1.5" MDF floating top should keep the top at the correct width. I don't have many clamps, but my vacuum pump holds about 25" Hg., or around 12 p.s.i., which figures out to be over 700 lbs. total pressure. I'll definitely make a short practice one 1st, as the final joiner will use somewhere around 175 , 40" x 50K tows. It can be disassembled to remove the joiner if necessary. What do you think? Thanks. Bob.
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Last edited by flyingfever; Feb 12, 2015 at 10:52 PM. Reason: sp
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Old Feb 13, 2015, 12:34 AM
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I think I'm reading your method right, so I'll throw this in: I made a joiner mold for RCRCM Typhoon joiner from MDF base, Aluminium angle sides screwed to the base. I put an existing joiner in the mould to work out the dihedral angle and to mould a "pusher" on top of the joiner. So, the wet out tows go into the joiner mould, the epoxy splooge "pusher" goes on top, then a pile of briks on top. The bricks push the "pusher" down compressing the tows, until the bricks rest on the ali joiner sides. Then the pusher can't be pushed any further and the joiner is at it's perfect thickness. Any excess epoxy was pushed out the ends of the mould and trimmed away. The mould's just unscrewed apart to release the part. The joiner fits and works perfect. An 8m Nimbus, I guess you'd use more substantial materials. Oh yeah, my mould took about 25 min of work to make .
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Old Feb 13, 2015, 03:50 AM
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Originally Posted by flyingfever View Post
Jim, ........................... What do you think? Thanks. Bob.
What do I think? A simple drawing would help to clarify what you mean considerably.
My old head tends to spin when trying to read the description.
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Old Feb 13, 2015, 09:33 AM
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Asher Carmichael's Avatar
Guntersville, AL
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Bob,
I think I understand and it should work. As Gizzo pointed out( and I mentioned earlier), a "limiting" cap will be needed to control the thickness. If possible, could you make the joiner thickness the height of the aluminum channel walls and simply cap on top of the channel edges? Not as elegant as a molded cap as mentioned by Gizzo but workable.
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Old Feb 13, 2015, 07:52 PM
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Joiner mold.

This will be a flat CF joiner, 50K tows with no dihedral. The joner thickness = the channel walls and form boards. The whole base & joiner asembly will be put in the vacuum bag. It's kinda hard for me to explain the plan, so here's a couple pictures of a quick mock-up of the joiner form. The top will "float", and as suggested, have a limiting stop, , or ledge to keep it from compressing unevenly. In order to make sure I have enough tow in there after compression, I could fill it to the top of the channel, cover the tow with a strip of porous release film and a strip of peelply, then put on the floating top, heldin place with some strips of tape, and vacuum compress it. Then, after a few minutes, or maybe even the next day, if it's not level with the top due to compression or resin seepage, remove strips and add tows to level it off and vacuum it again. I'll make a short trial joiner before the final 40" one. ALL ideas and posts much appreciated. Now, what do you think? Thanks. Don't know how I got 2 of each picture--sorry.
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Last edited by flyingfever; Feb 13, 2015 at 08:54 PM. Reason: pix
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