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Posted by CGordon | Sep 14, 2019 @ 11:24 AM | 884 Views
Continuing the experiments for using carbon tow stretched across a truss shape, this is a very early result for a fuselage side for my 24" Telemaster.

I used 1K tow, about 10ft, and wound it through the nails, doubling up in some places. With 1K, if you were organized you could come up with a winding pattern that reinforces areas according to the need.

As a first test, I was nearly sure that 1K would be a joke. But it might just work. The structure is stiffer than I expected and not a noodle. I do have commercial .010" rod and the cured tow isn't too far off the mark for stiffness. I used a tapered, stretched piece of heat-shrink to compress and round the tow.

I was going to do a 3K test next but instead I'm going to repeat it with 1K so I can assemble a full fuselage and get a feel for the handling characteristics. If it proves viable I'll need to keep practicing to improve my skills. This section was unglued in 7 points and needed CA . The points that did bond successfully are quite good and are "free" from a weight perspective.

All in all this has been a fun start.
Posted by CGordon | Aug 24, 2019 @ 03:41 PM | 662 Views
Remember those potholder looms for kids? It gave me an idea...

This is another application I have not seen - an all-carbon truss spar. Using wetted carbon tow (12K in this case), I wove a truss pattern though a set of brads. The idea (Pratt or Warren without verticals truss styles anyway) is to be able to slide ribs into a finished spar unit.

For fun I tried some different patterns with string (second photo). I notice you can take an extra round on the perimeter if you need more thickness.

In practice I found the tension tends to flatten the tow, and this isn't ideal since you'd like more of a round or square section. Despite this the test is very sturdy and certainly benefits from not having any glued joints.

I'm thinking about building a fuselage side like this for my 24" Telemaster. I could use my thickness sander to ensure a flat, uniform surface and make the carbon closer to a square profile.
Posted by CGordon | Aug 23, 2019 @ 09:28 PM | 897 Views
I'm surprised I haven't seen this before...using carbon fiber rods in a built-up spar. In this case, I used the very smallest rod I have (.020") on a 24" wing, along with depron for webs.

With no webs, it is almost uselessly floppy. Instead of using balsa, I opted for depron just to see what would happen. Surprisingly it is quite stiff within the normal parameters. At the edge of the limit, it has interesting characteristics! You can see in the video how the buckling behaves. I have no doubt the bottom spar (same size) is barely breaking a sweat. The cracking you hear is just the top of one of the ribs...still planning to fly with this thing!

I can see replacing the web with balsa...but perhaps depron would still be perfect for the outer half of the wing.

Construction is not hard. To get a consistent hole distance I added little bits of sharped rod to the rib template. It cuts the holes and also keeps the balsa from sliding while cutting. The ribs are then loaded on to the spars shish-kabob style. The webs are then loaded and glued to the rods.

This wing comes out at 6.7g and will be covered with tissue.

Carbon rod spar idea (0 min 20 sec)

Posted by CGordon | Apr 17, 2019 @ 07:07 PM | 1,284 Views
I wanted to try a darker hardwood and much thinner balsa than the last experiment. This time it is 1/16" balsa with teak veneer. The strips were glued with yellow glue and pressed overnight to dry. The teak was surfaced down to just a few thousandths on each side.

Next, a strength/weight test. I cut a very narrow strip and a wider strip of balsa of about the same weight. The balsa was slightly heavier and slightly stiffer, but I'd say these were fairly matched. No question that the teak was doing the work of carrying the tension and compression loads nicely.

There's probably a geometry of teak/balsa which exceeds plain balsa, but hard to optimize at this tiny scale.

I love the look of teak and I could create a slab fuselage with this technique that would look really cool. Tail pieces too. And I've proven I can do this without a weight penalty (especially if factoring in a lack of covering material).

As always, fun experiments with little need to have a practical application.
Posted by CGordon | Apr 10, 2019 @ 10:56 PM | 1,107 Views
I wanted to see if a small balsa square stick could be strengthened by a tiny amount of basswood on the outsides - much like a spar.

I glued thin (think veneer-like) strips to a balsa stick, and then I reduced the thickness of the finished glue-up the next day using my surface sander. I only left about .010" of basswood per side, which you can see along with the dark glue bands in the second photo. (Not the large balsa grain in the middle)

Results - this is much stiffer than plain balsa, but adds little weight. I went ahead and failed it, and predictably it failed in compression. The failure was much more friendly than the twig-like snap! you get from a balsa stick. If forces were coming from one direction, the optimum structure would have one side of basswood twice as thick as the other.

You could make a really strong truss with this material. It would also be good for tail construction, being both stiff and light.
Posted by CGordon | Apr 02, 2019 @ 09:04 PM | 1,155 Views
I enjoy preparing sections of material and surface sanding them down to see the unique resulting properties.

Typically with these tests I remove a huge amount of material because otherwise the material(s) would be too thin to hold together for sanding, or be glued with integrity, or with a perfect finish, etc. In the end something comes out that is somehow greater than the sum of its parts.

I have no practice application for this one in mind but thought it was interesting. Just sharing here.