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Carbon fiber over fiberglass wing tube?

Old 08-01-2008, 10:11 AM
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Default Carbon fiber over fiberglass wing tube?

I am trying to make a custom wing-tube and socket for a scratch build I'll be doing this fall and winter (scratch rebuild). In the original version, I used an aluminum wing tube and fiberglass sockets, but I'm looking to shave some weight. So I decided to make a fiberglass wing tube. I've been experimenting with wrapping the fiberglass (regular weave cloth) and am having good results, but I'm not happy with the resultant stiffness of the fiberglass tube (currently three full wraps). I'm using 3/4" electrical conduit to wrap around. My options are to wrap two or three more layers of fiberglass around the tube to increase the stiffness, or I have been thinking about a couple layers of the Carbon Fiber woven tubes on the outside of the fiberglass. I'm using Z-Poxy finishing resin.

Has anyone ever tried such a structure, and will the carbon fiber layers contibute significantly to the stiffness of the tube?

Is the resultant stiffness worth the expense of the CF?

How many layers of CF woven tube would be sufficient?

If I went with straight CF Woven Tube, how many layers would be sufficient (final weight target 7 lbs or less, 64" wing span)? (Note the weight of the original was 7 lbs 4 oz with a 24" 2024-T3 Aluminum Tube 0.625" OD x 0.058" wall)

Thanks,
Brad

Old 08-01-2008, 02:59 PM
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Default RE: Carbon fiber over fiberglass wing tube?

I am a little confused. I assume the wing tube slides into the socket. If you are wrapping fibreglass around a piece of conduit how do you control the outside diameter for a propper fit in the socket?

To answer the question.

Is the resultant stiffness worth the expense of the CF?
That is for you to decide.

How many layers of CF woven tube would be sufficient?
Assuming about two oz. carbon. two layers with one layer of regular glass cloth sandwiched.

Finaly, do not use finishing resin. Use proper laminating resin such as west Systems or MGS.

Ed S
Old 08-01-2008, 03:17 PM
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Default RE: Carbon fiber over fiberglass wing tube?

ORIGINAL: bkdavy

Has anyone ever tried such a structure, and will the carbon fiber layers contibute significantly to the stiffness of the tube?
Yes, it is significantly stiffer...however, note that you may trade additional stiffness for a more brittle structure if not built strongly enough.

Is the resultant stiffness worth the expense of the CF?
I'm a boat guy so I haven't a clue! ;-)


How many layers of CF woven tube would be sufficient?
There are a lot of factors that contribute to stiffness of a laminate. A big one is the amount of pressure under which it is cured. Professionally constructed tubing is done so either with a machine that tightly wraps a cellophane tape (no adhesive) around the lamination on a solid aluminum mandrel and then it's heated to cure the resin and expand the aluminum so it's easy to remove when it cools. Other processes involve vacuum bagging the laminate around the mandrel and placing it in an autoclave where the pressure and temperature are elevated substantially. If you are laminating with hand lay-ups and curing without any additional pressure you'll need to experiment since there are a lot of uncontrolled variables. Note, however, if you wrap your lamination with one or two layers of electrical tape (pulling tightly and overlapping each round) you will achieve a good bit of pressure and a decent exterior finish ready for a little finish work...the vinyl electrical tape releases cleanly. Punch holes in it with a woodpecker style balsa perforator and you will give excess resin a method to escape the layup.

If I went with straight CF Woven Tube, how many layers would be sufficient (final weight target 7 lbs or less, 64" wing span)? (Note the weight of the original was 7 lbs 4 oz with a 24" 2024-T3 Aluminum Tube 0.625" OD x 0.058" wall)
.

Not sure but you can plan on ~roughly~ a .75 to 1 weight ratio of resin to carbon fiber (not sure about those exact numbers - someone posted them on here not too long ago). Weigh the amount of carbon you are going to use, then plan on 75% to 100% of the same weight in resin. However, I believe you will find a significant weight savings and won't need a 7lb tube.
Old 08-01-2008, 07:49 PM
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Default RE: Carbon fiber over fiberglass wing tube?

The aluminum tube is not 7 lbs. The all up weight of the aircraft is targeted for 7 lbs. The current aluminum tube weighs in at around 7-8 ounces. I was hoping to cut that down significantly by using a fiberglass tube.

As far as controlling the OD to make the socket,, I plan to sand the exterior surface of the wing tube, then wrap it with wax paper. I will make the sockets by wrapping the end of the wing tube, then pulling it out before the resin has fully cured the same way I made the wing tube with the electrical conduit. I've made the wing sockets this way before, and figure I only need two layers of cloth, as the sockets will be epoxied into the wing structure, and only need to be about 10 inches long. The wax paper will give me the clearance and interior finish on the sockets. If I need to build up a little more, I can put on another layer of resin on the wing tube.

Brad

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