Firewall fibreglass

[trebor]

I spent a couple of hours trying to fibreglass the outside of a firewall engine mount box, gave up and spent a long time cleaning it off Maybe I used the wrong thing with a Deluxe wing joining kit. I put a layer across the engine mount face and rolled it over the box edge about 25mm on 4 sides. Then a wrap of fibreglass at 50mm round the box on top of the 25mm. For the life of me I could not stop the bandage lifting at the corners of the box creating air pockets. I kept dabbing down but they kept springing back up. Was the Deluxe fibreglass too thick and springy should I have used thinner stuff.

[Cuban8]

Was it chop-strand mat? if so, then it'll be impossible to get it to conform to a tight radius. Woven glass cloth (a heavy grade) works well and I've used it many times to beef up the front internals of fuselages.

[Chuck Plains]

My only experience with fibreglass to date is from repairing windsurf boards 2.5 decades ago. But yes, if the cloth is heavy, it needs a larger flat area to flatten itself out on after turning a sharp-ish corner. And two layers of thin glass cloth can be stronger than a single layer of double weight cloth.

But you mentioned bandage Treb. That's made me think that trimming off any woven edge on the cloth could help it to conform better by allowing the weave to open up on all sides.

[trebor]

Sorry, not sure what chop / strand is. It didn't say what thickness it is in the distructions. I had to cut it down to fit and I had problems then with the strands moving. It turned into a mess pretty quick.

[john stones 1 - Moderator]

Was it the bandage/cloth sort like you use on a foam wing or the car repair stuff copped strands ?

you already said

Edited By john stones 1 on 02/09/2016 17:42:08

[Alan Gorham_]

Chopped strand mat would be useless for wing joining anyway, so you are more likely to have a woven cloth and it's probably too heavy a weave to conform as you have found.

What size engine and what model is it?

[trebor]

It does say it's foam safe, the resin is also fuel proof.

[trebor]

It's a H9 Piper Pawnee with a saito 82 engine. The ply box is very thin

[Alan Gorham_]

For that sized model I'd be tempted to use 5 or 6 layers of 50g/square m cloth with perhaps some carbon tow or tape if the ply box is thin/flexible as you say. You could lay the carbon across the box as stiffening strips, while the glass cloth strengthens the box as a whole, plus the attachment to the fuselage.

[Geoff S]

Wing joining bandage is intended to be used flat and is too thick to conform successfully to the tight corners of a firewall to engine box joint. I would first fit the box with epoxy and then, once it was cured, reinforce the joint with lightweight (25gms/squ metre) cloth applied in several layers as Alan Gorham suggests. Though I favour even lighter cloth, I'm sure 50 gm would also work and need fewer layers.

Geoff

[McG 6969]

+1 for Geoff on this one.

'Bandage' is way to stiff as it is intended for flat surfaces like wings joining.

Go with a first layer of +/- 20-25gr/m² followed by a couple of layers of 50gr depending on the strength you need. Use as minimum resin between the layers as they are your 'obesity' enemy.

Use genuine 2K polyester resin if you want the best strength. Hard floor varnish or even HK 20 min 'finishing cure' are fine for surface protection, but not for the strength you expect in this case. Just my 2 €-cent...

Hakuna matata

Chris

BRU - BE / CTR Firewall Control

[trebor]

Thank you for your advise gents

[Geoff S]

Posted by trebor on 02/09/2016 17:36:25:

It does say it's foam safe, the resin is also fuel proof.

I f there's any foam involved don't use polyester resin; epoxy resin is the stuff to use. You can tell the difference by the sniff test after curing - polyester has a distinctive odour but cured epoxy is almost odour free.

Geoff