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Author Topic: Making carbon fiber wing posts  (Read 1766 times)
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jakepF1D
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« on: June 03, 2016, 12:11:53 PM »

I'm going to start this post with 2 disclaimers. 

First, while my bench testing shows them to be stiff and durable, I haven't actually flown a model with these posts yet.  They have about half the deflection of my old boron-balsa-boron posts, but they're also about twice the weight.

Second, and more importantly, this process involves sanding carbon fiber which will produce a lot of very fine carbon dust.  I don't think it would be good to inhale any of that dust.  I do this work outside, and wearing a dust mask is also a good idea.  I've also found the dust makes my hands itch, so some nitrile gloves are a good idea.

The process is really quite simple.  I start with some 1mm ID carbon pultrusion tube from CST.  I've been starting with a piece that's about 6.75" long, and I end up with about 6" of usable material in the end.  I cut a piece of 1mm OD wire to about 7" long and insert it into the pultrusion.  I leave a small amount of wire sticking out of each end.  I wrap one end with some masking tape so the tube can't rotate on the wire, and that end goes into the chuck of a drill.

For the sanding I made a couple small blocks from 1/4" birch plywood that are approximately 1" x 3".  Each one has some 120 grit silicon carbide sandpaper attached.  I then attach some 1.2mm OD wire (.047") at each end with some rubber bands.  This wire acts as a stop and prevents me from sanding any further than 1.2mm.  It takes me about 15-20 minutes running my drill on it's higher speed setting to get the tubing down to final size.  After sanding with the blocks I finish by hand with some 600 grit to remove any visible scratches and smooth the surface.

The end result is a tube that weighs around 11mg per inch, and as I mentioned it's about twice as stiff as my old wing posts.  I'm building a new motor tube now using these carbon tubes for the wing posts, and I plan to test them at Kibbie later this month.  I've attached a few photos of my setup for anyone that's interested.
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Making carbon fiber wing posts
Making carbon fiber wing posts
Making carbon fiber wing posts
Making carbon fiber wing posts
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Olbill
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« Reply #1 on: June 03, 2016, 01:25:52 PM »

Make them half as long and you break even on the weight!
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jakepF1D
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« Reply #2 on: June 03, 2016, 01:29:44 PM »

I am going shorter, but not half.  I think my old posts are 3.5", and the new posts will be 2.75".  Considering my old models are 1.2g, I don't think the extra 25mg is important.  I'll still end up using spacers and ballast.
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jakepF1D
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« Reply #3 on: June 03, 2016, 01:52:38 PM »

I should also note I plan to use these carbon wing posts to support my bracing wire.  That will allow me to eliminate my normal center post which will offset some of the weight penalty. 

I'm also working on a method to allow easy and precise wing wash adjustment.  I'll post something about that very soon.
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jakepF1D
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« Reply #4 on: January 03, 2017, 06:40:57 PM »

I used these posts at a couple competitions last year, and I won't go back to balsa and boron for F1D.  The carbon posts are incredibly strong, and completely consistent.  No more repairs on cracked wing posts, or searching for loose boron.

I'm also using these tubes for prop spars, and again I probably won't go back to balsa.  They do carry a weight penalty, but they're so much easier for me to make than a matched set of balsa spars. 
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brabazon
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« Reply #5 on: January 22, 2017, 10:08:13 AM »

I used these posts at a couple competitions last year, and I won't go back to balsa and boron for F1D.  The carbon posts are incredibly strong, and completely consistent.  No more repairs on cracked wing posts, or searching for loose boron.

I'm also using these tubes for prop spars, and again I probably won't go back to balsa.  They do carry a weight penalty, but they're so much easier for me to make than a matched set of balsa spars. 

Having recently tried a wingpost made out of fibreglass prop blade tubing, I totally see your point. I thought of trying the pultruded tube next, but I was thinking of reaming it with a long drill. I have found such drills on ebay. Have you found your sanding works better?
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jakepF1D
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« Reply #6 on: January 24, 2017, 05:12:09 PM »

Drilling requires carbide as HSS is quickly dulled by carbon.  I'm not sure bits long enough to do this work exist in carbide, but if they do they would be expensive and easy to break.  It's also almost impossible to drill the ID of a pultrusion tube without splitting it.  Long story short, stick with sanding.
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brabazon
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« Reply #7 on: January 31, 2017, 03:11:51 PM »

if you start making your posts with the prepreg and the female mould you have, I would certainly be interested in hearing how you do it. Unfortunately I still have no prepreg as the order appears to have disappeared in the post. Undecided
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jakepF1D
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« Reply #8 on: January 31, 2017, 03:51:04 PM »

While the mold has channels for 1.25mm and 1.5mm tubes, I probably won't ever use them.  Making tubes that small is difficult, and the entire process takes several hours to complete.  It also uses expensive material I had to import from Europe.

By contrast pultrusion tubes are cheap, readily available in the US, easy to sand in less than 10 minutes, and my last set only weighed 8mg per inch.
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