Foam wing core "filler" before glassing - RC Groups
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Jul 10, 2005, 02:23 AM
Nucking Futs
I'm Rick James's Avatar

Foam wing core "filler" before glassing

I have a pair of blue foam wings that I cut last week and am about ready to build. I wanted to glass directly over the foam but there are some waves in there that I would like to fill or sand out. Can I just use some spackle and fill them in and then glass?

Also, I have a hardwood LE and Balsa TE. Do I need a spar? This is a 60" wing on a 50oz PSS plane. I will be using some fairly heavy glass and I guess I have the option of bagging them up. I have the stuff just never used it before. I have a bunch of CF tow, fiberglass rods and a few carbon rods as well.

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Jul 10, 2005, 06:46 AM
Registered User
"Can I just use some spackle and fill them in and then glass?"


"Do I need a spar?"

It depends on the airfoil thickness, glass skin thickness, airspeed (or maximum G's).
Jul 10, 2005, 01:09 PM
Registered User
Make sure the spakle is good and dry before bagging the wings.
I have had a couple bagged wings that developed bubbles under the skin after
owning them for a while. It was explained to me that they may have been bagged too soon after the spakle was applied, and the spakle will "GAS" whie it is curing.
So far, to my knowlege, none of the wings I have bagged have done this, but I always wait a few days after using the spakle before I sand them and then bag them.
Maybe other wing makers could tell us more about this.

Jul 10, 2005, 03:29 PM
Knowledge increases life's FOV
Sled Driver's Avatar
Originally Posted by I'm Rick James
Can I just use some spackle and fill them in and then glass?
If you are using a better structural foam, e.g., HiLoad 60, and you would like to maintain the structural integrity of a stressed skin wing, then I would suggest that you avoid using spackle as much as possible. This is probably not an issue, if you have typical foam, e.g., Roofmate, cores or need to do minor filling - avoid filling near the high point of the airfoil near the root of the wing.

Originally Posted by I'm Rick James
Also, I have a hardwood LE and Balsa TE. Do I need a spar? This is a 60" wing on a 50oz PSS plane.
For all the wing designs that I've run through the computer, I have not found a lead sled wing design where a spar was necessary for wingspans less than 60-inches - if the skin is designed right. A couple qualifiers: 1) Foam - analysis based on using a better structural foams, e.g., Foamular 400 or better, 2) Laminate - two layer lay-up, a layer of uni-directional for bending and a layer of bidectional on bias for torsion, 3) Building quality - it is reasonably well bagged with no significant flaws.

For my lead sleads, I typically use 3 oz uni-CF or 4 to 6 oz uni-S-glass and one layer of tight weave 3 or 4 oz bidirectional glass on bias (match the direction of the warp yarns on the top and bottom or the wings will have a tendancy to twist as the resin cures and shrinks).
Jul 11, 2005, 12:53 AM
Nucking Futs
I'm Rick James's Avatar
Great, thanks for the info. The filling is very minor. I plan on putting this one through a few DS loops at some point so I'll go on ahead and slip a spar system in there.

Thanks, RJ
Jul 13, 2005, 02:40 AM
Creations Aberration
Aethertek's Avatar
Better than spackle.
Jul 14, 2005, 07:40 AM
The reviewer
XJet's Avatar
I have had great success laying up straight over the raw EP cores.

It is important to get the cores as smooth as possible (since any imperfections *will* show up after glassing) but a light-sand was all I needed on these cores. If you're getting waves you ought to practice your cutting technique until you can cut cores that are perfectly smooth -- it's not hard once you get the hang of it, even if there's a big taper involved (as was the case with part of my most recent wing).

The last bird I built like this had an 8% thick symetrical section with a 1.4m (55 inch) span and I used just one layer of 6-oz cloth and West System epoxy.

It wasn't bagged and as much of the resin was squeegied out as possible so as to keep the weight down. A fill-layer of very stiff microballoons and epoxy was then used to fill the weave of the cloth (post cure) and a bit of sanding brought the whole thing up very nice.

No inset spars or LE/TE wood was used, it's just foam and glass. The top skin of the wing also forms the hinges for the elevons.

The result was a very light, very stiff wing that was quick to build and has a nice finish.

There's a 3.6MB video of this bird in flight at:
Jul 14, 2005, 02:11 PM
Need 4 Speed!
pdawg's Avatar
Why not glass the wing 1st to get a good bond between the fiberglass and the cores? Once the strength has been established, then spackle (or even bondo) on top of the glass to achieve the desired shape. I personally believe this would be a better sequence of events for avoiding delamination.

By the way, this is a very good question.
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Jul 15, 2005, 10:45 AM
Nucking Futs
I'm Rick James's Avatar
Cool pdawg, I like that idea myself. I guess I was thinking along the lines of EPP foamies where you tape, spackle and then cover. I've done it but also thought from day one that the filler might not be a very good thing to bond to as opposed to the substrate foam.

Xjet, we cut these cores using my buddies CNC wing cutter. We didn't have the temperature perfect for cutting hiload 40 so there were some minor imperfections. I think we're learning that the trick is to keep the heat as low as possible but still allow burn and keeping the speeds really low as well. I think we've been too hot and fast for the proper cuts.


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