I'm building a quarterscale Fly Baby. I'll have to bring the engine forward about 2 1/4" to poke the prop thru the cowl. I can't see gluing up that much plywood. I recall glancing at a picture of a box-like structure mounted on the firewall. If any of you have used these, I'dd appreciate your thoughts. In addition; 1) would 3/8" 5-ply birch be strong enough to support a 130 four-stroker? 2) Would you epoxy the box to the firewall and mount the engine to the box only...or through-bolt the engine to the firewall? Thanks.

I built a couple of motor boxes. On a RadioCraft designed for a big gasser it was still 1/4 in ply, but corner braced with 1/2 in hard triangle stock all around. Same with the box to the fuse, but the sides were cut into the firewall too. Nosen Citabria uses one, never had any problems with that with a pound and a half of engine and reduction drive.

I think if you do your woodworking nicely, 1/4 in with triangle stock works fine. I through bolted the Citabria and it wasn't all that difficult to do with threaded rod stock and lock nuts. Makes it a little more solid and doesn't cost much.

What you are describing is common on giant scale planes. You have a couple options, you can build and engine box or use standoffs. The engine box can be made of 1/4" ply for the sides and 3/8"-1/2" for the firewall. I would use 1/4" for the back and through bolt it with 10-32 bolts and blind nuts. I would then re-enforce the area where the the box meets the original firewall with 1/2" triangle balsa on both sides and the bottom. Cut some lightening holes into the sides, bottom and top of the box. I like to coat the inside of the box with thinned epoxy and glass cloth. The outside can be covered with carbon fiber cloth if you like or just thinned epoxy. You may also want to use some 3/16" dowl to pin the firewall to the box sides. The result will be a lightweight and very rigid engine box. Your other option is to use standoffs. This are usually made of aluminum and some even have the thrust angle machined in. Check out http://www.aerografixs.com/ they have a nice install using standoffs.

You are using a micro engine by Giant Scale standards. Please don't overdo the construction. Example - my 1/3 Scale 96" Laser has a ZDZ 80 (80 cc) with a 26" prop on the front end bolted to an engine box with a 50% cut-out (big hole in the middle) 3/8" firewall. The sides are 1/8" light ply supported with a light ply doubler with big holes cut out of it. The bottom in 1/4" light ply with four big lightening holes cut out of it as well. The box has no top. Works great!

For a 130 FS (21 cc) I think you can easily get away with a 1/4 birch firewall corner braced to 1/4" light ply sides and bottom. The top can stay open. You could even add some lightening holes on the sides.

Light planes fly better than heavy ones!

Thank you all, gentlemen, you've provided the info I needed!

Mr Roseberry,
Without seeing the specific application I was simply giving an overview of the building process. I have a Lanier 35% giles with a zdz 80 installed, believe me when I tell you that the motor box had quite a bit of flex in it before it was reinforced. At minimum you need triangle stock around the motor box sides and bottom. Did you fuel proof your motor box and firewall? Lanier even recommends pinning the firewall to the box sides. I am confident that the engine box on my plane will not flex or break. I would rather add a few ounces of weight rather than have the motor box fail.