F18 Build - Part 4

I did not have a huge amount of time tonight but managed to lay up the top spars into the wing. I decided to use 12K tows for this as it is less "ropey" than the 60K tows I have and it is easier to get the epoxy to properly penetrate the carbon strands without making one moer-of-a-mess ;o)

First step is to decide how many tows to use. As I have always used spruce spars in the past in all my slope models I had no clue as to how many is enough. I just cut some small pieces and stacked them together until I got to about what I thought "looked" strong enough. Based on the "long-term" Allegro build I am also busy with, which used 4 layers of uni-directional carbon to get to the prescribed 0.7mm thickness I opted for six of the 12K tows. In hindsight If I had enought time I would have opted to use the same technique to make pre-pregnated strips (as described here) from UD carbon and just epoxy them in afterwards.

The depth of the groove was just right as the 1.5mm balsa cap I put on top stood just proud of the wing top surface.

The last bit of epoxy mix was thickened to a syruppy consistence slurry with microballoons and used as a filler to close up the tiny gaps. A maxed-out useless credit card took care of the excess epoxy to prevent too much sanding afterwards. To prevent the wing "negatives" sticking to this beautiful piece of handiwork I cut two narrow strips of Mylar which I put over the spars.

And the last task of the evening before I get some much needed sleep was to weigh the whole setup down. This is where those old UPS sealed lead-acid batteries come in very handy. Tomorrow evening I do the bottom spars.