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Posted by CGordon | May 13, 2021 @ 07:46 PM | 18,053 Views
I'm working on a comic-scale version of the Mambo with 28" low AR wing and 1/16" slab side fuselage.

Since I can't leave things alone, even the well-proven fuselage box, I decided to work in some truss members behind the wing. Obviously the grain wouldn't work well as-is, so I came up with 9 different options for the truss members.

Results on the third photo.

I list them here in roughly ascending order of preference. I will be going with #8, since it feels like the right amount of stiffness and durability. Definitely I will be saving some balsa weight from the cutout sections and even what would have been some balsa truss.

I've used similar techniques in a very small Telemaster and a 92" Guillows Javelin (400%) with a 7-foot long fuselage. The balsa/carbon bond (with CA) is not a problem but could be reinforced if needed.
Posted by CGordon | Feb 23, 2021 @ 10:36 AM | 10,191 Views
I couldn't find anything on it, but it seems odd to me that ribs are so often designed with fixed width between them, regardless of how the chord may taper down. I'm not even sure this is the "right" design decision from a fabric-sag perspective (which could be another can of worms).

I'm interested in the aesthetics of this, so I mocked it up in DevWing and Excel using some simple formulas.

First up was the fixed width ribs (top), which doesn't exactly look wrong.
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Next up is the proportional spacing, which is exactly proportional to the chord. I used a factor of 1:2.21 spacing vs chord, to keep the number ribs the same for this sample.
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This one definitely didn't look right to me. But a weird thing happened. When I went back to studying the fixed width ribs, they now looked horrible to me.

Next, I tried something I didn't expect to do when I started this...I averaged the values between these. What do you think? To my eye, this looks very natural and pleasing. In fact, it rather tricks the eye into thinking the spacing is "fixed" even though it changes quite substantially.
...Continue Reading
Posted by CGordon | Mar 29, 2020 @ 04:18 PM | 12,554 Views
UPDATE - DON'T DO THIS. During the hot, dry summer in the garage, the thin MDF shrunk and the metal buckled and delaminated. It's useless now. I'm going to try something else. First, the steel was too thin, and secondly to address the realities of expansion/compression of dissimilar materials. My plan is to pick up a 14 gauge steel sheet, and 3/4" MDF. Then I will embed a matrix of countersunk round magnets into the MDF, which will secure the steel...but allow it to freely slide. I think this will work well.


While I'm waiting to get the Magnetic Fixture System, I built a magnetic board from local Home Depot supplies. I got a 26 gauge steel panel (30" x 24") and a 1/4" MDF project board which was supposedly 36" x 24" but was oversize. The oversize was nice because I was able to have a small border - no worries about fingers on the metal edges. I glued it together with a thin layer of epoxy.

The 26 gauge is thin and gives less than half the strength of the yellow "quad hands" plate. I hope I'm not disappointed in the holding ability. If so I'll have to do more legwork to track down a thicker sheet.
Posted by CGordon | Sep 22, 2019 @ 10:20 AM | 8,414 Views
Proof of concept for a carbon fiber space frame for a 24" model continues. Made of a mix of 1K tow and .010" and 0.030" commercial rod.

I will be adding upper/lower diagonals to this but wanted to capture the interesting movement of the truss in torsion.

Quite a bit more construction and reinforcement will be added but it just under 1g currently.

Carbon Fiber Space Frame Fuselage (0 min 17 sec)

Posted by CGordon | Sep 14, 2019 @ 11:24 AM | 9,134 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 | 8,804 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 | 9,204 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 | 9,508 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 | 9,370 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 | 9,969 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.