Hi,
I am installing 30cc gas engine ( SAP-180 ) on Zen-120. i had a thought about reinforcing the firewall,
what whould you think will be the best way ? ( attached the current firewall structure )

Thanks,

If it were my plane, I'd add some FG cloth and resin to the firewall making sure it wrapped around some solid structure on the edges.

Just my .02

There is no access to the firewall from the inner side. do you mean to remove the cover from the fuselage sides ( near the firewall ) , and then to use two FG strips ? ( see attached file )

Just for clarification....
the firewall "cheeks" ( the part that the firewall mounts to) are balsa??????

Here is the way I did mine. Cut a hole in the firewall for the carb to stick through into the fuse. I used balsa triangle epoxied into place all around the motor mount box. For a 30cc should be plenty strong.

What I do is put the FG cloth on the outside, right on the firewall, no inner access needed.

IMHOyou will need to do better than 2mm balsa to anchor the firewall ofa 30cc engine to. I'd recommend 1/8" light ply cheeks as a minimumglued to the balsa sheeting for the front 4" or so of the model (behind &up to the firewall). Plus tri-stock inside. I would then drill holes through the cheeks/balsa into the firewall and epoxy toothpicks or (better) bamboo skewers in as reinforcement fasteners. You can see the ends of the dowels in this image. The plugs still sticking out are to keep the engine bolts from crushing the firewall.

That's way too much weight and vibration for 2mm sheet balsa. Onehard landing and the engine will flop off after it tears the balsa.

Listen to Charlie....thats exactly what I do on gassers. I've used bamboo skewers as well as toothpicks too. Thanks for the pics Charlie!

Like so:

Yep, I cover the whole front of the firewall as well. This is not a "strip" thing for me. Get a piece that will cover the whole firewall.

o Do the toothpick/skewer mod that Charlie described
o Cut your glass cloth (light stuff will do) to the size you need to cover the whole firewall and a little on the sides
o Mix up some 30 minute epoxy diluted with alcohol. The alcohol helps the epoxy to flow into the glass
o Coat the firewall with your epoxy mix
o Lay the glass on the just applied epoxy making sure it gets soaked all the way through the cloth
o Add a little more epoxy if needed on top
o Let it dry and then drill holes where needed (existing holes in firewall)

Done! Others may have slightly differant approaches but in the end, its about keeping the gasser from vibrating your firewall loose causing bad things to happen

I would use aluminum "L" brackets drilled to accept a couple of nuts and bolts. Easy and cheap. Works fine on my Extra300 powered with a DLE 30.

Thank you all !