Silly construction question: How exactly do people build those box-style recessed firewalls? What thickness of ply is required (on a 40-60 size model, for example)? Is it enough just to butt-join the parts of the box with epoxy or is it necessary to build in dove-tail style interlocking pieces?

Photos gleefully accepted!

the sides and top and bottom can be 1/8 lite ply and use 1/4 for the firewall and drill and pin it with 1/8th dowl wood then put triangle stock in the four inside joints

Like this? I used 1/4 inch on all of it, butt jointed, epoxied, and screwed thru the joints with #0 screws, then a layer of fiberglass (1/2 oz) on the backside of the joints. It's probably overkill; it's only a .65 bolted to it. The stock firewall is next to it in one picture.

Depending on how much of a recess you need, you can just cut a hole in the original firewall large enough to allow the mount to fit into, then laminate another piece of ply on the back. With a 1/4" ply firewall this would give you a 1/4" recess. The laminate on the back need not be as large as the original firewall. Continue the process and each layer will yield another 1/4". This method is heavier so use it on airplanes that will need nose weight anyway, like WWI biplanes!

Randy

Randy, that's an interesting idea. I wouldn't have though of that. In the case of my Flair Legionaire I need to be able to recess the engine+mount (Saito 56 with standard nylon filled mount) a full 6cm. So imagine it might be a bit much to put a 6cm thick block of wood up front.

I have consider using the method recommended in the instructions, namely bolting an aluminum plate directly to the back of the engine crankcase and then screwing the plate onto the firewall (thereby eliminating the mount entirely). Doing that would cut off nearly 5cm in length (not to mention weight) and require only a few cutouts (and small box) around the carb. But I don't really have any experience or confidence with this system. Plus with the lenght of the Saito and the shallowness of the cowl I'd still have to recess the firewall a bit -- though Randy's trick would now work.

And of course the engine+mount makes for a very long unit that will necessarily push the other component further back in the fuse in the fuse which can only be a bad thing.

Here is something else that I was going to try on the Hanriot, but it just didn't work for me. I was able to reduce the overall length of the Saito 180/mount combo by sliding the motor back on the mount, after removing the velocity stack from the carb. Is your Saito similar? The front hole of the engine lug is in the back hole of the mount. The location of the front hole in the mount is marked in black. This saved me almost 1/2" but it still wasn't enough...

If the first picture is still here it is because I don't know how to delete it....the second picture is the Saito....

Mine is the newer model Saito (actually I've got two 56's one in silver and one black/gold). I'm afraid I'm not experienced enough with engines to go removing parts I can't even name! Obviously, a different style engine mount could help but I'm a bit limited here in Japan.

Also on the topic of engine mounting and radial cowls, how do you deal with the center offset caused by side and down thrust? How do you go about figuring out how much you need to relocated the mount to compensate?

Another question is the size and shape of the box. I'm having a little difficulty getting my brain around this. Does this diagram look about right? I also wonder whether I shouldn't just build in a whole second firewall and just leave the current one as a "false firewall." I'd still be able to put the battery and possible also receiver forward of the new firewall.

That's a tough one, isn't it? What about hardwood beams as a motor mount? I think maple was/is popular. Maybe something similar to the Pup arrangement but without that crazy box. You could support them on the backside and front of the firewall with triangular gussets and maybe another former an inch or so behind the original firewall. It would not need to be a full former so that would leave room for the fuel tank . If you extend the beams back far enough the tank could rest on top of them. Trying to accomodate that motor mount is going to be tough.

On second thought maybe that box from the Pup would be just the ticket....You could cut away parts of the firewall to clear the engine..once assembled I don't think strength would be an issue...

Randy

Interesting thread, my question is, how will this affect the CG when the engine is moved back into the fuse?

Joe

Doesn't seem to me that the CG will be effected in any major way because the engine itself is still in the same location as the engine on the plans (though of course probably a bit heavier since they use an OS 40 FS). Obviously there's the weight of the mount itself. But the location of the CG shouldn't change.

Randy, I'll have to look harder at the Puppeteer instructions/drawings to see how the box idea might be an option. I'll also think about the idea of wooden beams -- though as far as I know there aren't any of the usual hardwoods available here in "model sizes." There is one dark, dense wood with a Japanese name I haven't been able to translate yet that might work for beams.