Wing damage repair advice wanted!

[Aerotec]

Hi all,

Sadly my model got in to a fight with a fence post after an extended landing. New model, second flight and misjudged the glide ability of this model!

I have pranged the wing off a fence post and it's a foam core wing. Looking for advice on best way to fix it? Cut a block section out and trim some white foam to fit then sheet with balsa? Or cut foam section out and install some ribs embedded in to the foam to support the sheeting?

I'll also need a new wing tube. It's 475mm long and 18mm x 1.6/2mm. Would it be OK to replace with a Carbon Fibre wing tube? I can get a 450mm 18mm x 4mm off ebay which sounds robust enough?

Many thanks! Richard

Edited By Aerotec on 06/09/2020 21:18:38

[Aerotec]

Yes and it ripped the undercarriage off too. ​​​​​​

Gutted as it was a greaser of a landing but the far end of our WW2 airfield taxiway that we fly off has a post and rope section to stop boy racers using the site as a race track. Ran right in to it!

[Geoff S]

Not sure what you mean by the dimensions of the wing tube. I'm guessing 475mm long x 18mm diameter, and a tube with 4mm wall thickness (so a 10mm dia hole). I assume it's alloy so I'd agree the carbon you suggest as a replacement should be adequate but I'm no expert on meterial strength other than 'common sense' .

You really need to remove the covering and expose the damage to the wing itself. I'd be inclined to cut square the damaged foam and glue in oversize foam with Gorilla glue, sand/Permagrit to shape and recover the sheeting (1.5mm balsa probably) glued with thin epoxy or contact adhesive and then replace the leading edge. At least the film covering looks to be a simple design which should be relatively easy to match in.

I don't think it would be a difficult repair at first glance.

Geoff

[Richard Wills 2]

Cut out all the damaged area, glue in new foam and sand to profile. Remember veneered foam is a stressed skin construction so when you replace the veneer you will need to replace more than just the damaged area. You can have joins along the span of the wing (with the grain), but not across the chord.

[Martin Harris - Moderator]

Ideally, you should scarf in any sheeting replaced. i.e. sand a taper to the existing sheeting either side of the join and a matching one on the underside of the repair patch - less important to do this on the spanwise joins but better if you can. A 10:1 scarf slope would be fine for a model - so for example with 1mm veneer, sand the scarf joint 10mm wide.

[FlyinBrian]

Cut out the damage so that you have a sensible shape to work on, rectangular is easiest

Remove about three cm of wing skin either side of the cutouts.

Make up replacement foam rectangular sections to fit and insert into the cutouts.

Mark the wing section onto the new piece/s of foam then trim to shape.

Fit foam pieces using PVA and give a final sand to shape.

Use 25g/cm glass cloth and cut to fit between the wing skin, use regular 15 - 30 min epoxy or even PVA to lay this in place and leave to dry.

Cut new piece/s of wing skin to fit, lightly sand underlying glass cloth and glue skins in place, if you used epoxy for the glass cloth use it also for the skin, PVA for glass cloth? then use that to glue skins in place. Use sellotape or similar to hold all the skin tight to the wing and leave to dry, ( for a couple of days if using PVA. )

Remove tape and using a sanding block smooth and blend the repair into the wing,.

Cover and decorate

Fly

[Nigel R]

"Use 25g/cm glass cloth and cut to fit between the wing skin, use regular 15 - 30 min epoxy or even PVA to lay this in place and leave to dry."

How does this bit work, do you lift up the existing skin to slide the glass underneath it?

I'd probably just glass over the outside of the join in the wing skins. Battle scar.

Or maybe use a patch of 1/32 ply, edges feathered down to near nothing, to wrap around the LE where the inner and outer fore/aft joins are.

Not sure I'd do much about the spanwise joint, just make sure the skin fits neatly and glue in place with some resin to make sure the join is decent.

[Bob Cotsford]

I'd repair the foam by cutting in a new section of foam as previously described, then I'd peel off a strip of veneer down to the wing root wide enough to take in the damage and replace that with new wood. If I was just going to repair only the damaged area I'd cut in the new veneer at an angle of around 30 degrees to the span. It would however be a compromise of practicality over restoring maximizing strength, but unless you intend stressing it with high G maneuvers it should be ok. Whichever method you use the most important thing would be to jig up the wing while bonding the veneer to avoid inducing a warp.

[Andy Stephenson]

Just to add my 2p worth...

Using PVA or other water based adhesive to glue foam to foam takes for ever to set as the water has nowhere to get out easily. Foam to wood no problem but foam to foam not good IMHO.

Gorilla foaming PU glue is by far the best for a foam to foam joint.

A.

Edited By Andy Stephenson on 07/09/2020 13:27:40

[Frank Skilbeck]

When I've done this type of repair in the past, I've cut the section of damaged foam out with a 30ish deg angle, sort of a trapezodial shape, glues in a piece of foam (I used epoxy but would now use Gorilla glue), shape and put a balsa skin on it and a new leading edge with scarf joints to the existing leading edge, then finished sanding down the new balsa skin and leading edge and recover.

Looking at your photo, I'd cut out for one or two foam repair pieces depending on the inspection once the covering is stripped back, but in either case replace the leading edge with one piece spanning either side of the full repair.

Do a lateral balance of the full model after repair and add some weight to the good wing if needed.

Your proposed new wing tube sounds fine.

[PatMc]

Posted by Frank Skilbeck on 07/09/2020 16:10:21:

When I've done this type of repair in the past, I've cut the section of damaged foam out with a 30ish deg angle, sort of a trapezodial shape, glues in a piece of foam (I used epoxy but would now use Gorilla glue), shape and put a balsa skin on it and a new leading edge with scarf joints to the existing leading edge, then finished sanding down the new balsa skin and leading edge and recover.

Looking at your photo, I'd cut out for one or two foam repair pieces depending on the inspection once the covering is stripped back, but in either case replace the leading edge with one piece spanning either side of the full repair.

Do a lateral balance of the full model after repair and add some weight to the good wing if needed.

Your proposed new wing tube sounds fine.

Similar to Frank but with a few differences - only shape the top part of the foam & skin with soft 1/8" balsa.
Next hold in place with the balsa skin flush with the wing upper surface, now shape the lower part of the foam flush with the lower wing surface.
Skin lower foam with 1/16" balsa.
Glue in place with the lower 1/16" balsa flush with or very slightly proud of the wing lower surface. I used foam friendly contact adhesive for the skinning & aliphatic to glue the foam in place (It does take longer but not that long) held in place with masking tape. Never tried gorilla glue as it wasn't around when I've done this type of repair.
Sand the balsa skins & the LE.
If the are any small gaps fill them with a mix of a little aliphatic & a water based filler.

Final sand, vacuum &/or tack cloth then cover.

[Aerotec]

Hello all,

Thanks so much for the detailed responses! I will take the advice and get started on a repair when I get some free time and report back here! I was planning to pick up the required materials at Weston Park but we have decided to play it safe and not attend the rescheduled show this year as I was going with my elderly father. I'll get some ordered up online instead.

I have found a Carbon Fibre manufacturer who has a nice 1m x 18mm x 16mm that looks perfect for the wing tube.

Does anyone have any suppliers for wing core foam as I am guessing it is denser than packaging foam. And any suggestions on how to cut the foam to shape without making a wire cutter and jig? Could I use expanding foam as that can be sanded but guessing it wouldn't be dense enough to provide the correct support?

Edit: I have found grey styrofoam on the Balsa Cabin website. 

https://www.balsacabin.co.uk/product-category/foam-sheets/blue-styrfoam/

Cheers all! Richard

Edited By Aerotec on 13/09/2020 14:02:10

Edited By Aerotec on 13/09/2020 14:02:25

Edited By Aerotec on 13/09/2020 14:05:35

[Aerotec]

Hmmm,

Looking to replace the 18mm wingtube. I can get a few options of CF tube...

Hobbyking 500mm x 18mm x 16mm for around £6 + Postage:

https://hobbyking.com/en_us/carbon-fiber-round-tubes-18-16-500mm.html?wrh_pdp=7

Easy Composites 1m x 18mm x 16mm for around £20:

https://www.easycomposites.co.uk/#!/cured-carbon-fibre-products/carbon-fibre-tube/woven-finish-carbon-fibre-tube/glossy-3k-woven-finish-18mm-16mm.html

Or a thicker 500mm x 18mm x 15mm for around £16:

https://www.ecfibreglasssupplies.co.uk/product/carbon-fibre-tube-18mm-15mm

Should I play it safe with a 3mm thickness?

Richard

[Chris Walby]

Hi,

As the additional weight is on/very close to the C of G I would go for the thicker tube.

PS - Don't every try to fit a carbon tube inside another carbon tube...they bind and are very difficult/impossible to part...

[Aerotec]

Ah 18mm x 15mm is a 1.5mm thickness not 3mm! I found an 18mm x 14mm I will go for instead,

[Aerotec]

Hi all,

 

OK job done! I didn't cut out a full section as on inspection the compressed foam still held the shape of the wing. I cut out the two damaged sections of skin and sheeted one side then filled out any foam gaps with new foam. I used gorilla glue which is superb as it foams up and fills gaps nicely.

 

I then fitted the top balsa sheeting. Applying a chamfer to increase strength. Sanded and covered. I'm pleased with the result. First time doing iron on covering in probably 25yrs 😁

 

Its not as robust a repair as some suggested like full length sheeting or fibreglassing but as it's the leading edge section and not the central section of the wing I'm hoping the lapped wood, foamed gorilla glue with additional cyano to the wood will help hold it all together.

 

Now to hunt down some matching red trim.

 

 

 

 

Edited By Aerotec on 20/11/2020 21:59:13

[john stones 1 - Moderator]

Looks good to me.

[GrumpyGnome]

Ditto!

Well done.

GG