Strong, lightweight material to patch LARGE hole in balsa?

[Rktman]

Hi All,

Need some help with repairs on my glider, so hopefully this is the right Forum section to get that. As you can see from the pics, my upscale Manta suffered a shattered fuselage from using a too powerful motor (underside of fuse is at the top).

I couldn’t find the missing chunk of balsa so I bridged the gap with a thin 1.5mm carbon rod after gluing the fuse together as best I could. I intend to reinforce the area on either side with two plates of either 1/64” or 1/32” ply, but I suspect it won’t be enough with that gaping hole still separating the forward and back halves of the fuse.

Any ideas on what to fill it with to strengthen that area without adding a whole lot of extra weight to the glider?

 

[James Owen]

I haven't done this with balsa, but I have patched holes in wood parts before. I generally (over)fill the hole with copious amounts of wood filler (the kind that comes in what looks like a big toothpaste tube), let that dry, sand it down, and then do a light coat of some fast curing epoxy for a smoother finish (the epoxy also increases the strength of the patched hole).

hope this helps.

 

[Rktman]

I haven't done this with balsa, but I have patched holes in wood parts before. I generally (over)fill the hole with copious amounts of wood filler (the kind that comes in what looks like a big toothpaste tube), let that dry, sand it down, and then do a light coat of some fast curing epoxy for a smoother finish (the epoxy also increases the strength of the patched hole).

hope this helps.

Wood filler like Elmer's CWF? Doesn't that dry kind of heavy? I like the idea of the epoxy. Do you think it would be possible to mix the wood putty with a small amount of epoxy?

 

[lakeroadster]

I'd get a piece of balsa and make sawdust out of it using a back saw. I'd then mix it up with some gorilla wood glue and make a paste of it. Then I'd use that to fill the void. You may have to use some wax paper to make a form to keep the paste in place on both sides of the part.

 

[James Owen]

Wood filler like Elmer's CWF? Doesn't that dry kind of heavy? I like the idea of the epoxy. Do you think it would be possible to mix the wood putty with a small amount of epoxy?

here's the stuff I use. it dries pretty light.

 

[rocketguy101]

you might be able to use epoxy and glass beads.** I don't recall where I got this, Dave Bucher posted the technique either on Info-Central or Facebook, but I used it to fix the tip of a fin that broke off during a hard landing. The end result was semi-flexible, which you might not want for a fuselage, but the stiffness might be adjusted by changing the ratio of glass beads to epoxy...

**Edit: I called them glass beads... waltr used the correct micro balloons (hollow glass spheres!)

 

[David_Stack]

I think that if you do resort to a lightweight filler to address the void, give strong consideration to using a single piece of lightweight fiberglass cloth (or even better carbon weave) to wrap the three sides you can get to; down one side, over the bottom, and up the other side. Use toilet paper to blot it after it is affixed to the fuselage to soak up as much excess resin as possible.

There likely is not much bending force applied to the fuselage during flight, but having that 'splint' may go a long ways toward preventing it breaking in that spot again.

 

[waltr]

Mix Micro-ballons into the epoxy for strong lighter weight fix.

 

[G_T]

How about reshaping the damaged area, and putting in a scarf joint sandwich of balsa? Almost no weight gain, or strength loss, that way.

Gerald

 

[SolarYellow]

How about reshaping the damaged area, and putting in a scarf joint sandwich of balsa? Almost no weight gain, or strength loss, that way.

Gerald

This. Replace the balsa with balsa and leave the fillers for smoothing it out.

 

[Rktman]

What do you guys think of Gorilla Glue foamed up with balsa dust? How strong, and what's the weight like when cured?

 

[bibbster]

Build a dam on each side with painters tape. Then spoon in some baking soda, filling it about 1/3 of the way. Tap the fuselage on the desk to settle the baking soda. Then drop in thin CA until saturated. How much is that? You'll know it when you see it. Repeat for the remaining 2/3. Peel off the tape after 10 or so minutes and sand. Lightweight, strong and easy. You can use the same process on the cracked area. Just put a thin layer of baking soda and drip thin CA on it. Sand and Bob's your uncle.

I always keep CA and baking soda in my range box. Simple strong repairs can be done quickly. You can also apply the CA and then sprinkle with baking soda to make a strong bond.

 

[Back_at_it]

I personally would cut out the old and splice in a new piece then sand to shape. I've done this a number of times when fins are damaged. Will be lighter and stronger than using wood filler and probably as strong as filling it with epoxy.

 

[David Schwantz]

Put in a new piece of balsa and then add a piece of 1/64th ply on both sides. Will weigh a few grams more only.

 

[Rktman]

Much thanks for all your suggestions and advice gents! Really appreciate it.

 

[BABAR]

Much thanks for all your suggestions and advice gents! Really appreciate it.

late as usual to the party.

hmmm, you have carbon fiber to replace the strength.

you are going to put a 1/64” balsa around all the open sides anyway.

why do you need ANYTHING in the middle?

if you WEREN’T putting the balsa around it, I’d probably add 1 or 2 more 1mm carbon rods (they weigh next to nothing, spackle it to provide volume/smooth airflow, and maybe coat it with the thin layer of wood glue just to keep the spackle from crumbling.

 

[Rktman]

late as usual to the party.

hmmm, you have carbon fiber to replace the strength.

you are going to put a 1/64” balsa around all the open sides anyway.

why do you need ANYTHING in the middle?

if you WEREN’T putting the balsa around it, I’d probably add 1 or 2 more 1mm carbon rods (they weigh next to nothing, spackle it to provide volume/smooth airflow, and maybe coat it with the thin layer of wood glue just to keep the spackle from crumbling.

I was worried that even with the ply sides, that gaping hole would make the fuselage vulnerable to any linear flexing, and that filling it would better spread the stresses from boost and the oftentimes too energetic ejection charge along the entire fuselage, making the front half less likely to snap off again.

I think it was actually the shotgun ejection charge from the E20 that shattered the fuselage. Everything came apart after that overly loud bang that not only shredded my chute, it tore my nosecone off the 250lb kevlar shock cord and sent it into the next county. Guess I'm being overly cautious but I want to make sure the repaired fuselage's integrity is as strong as I can get it to avoid the same catastrophe.

 

[BABAR]

I was worried that even with the ply sides, that gaping hole would make the fuselage vulnerable to any linear flexing, and that filling it would better spread the stresses from boost and the oftentimes too energetic ejection charge along the entire fuselage, making the front half less likely to snap off again.

I think it was actually the shotgun ejection charge from the E20 that shattered the fuselage. Everything came apart after that overly loud bang that not only shredded my chute, it tore my nosecone off the 250lb kevlar shock cord and sent it into the next county. Guess I'm being overly cautious but I want to make sure the repaired fuselage's integrity is as strong as I can get it to avoid the same catastrophe.

Answer=big carbon fiber tube

 

[jqavins]

You know what's as light as balsa, as strong as balsa, and as easy to shape as balsa? Balsa!

I'd remove the CF rod, clean up the edges of the hole, and carefully shape a new piece of wood to fill the space. Glue it in all around, and you'll probably have little grooves where be beveling the edges was unavoidable. Fill the grooves with something that's both filler and adhesive, like the saw dust and wood glue @lakeroadster suggested. That by itself would probably provide you all of the original strength, and if you're going to laminate it with 1/64" plywood on each side, then you'll be better than new.

EDIT: I see a few people have beaten me to the same advice.

 

[Curtis Enlow]

The color changing wood filler dries very light and is easy sanding. I actually used it on the fin fillets on my MPR Super Big Bertha and it was great. It's water based so you can thin it down and use a wetted finger tip to smooth it.

 

[Rktman]

The color changing wood filler dries very light and is easy sanding. I actually used it on the fin fillets on my MPR Super Big Bertha and it was great. It's water based so you can thin it down and use a wetted finger tip to smooth it.

Thanks. What's the name/brand of the wood filler?

 

[Curtis Enlow]

Thanks. What's the name/brand of the wood filler?

This stuff.

https://www.lowes.com/pd/Minwax-Col...WncnBXsbayenLY3E3phoCMUIQAvD_BwE&gclsrc=aw.ds

 

[Rktman]

This stuff.

https://www.lowes.com/pd/Minwax-Col...WncnBXsbayenLY3E3phoCMUIQAvD_BwE&gclsrc=aw.ds

Thanks, appreciate that.