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Archive for August, 2019
Posted by CGordon | Aug 24, 2019 @ 03:41 PM | 810 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 | 1,048 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)