Building Aero 3D Day 20: Scarf Joints

Scarf Joint Marked

Yesterday I learned how to make a scarf joint and then proceeded to make four of them. For this model the wing is one piece; however, the spars are not long enough in a single piece so need to make them out of two pieces. Since need one for top and bottom need two of each. The main spars are made out of 1/2” x 1/4” balsa and the other two out of 1/4” x 1/4” balsa.

The rule of thumb (apparently with full scall aircraft too) is a ratio of 12:1. That is for every 1 unit of thickness you need 12 units of overlapping length. I may not have explained that well so here are the numbers for my pieces.

For the 1/4” x 1/4”, since it is square the numbers work the same and for the 1/2” x 1/4” you go by when the piece is sitting in place what is the height, which is a 1/4” in our cas. So applying the rule of thumb the length of overlap needs to be 12 x 1/4” = 3″.

With that number I measured 3″ on one end of each piece. When pairing them up and doing this want to make sure you keep the two pieces you worked on together. Once I measured the length and made markings on the sides I made a diagonal line from 3 inches in to the end. You can see this in the featured image above, along with the markings of an ‘X’ on the side I wish to keep as I want to make sure they fit on top of each other.

I then sand the ends down to make a triangle and keep trial fitting until I am happy with the fit. Using the penned lines (on each side) as a guide for how much to take off and ensuring I don’t take off more on one side then the other.

Scarf Joint Sanded
Scarf Joint Sanded

Once all the pieces have been satisfactorily trial fitted it is time to glue the matching pieces together. For this I mixed together 105 epoxy risen and 205 fast hardener to glue all the scarf joints togther. You can see the end result below. There are three clamps per joint, each end and one in the middle, as well as pins to keep things in place along wtih weights as don’t want them to twist.

Scarf Joint Glued
Scarf Joint Glued

That ended another building session and wil have to see where we get on the wing during todays building session!

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