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May 11, 2016, 06:07 PM
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Question

Tubes for Carbon Fiber Spars (in removable wings)


Hi,

Simple question but important nonetheless. I am building an aircraft this summer that features removable wings. The design is mostly done, but I need to figure out the wing removal part. Since this plane is built-up balsa/bass construction, I need something to run through the spars that can serve as a guide for the 5mm OD carbon fiber spar. Obviously, I need some type of tube that is slightly over 5mm ID (or exactly 5mm ID, in which case I could sand down the CF somewhat.)

Any ideas on this? What do people use? Where do ARF manufacturers get these tubes?

Thanks!

Jim
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May 12, 2016, 06:16 AM
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I'm planning a similar feature for my new build. I got both carbon tubes at
my local hobby shop. Not a perfect fit, but with some shims it should be a firm, smooth fit.

My design has a glued-in smaller tube through the center section at 90 degrees to everything.
The wings will have the outer tube part-way out the wingspan, at an angle highest inboard
to establish the dihedral angle, passing through holes in the ribs just behind the main spar.
Bonded to both ribs and spar, nothing will be moving.

Support for the "round the tube" axis will be small hardwood dowels fore and aft at the inside rib
into the fuselage; all held in place when installed by a screw from the inside into a thread into
the wing root. Torque from the wing-tip lift should bind the tubes on the inside with friction, so
all else is there to hold position.

Hope this helps.

Robyn
May 12, 2016, 02:15 PM
An itch?. Scratch build.
eflightray's Avatar
I generally use paper tubes as 'guides' through the ribs, the ribs and the main spar do all the load work.


Ray.
May 12, 2016, 03:43 PM
Registered User
Thread OP
Thanks guys, both good suggestions. I tried a method similar to Robyn's, but the issue with that is it's not always easy to find CF rods with inner diameters equal to the necessary outer diameter.

That paper tube method looks plausible though. I may give that a try later.

Thanks!
May 13, 2016, 10:09 PM
Registered User

google is your friend


Google "telescoping carbon fiber tubes". There are a bunch of vendors out there. D.H.
May 13, 2016, 11:01 PM
The Junk Man
The soaring community has making glass sockets for carbon tubes down to a science. Check them out for a bunch of "how to" posts.

Tom
May 23, 2016, 07:55 PM
Registered User

paper guide tubes


Hello Jim,
This is probably late but I just found your thread. I use two or three layers of printer paper to make my tubes and then tie them to the ribs, spars and-or spar webs for support. Almost all of the loads are transferred to the wing and not the tubing. The following is what I do;

Place a towel on your work bench for a softer surface. Using your joiner (carbon fibre, aluminium tubes or dowel) as a tool, place a sheet of copier paper on the towel, roll the joiner over the paper several times. The paper will curl on its own.

Now wrap the paper tightly around the joiner forming a tube. Mark the width of the paper to get the number of layers you want, unwrap the paper and cut it to width.

Re-wrap the paper to a tight fit. You can use a slight twisting motion to make the paper tubing a very tight fit with no slop. When you are satisfied with the fit ( your joiner should slide in and out with only a bit of friction), lay your joiner- tubing on a hard surface, unroll about a half inch of paper and apply some glue with a glue stick...Elmers, ect. Re-wrap the edge to the paper tube and press the glue into the paper.

Retract the joiner inside the paper tube and carefully apply a small drop of thin super glue to the inside over-lap. Take care not to glue your joiner to the paper. After the glue has cured, repeat at the other end. Remove the joiner and apply thin SG to the outer over-lap. Cut the paper tubing to length, glueing the inner over-lap the same as before.

I have used this method on power aircraft up to 70 inch span and gliders over 100 inches, using spar joiners up to 1 inch in diameter. The paper will take any glue and has held up well over the years.

Paul
Aug 07, 2016, 12:44 AM
Registered User
I got mine at Rock West. I like how they give some reference data for most of their tubes (only for standard modulus tubes. None for intermediate modulus yet). I just do a simple stress calculation (sigma=M*y/I) to make sure the expect stress level is less than the flexural strength with a factor of safety.

Plenty of other places also sell tubes but not too many give the technical data.


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