RE: 4*120 Build Thread
Alright!! Here's some more pics of the wings. I've got a bit more sheeting to do, but I can't complete it until I pull the wing off the jig.
As you can see, I've added some diagonal braces in between the rib bays. This is the same way that the 4*60 wing is built. These diagonal braces will add a tremendous amount of torsional stiffness to the wing.
Before I put the torsional braces in--the first wing off the jig was VERY flimsy feeling. I was able to hold the tip and root ribs and twist the wing quite a lot. After installation of the torsional braces--I can't budge it unless I twist pretty hard. Lots of strength, for a minimal amount of weight. The torsional braces are made from 3/32" balsa.
At the LE of the root rib bay, I've added a small balsa filler block. This is the area where the wing dowel will be epoxied into the LE of the wing. The plans only have a small lite-ply brace thats glued in between the top and bottom of the first stringer. I don't trust this little piece of lite-ply as my only means of keeping the LE attached to the wing when pulling high G maneuvers. The balsa block will add some additional glueing surface to the dowel and frontal area. Again, a minimal weight gain for a tremendous gain in strength.
Look at the 2nd and 4th pics:
This is the dihedral brace area. The dihedral brace goes into the first 2 rib bays. The brace is only 5/32" wide. But, the main spars are 1/2" wide. According to the instructions and plans, you simply notch out the first 2 ribs for clearance, and then slide the dihedral brace in untill it touches the 3rd rib. Epoxy it.
Thats not going to work for me. The problem is that there is not a "pocket" for the brace to glue into. By just sliding the brace in and glueing it to the top and bottom spars--you will get minimal glueing area. This is because the dihedral brace will only be glued to the spars. Nothing else. Remember, this brace is only 5/32" wide. Just barely over 1/4" in thickness. You have no way to contain the epoxy once you slip the brace into place. The epoxy if free to run all over inside the wing, and get on anything BUT the area where you want it.
My solution:
I've added a simple balsa web--similar to a shear web. There are 2 balsa webs. One in the front of the dihedral brace and one behind it. The front balsa web runs from the top spar to the bottom spar, and is flush with the front of the spars. The rear web is placed so that it is just slightly snug to the dihedral brace when it's inserted.
What I've essentially created is a boxed pocket for the brace to glue into. By putting balsa webs in front of the brace and behind it, there will be no place for the epoxy to run out. All your epoxy will be contained to the area immediately surrounding the dihedral brace. This provides additional glueing area for the brace as well. You will not be relying solely on the glue where the dihedral brace contacts the top and bottom spars. I now have a complete boxed pocket for the brace to glue into. Tremendous glueing area.
I used only 2 or 3 drops of thin CA to install these additional webs. They just need to hold in place. If you flood the area with CA, your likely to end up with a big goober of hardened CA INSIDE the pocket that we're trying to creat. If you get a big goober of glue in there---your dihedral brace is going to hang up and it won't fit anymore. This would be BAD.
Just tack the webs in with enough CA to hold them in place. The epoxy will do the rest when you glue the dihedral brace in.
Again, minimal weight gain for a tremendous gain in strength.
And the pics: