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Fiberglass Firewall

Trying my hand at building a big slow foam board plane. 5ft wingspan and about 3ft long w/ a weird hybrid split v tail section that I made up. I've got the majority of the air frame built and I'm waiting on the motor and ESC to show up. I got to the part of construction where I needed to worry about a motor mount for the front of the plane. I don't have any thin plywood lying around and I'm to lazy to track down a store that sells it so I came up with an alternative: Fiberglass.

I grabbed a scrap chunk of MDF about 1 foot square and covered the surface with packing tape to get a smooth, non porous surface, then hosed it down with mold release. I threw down a layer of fiberglass cloth (can't remember the Oz rating) then a layer of chopped fiberglass mat, then another final layer of cloth. Soaked it through with marine grade resin and squeezed out all the air bubbles. Once it's done curing I'll pry it off the MDF and clean up the edges with a pneumatic cutoff wheel I have floating around somewhere. Once the motor comes in I'll mark off the mounting holes and affix it to the air frame.

Has anyone made/used a fiberglass firewall/engine mount? If so did you see any benefits over the plywood?

Also, if anyone is curious how it turned out I'll try to remember and post pictures and a follow up once I get it all done.


Skill Collector
I'm pretty sure I've seen this done in the past, and it should work out just fine. Heck, I've used coffee shop gift cards for firewalls and those did just fine - your fiberglass solution is probably going to be a lot stronger than that :D


Master member
It is likely to be plenty strong enough.
The only issue I can see is how well it will stick to the foam. Wood is slightly porous so giving adequate glue area.
Fiberglass is completely non porous so you will be relying just on the ability of the glue to stick directly to resin and not much does!


Skill Collector
Good point. If it shows any weakness, could add pinning the edges with toothpicks - drill little hole in fiberglass that toothpick sticks out of - other end driven deep into the foam board.
The plan was to scuff up the edges of the fiberglass w/ 80 grit for good bonding, then gorilla glue to the foam board. I've had really good luck with gorilla glue. Since it expands it pretty much locks itself into the pours of the foam. I'm hoping the rough edge of the fiberglass will give it a nice a stout hold.

The toothpick idea is good (I'll probably use BBQ skews). That's a good fall back if what I plan on doing flops or the foam doesn't have enough strength for surface mounting.

Stay tuned
Bellow is the plane I've been working on.

This is the fiberglass. It's pretty well cured at this point. My goal is to chop it up into a few pieces to use for various projects. I could see a lot of good uses for square/rectangular chunks of fiberglass added in a few places for support and strength.
Had some time today and managed to get the firewall and motor mounted.

Bellow is the piece I cut out with the motor mount sized up on top of it

Holes were drilled and the motor was mounted to the bracket, bracket mounted to firewall, and firewall mounted to the plane. Here are the results:


As far as I can tell its strong as all get out. Very happy with how it turned out. And after 3 days all the fumes have finally cleared out of my garage and it no longer smells like resin lol.