Spar reinforcement for Simple Cub

[Andy.T.]

I'm building a Simple Cub right now, and pondering something for not the first build without feeling like I have a good solution. The wing for the Cub is not straight. How would one go about reinforcing the spar on a wing like that (high wing, joined in the middle, with one or two angles in it)? I don't want to worry about whether or not a foam-only wing is going to hold up when I'm flying w/ floats on or hanging some bombs from under the wing, etc. But, the wing is glued and folded shut, then the poly or dihedral is added. When I was trying to decide what to do w/ the Explorer when I had to rebuild a wing with more reinforcement than just the foam spar, I opted for just building the wing flat, no dihedral. But, I am thinking I'd prefer to build the Cub with dihedral, so now I feel like I need to solve the problem of reinforcing the spar before actually gluing the wing shut. Or do people tend to glue shut, add, dihedral, and then cut into the wing again to add the spar reinforcement?
Always interested to hear others thoughts.

 

[Merv]

I use barbecue skewers, imbeded into the foam. Before you fold the wing, cut & dig a little slot for the skewers to sit in. Similar to a slot for an A or B fold but narrower, just wide for the skewer. You want them to be directly on top & bottom of the foam spar. Fold the wing as normally & install the skewers as you join the wing halves. Use the glue of your choice, I prefer polyurethane for the longer working time. The skewers are flexible enough that the dihedral is no problem. For normal flying one on top & one on the bottom should be enough. I fly high G maneuvers & use two top & bottom, placed a couple of inches apart. I have not been able to break it yet but I'll keep trying.

 

[Tench745]

The wing is pretty strong as is, without extra reinforcement, so long as you tape over the center joint.
You could make some kind of wood or plywood doubler, glue it to the spars, then fold over each wing panel one side at a time. Might take a little finessing go get the skins to lay nice along the center seam, but nothing too difficult.
I have had luck on other planes, cutting a slot in the lower wing skin, smearing gorilla glue on a popsicle stick, and then sliding it up into the wing to reinforce the center join after everything is assembled.

 

[Foamforce]

I’ve done lots of loops in two different cubs and never had a wing fold. I only have a normal 1000kv B pack, so it’s not a fast plane, but I’ve done dives into loops. I’ve always been surprised how strong these wings are without some sort of additional spar.

 

[Spitfire222]

You could make some kind of wood or plywood doubler, glue it to the spars, then fold over each wing panel one side at a time. Might take a little finessing go get the skins to lay nice along the center seam, but nothing too difficult.

This is what I did when I built my first Simple Cub a week or two ago.

Here are the steps I followed:
1. Glue the wing halves together on a flat surface, as normal.
2. Cut ONLY THE UPPER WINGS all the way through at the dihedral lines.
3. Score the bottom wings on the dihedral line
4. Using the dihedral gauges, raise the wing tips and place a weight on the center section to keep it flat. Apply glue on the seams to get it to hold that angle, let it dry thoroughly.
5. Glue in the foam C spar. You can make some score cuts to help it follow the dihedral, but I found the spar bent fine without it.
6. The main step: use a hardwood dowel rod, or small section of carbon fiber tube as a spar. Not a bamboo skewer, they're too flexible. You don't need it to get from tip-to-tip, I cut mine to a length that keeps it within the wing. (See image for detail)
7. Fold over and glue the upper, middle section of wing.
8. Fold over and glue each half of the remaining upper skin, trimming the inner edge as needed for a nice joint with the center section.
9. Add packing tape chord-wise to reinforce the joints.

Hope this helps! That added solid spar really gives me more piece of mind.

 

[Piotrsko]

If you do the typical cub wing and attach it with 4 only #32 rubber bands, it will probably seperate the wing before it folds unless youre attempting a square loop with a big battery. Probably separate the rubber bands first. Folding a cub wing will require you to collapse the top skin and or the spar by compression. Second failure is by the bottom wing joint comming apart. Third point of failure is having 6 #64 or 8 #32 rubber bands and compressing the top skin.... reinforcing the spar with pieces that make the spar an I-beam means you'll need to break something else.

 

[Andy.T.]

super interesting, thanks as always guys. Next wing I'll get over myself and cut into it after I put it together with the correct dihedral.