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Foam Model Repair

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Alan Thorpe14/11/2019 11:13:36
228 forum posts
77 photos

Hi everyone,

I am in process of repairing a foam Focke Wolf after an unscheduled meeting with rocks on landing!

The problem I have is that some of the foam at the wing root leading edge is missing, I couldn't find it at the crash site.

What would be the best way to build it back up? I'm guessing with blue foam or balsa block, but not sure.

The area is at the leading edge wing root and is immediately forward of the landing gear bay, as circled in red below.

Thanks!
Alan

focke-wolfe-190-a3[1].jpg

Plummet14/11/2019 14:23:26
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1418 forum posts
41 photos

I have no experience with it, but just the other day I saw a video on You Tube of a bloke making a foam cored wing using canned expanding foam.

Is there any way you could create a mould or skin to enclose the space to be filled and to then inject some foam. As the foam expands, you would need to have bleed holes to allow excess foam to escape.

Plummet

Colin Carpenter14/11/2019 14:29:12
648 forum posts
36 photos

I have just rebuilt a front end of a Visionaire where a burning esc melted the nose left side completely! If the missing foam is non structural then lightweight filler from a diy store will do. Then just sand and paint. Otherwise scrounge bits of foam from fellow flyers to shape and glue in. Colin

Steve J14/11/2019 14:32:24
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1986 forum posts
54 photos

How big a bit is missing? Fill most of the hole with scrap foam or light balsa and then fill with light weight filler (the stuff where it feels like the pot is empty) and rub back. Seal the filler with water based varnish (Ronseal Diamond) before painting.

Alan Thorpe15/11/2019 00:40:34
228 forum posts
77 photos

20191115_003725.jpgThanks for the helpful replies everyone.

I've attached a few photos here...it looks worse than reality as the missing retract lives in the wheel well.

I have glued in a new spar and 2 short trailing edge spars/joiners to strenthen the wing.

20191115_003655.jpg

Phil B15/11/2019 08:06:11
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212 forum posts
196 photos

In my experience, that area is much too big to fill with DIY expanding foam.

You may be able to use blue foam. Personally I always go for soft balsa block. The advantages of blue foam are lightweight and easy sanding to shape. Disadvantages, unless you fibreglass coat it, it will dent more easily than the original foam.

Whichever you use, cut out enough of the original foam to give some straight edges to bond too. Use PU expanding glue (eg gorilla glue) to bond in your block, which is cut to fit in nice and tight, probably over size where you'll be sanding it back later. If possible wrap the whole repair area with masking tape which will prevent the foam creeping everywhere. Allow to dry and then use knife and sanding block to get the shape back.

I usually finish by covering the affected area with lightweight fibreglass cloth, but you may get away with primer and paint.

Alan Thorpe15/11/2019 09:32:29
228 forum posts
77 photos

Hi Phil,

That's great, thanks very much for the helpful reply. I'll try the balsa block and fibre glass it, sounds like a strong repair.

Thanks a million!
Alan

Engine Doctor15/11/2019 11:03:10
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2541 forum posts
40 photos

A bit of bad luck there .Looking at the pic I would if possible, in addition to the foam replacement fit a carbon spar/tube into the wing , extending as far as possible into the wings and through the repair as you can without compromising the retract units . As the wing is cracked right on the fuselage joint it will have to support the flying loads so a spar might help stiffen a compromised wing.. Good luck and show us any repairs as I am sure plenty of us ill be interested how you get on .

E.D.

Erfolg15/11/2019 20:12:47
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11781 forum posts
1340 photos

I have due to my piloting skills undertaken a number of foam repairs.

wp_20191009_12_16_05_pro.jpg

In this case a new nose, the mangled one is to the side. There were numerous other small repairs involved.

wp_20160502_15_11_36_pro.jpg

This a wing tip repair.

wp_20160503_18_33_07_pro.jpg

when finished.

If you look at my repairs, I tend to use quite different methods and materials dependent on the issues. Perhaps stating the obvious.

The Do335 has a ply box around which blue foam is stuck, using polyurethane glue. I try not to gap fill, making the best joints I can.

The Delta is constructed from Depron, sticking together using PVA. With PVA I rough up both surfaces, weight down, then wait days for the glue to go off.

I did have a wing fold previously on the 335, in that case the glued in spar, debonded, plus there was a poor spar arrangement. In that case I just put it back in place. I used a good glue, in that case Cyno, sorted the spar arrangement out.

You will possibly notice that I have skinned the areas with glass cloth, using Water Based Varnish, to get an OK finish.

There is no one way.

As has been suggested filling big holes with expanding foam, does not fit the bill, IMO.

I would bridge gaps using ply as spars.I possibly would bit by bit make the spars into "I" type beams, The emphasis on getting the wing and body geometries right. I would then fill the voids, probably using "blue foam" where large bits are missing, and roughly oversize sand to shape. I would fill the thinner sections with balsa sheet.

I would then sand to my perfect (or acceptable, in my case) size and finish.

Time is probably the most used asset.

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