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Joining Hot Wire Cut Wing Pannels

Old 04-04-2019, 03:24 AM
  #1  
Silverback
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Default Joining Hot Wire Cut Wing Pannels

I could use some suggestions on how to join hot wire cut Owens Corning pink foam (fomular 250) covered in white glue/brown masking paper together at the center to hold a few degrees dihedral and how much bracing it will need. The plane is a Cub/tundra plane type profile, high wing, 48" span, 9.25" chord, with ailerons and flaps (I think I'll cut servos in after I join them, not sure). The wing halves have no bracing right now, just foam/paper covering, and honestly, they feel pretty rigid, they don't seem to need a spar, but I'm pretty nervous about the center joint, especially because I'm hoping to put about 4* of dihedral in it.

What would you use? How would you do it? Anything to look out for?
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Old 04-04-2019, 06:04 AM
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A few pics so you can see what I'm dealing with...


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Old 04-04-2019, 09:03 AM
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solentlife
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Its as well to go back to old days methods .....

Kits 20 - 30yrs ago were common with veneered foam wings ... in fact I had a small 'business' called Wings and Things cutting cores for local modellers back then in the UK.

Joining core wings :

Dihedral angle sanded into each panel end.

Slot cut for plywood brace to cross the joint in its deepest section. A 3 or 4mm thick ply brace is more than enough with depth equal to the core depth. The ply brace cut to match dihedral angle.

Lightweight Glassfibre bandage about 2" (5cm) wide and length to fold completely round the joint.

Two part resin to fix bandage. (Old days we used Polyester for its quick setting - but it eats foam ! so the joint had to be sealed well before application). Laminating Epoxy would be safer.

OK ... how :

Epoxy join the wing halves with ply brace epoxied into its slot. making sure epoxy oozes out all along the joint to seal it. Scrape away excess before it sets. I used to spread it a few mm's either side of joint to be sure.
Once set and wing fixed .... then apply bandage with resin ... wetting out fully and sqeeg'ing to make sure fully contacted to wing skin. Smooth and leave to set.

Viola as they say ...
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Old 04-04-2019, 12:53 PM
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Originally Posted by solentlife View Post
Its as well to go back to old days methods .....

Kits 20 - 30yrs ago were common with veneered foam wings ... in fact I had a small 'business' called Wings and Things cutting cores for local modellers back then in the UK.

Joining core wings :

Dihedral angle sanded into each panel end.

Slot cut for plywood brace to cross the joint in its deepest section. A 3 or 4mm thick ply brace is more than enough with depth equal to the core depth. The ply brace cut to match dihedral angle.
"A 3 or 4mm thick ply brace is more than enough with depth equal to the core depth." - Depth of what? What is the core depth? Do you mean a brace the same height as the thickness of the core? Or going the thickness of the core into the wing?

My best guess is to cut a brace as tall as the thickness of the wing core airfoil but I'm not sure how far into the wing... Going back to my stick building days typically the center of the wing would have a ply brace the height of the top to bottom spar going out a rib bay or 2... I'm guessing this is about what we're going for?

Lightweight Glassfibre bandage about 2" (5cm) wide and length to fold completely round the joint.

Two part resin to fix bandage. (Old days we used Polyester for its quick setting - but it eats foam ! so the joint had to be sealed well before application). Laminating Epoxy would be safer.

OK ... how :

Epoxy join the wing halves with ply brace epoxied into its slot. making sure epoxy oozes out all along the joint to seal it. Scrape away excess before it sets. I used to spread it a few mm's either side of joint to be sure.
Once set and wing fixed .... then apply bandage with resin ... wetting out fully and sqeeg'ing to make sure fully contacted to wing skin. Smooth and leave to set.
Any thoughts on using polyurethane in the place of epoxy? I've been on a "keeping this thing light" kick with this build and have mostly stayed away from epoxy and hot glue because of it. I shouldn't have any significant gaps (I cut the dihedral angle using a jig on a table saw, got a perfect cut, I'm very happy with how that turned out), but even so, polyurethane (GG) should fill small gaps and should still be stronger than the foam I'm gluing to.

Viola as they say ...
For some reason that made me laugh... thanks
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Old 04-04-2019, 02:15 PM
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solentlife
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Originally Posted by Silverback View Post
"A 3 or 4mm thick ply brace is more than enough with depth equal to the core depth." - Depth of what? What is the core depth? Do you mean a brace the same height as the thickness of the core? Or going the thickness of the core into the wing?

My best guess is to cut a brace as tall as the thickness of the wing core airfoil but I'm not sure how far into the wing... Going back to my stick building days typically the center of the wing would have a ply brace the height of the top to bottom spar going out a rib bay or 2... I'm guessing this is about what we're going for?
Depth would be top to bottom surface - so yes depth of the wing.

How far into the wing ... thinking about it - usually it would be anywhere from 1/2 to equal to chord of the wing root ... it really depends on what stress / style of flying. If sunday flying - then a simple brace of about 3/4 chord into each wing half is fine .... if you are into aerobatics (given the wing in your post - I doubt it !) then full chord or even more length into wing half would be sensible.

Any thoughts on using polyurethane in the place of epoxy? I've been on a "keeping this thing light" kick with this build and have mostly stayed away from epoxy and hot glue because of it. I shouldn't have any significant gaps (I cut the dihedral angle using a jig on a table saw, got a perfect cut, I'm very happy with how that turned out), but even so, polyurethane (GG) should fill small gaps and should still be stronger than the foam I'm gluing to.
You could do away with the bandage ... if you created holes into the cores roots and the GG was allowed to 'create foam spars' into them ....
Or
Taking CF rods and tubes ... you could glue tubes into each wing and have a CF rod bridging the joint ... GG to join it all.

But I would not use GG with the bandage instead of resin .... the GG expands and would not anchor the bandage correctly - that's if you could wet it out even. There is another way to bandage ..... using nylon and PVA white glue watered down ... But then it takes time to dry out and cure.

For some reason that made me laugh... thanks
We all need a smile now and again ...
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Old 04-04-2019, 08:08 PM
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pmullen503
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I see you have a table saw... What I've done in that situation is glue the cores together with proper dihedral etc. Then prop the wing over the table saw (lots of sand bags etc. so it can't move) such that when the blade is raised it cuts a slot across the center. Now turn on the saw and slowly raise the blade to just below the height of the wing. You now have a straight slot for your wing brace. Extend the slot if you feel its needed and cut out a brace to fit in the slot and glue it in. Then epoxy on the bandage.

You could probably get away with just the bandage on a wing of that size with it's thick airfoil.
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Old 04-06-2019, 03:25 AM
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Originally Posted by solentlife View Post
Depth would be top to bottom surface - so yes depth of the wing.

How far into the wing ... thinking about it - usually it would be anywhere from 1/2 to equal to chord of the wing root ... it really depends on what stress / style of flying. If sunday flying - then a simple brace of about 3/4 chord into each wing half is fine .... if you are into aerobatics (given the wing in your post - I doubt it !) then full chord or even more length into wing half would be sensible.
LOL, you used the word sensible... most people that know me... well...

The airfoil is basically a Clark Y thickened to 15%, which is basically similar to a lot of high lift/heavy lifting airfoils that I found without an under chamber (didn't want to deal with that on my first try cutting wing cores). It will likely go on the HZ Cub fuselage you see in the background of one of the pictures, same span as stock, 2" more chord and thicker (both airfoil % and physical actual thickness wise), hoping to get something that will fly slower, with more lift and more lifting capability. With the hefty cross-section, the big drop down flaps and possibly leading edge slats or VGs I'm hoping to get STOL performance and possibly play with some heavy payloads otherwise. I don't expect any kind of aerobatics out of it, but I may end up putting some hefty strain on that wing joint (I'd love to be able to take off in less than a yard and be able to drop flaps, nose down, pull up last second and land in a few feet).

You could do away with the bandage ... if you created holes into the cores roots and the GG was allowed to 'create foam spars' into them ....
Or
Taking CF rods and tubes ... you could glue tubes into each wing and have a CF rod bridging the joint ... GG to join it all.
It's funny how in different disciplines the same thing is treated completely different. In the woodworking world GG foam in considered not structural and as having no strength, where here it's a reinforcing foam spar

But I would not use GG with the bandage instead of resin .... the GG expands and would not anchor the bandage correctly - that's if you could wet it out even. There is another way to bandage ..... using nylon and PVA white glue watered down ... But then it takes time to dry out and cure.
I know I've read of someone using GG to wet out FG, but I don't remember the details. I suspect that the way to make it work is to mix the GG with some water or white glue/wood glue, let it foam and then use the foam to wet out the FG, I know that kind of thing works with 2 part expanding urethane foam- you sort of smash it down again after it starts foaming and you get a really strong adhesive. I'd be more likely to try to use CA (I've done that on stick built wings) or water-based polyurethane to wet out the FG if I wasn't going to use epoxy...

Right now I'm not really sure where to get FG cloth that thin/light either locally or in a few days (amazon prime...).

I also did some searching for nylon and PVA with no luck. What kind of nylon?

Originally Posted by pmullen503 View Post
I see you have a table saw... What I've done in that situation is glue the cores together with proper dihedral etc. Then prop the wing over the table saw (lots of sand bags etc. so it can't move) such that when the blade is raised it cuts a slot across the center. Now turn on the saw and slowly raise the blade to just below the height of the wing. You now have a straight slot for your wing brace. Extend the slot if you feel its needed and cut out a brace to fit in the slot and glue it in. Then epoxy on the bandage.
_I think_ I'm picturing what you're describing- having the length of the wing facing the same direction as the table saw blade, I'm guessing dihedral up (center down, tips up, right side up) and plunging the blade into the bottom of the wing... If that's the case you'll cut out a radiused slot and I"m assuming that you're talking about making a radiused brace. Interesting idea, I'll have to go see if I have any blades that have similar curfs to the plywood I have around, but this has me wondering if it even really needs to be plywood- plywood is nice because it's equally strong in all directions, but in this case we're just trying to resist bending in one direction (it's sandwiched inside the core preventing it from moving in any other direction), I'm wondering if I could get away with just a thin slat cut out of a nice piece of straight-grained hardwood.

You could probably get away with just the bandage on a wing of that size with it's thick airfoil.
I've been wondering if someone was going to say that... in theory, almost anything I glued that with should be stronger than the foam, and as long as I patch the skin to be at least as strong as the rest of it I have a feeling that it will be plenty rigid, but I'm not sure it fit would tend to break at the joint more... I don't think I need that joint to be any stronger than it would be if it just was the same as the rest of the wing.
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Old 04-06-2019, 03:55 AM
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I always use solid wood for spars/braces. Plywood has half it's plys running in the wrong direction. Solid wood is stronger in the direction you need compared to the same weight of plywood.
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Old 04-06-2019, 10:55 AM
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solentlife
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Originally Posted by Silverback View Post
LOL, you used the word sensible... most people that know me... well...
Yeh I know !!

It's funny how in different disciplines the same thing is treated completely different. In the woodworking world GG foam in considered not structural and as having no strength, where here it's a reinforcing foam spar
If GG is sprayed with water - I agree it is a filling type of adhesive and non structural, as it foams up significantly. But try it without water and then see what it does ... it will be stronger than your foam core and paper covering by a large margin.
I have used straight PU Glue (I refuse to pay GG inflated price ! same generic glue by Bison is 1/3rd price for 2x volume !) to anchor retracts, wing bolts, all sorts of load bearing items.

I know I've read of someone using GG to wet out FG, but I don't remember the details. I suspect that the way to make it work is to mix the GG with some water or white glue/wood glue, let it foam and then use the foam to wet out the FG, I know that kind of thing works with 2 part expanding urethane foam- you sort of smash it down again after it starts foaming and you get a really strong adhesive. I'd be more likely to try to use CA (I've done that on stick built wings) or water-based polyurethane to wet out the FG if I wasn't going to use epoxy...
Personally I would use PVA White glue if not epoxy or Ployester.

Right now I'm not really sure where to get FG cloth that thin/light either locally or in a few days (amazon prime...).
Normal woven medium weight bandage is fine

I also did some searching for nylon and PVA with no luck. What kind of nylon?
I buy the lightest plain Net Curtain nylon I can find in local Household shop ....

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