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Jul 03, 2019, 10:02 AM
What could possibly go wrong?
Thread OP

Swappable engine mounts or firewalls

I'm looking for ideas to help make interchangeable engine mounts for my current build. It's just a standard ply firewall, and I'm fitting a Saito 130TD (4-stroke glow twin cylinder). That should be a good match for this model but I have 2 other engines I want to use some time too, an FA-182TD and a YS FZ91AC. The 182 is very similar in weight and dimensions but it has a different mounting bolt pattern, though they both use a similar radial-type mount. The YS is a single cylinder 4-stroke and needs a standard store-bought 2 beam engine mount, but once again that mount will have another bolt pattern too.

There isn't room to drill all the different holes in the firewall and fit blind nuts, in fact for beam mounts I could even end up with holes overlapping. It's possible I could convert from one engine to another by removing the blind nuts and filling the holes, then drilling for the 182 or another mount, but it's a one-off, I couldn't go back to the 130 again later. The model has been designed around that engine so using the 182 with it is not a priority, but that engine and the YS haven't been used yet and it seems handy to use an existing model to try them with until I can build something else for each. The YS will hopefully get a Kaos 90 made for it. OTOH the Fa-130td would be worth trying in that model too, so if I had a reasonable method for swapping either the engine mount or the firewall itself I could get more use out of all 3 of my "big" engines, and there are other obvious reasons.

But every idea I come up with falls short. I've posted some in the log at the link above. The first problem is weight with all the bolts and extra wood required, the other is the work involved. The engine box mount looks nice, but each box is a lot of work, heavy and needs a lot of fasteners. It also means shortening the nose section, which works for this model but not universally.

There's also swapping the firewall, so I can make a new one with different bolt patterns for the different engines or beam mounts. I haven't sketched it, I imagine it would have wood flanges on the rear side that slides into the fuselage and you then bolt through from the sides, top and bottom into the flanges. But that means a lot of bolts and nuts, adding weight.

Finally I could make a different radial type mount for the 130 so both it and the 182 can fit the same bolt holes. The 182 has its mount cast in so I can't easily change it, but AFAIK it's a removable piece on the 130 that I could replace. But making a new one is non trivial for me, I'm not really set up for that. Secondly that still doesn't let me use any other engines but those 2 twins; the YS is out, and if I got something new like a Saito FA-125 (glow) or FG-21, even though they might haul the model i would have to make major modifications to fit them.

Has anyone come up with a solution for this? I know it's tricky but if there's a way to do a swappable firewall or mounting box etc I would consider it.

If it's really no go and once I fit the 130 I'm committed to it in that model, that's OK too. It's a beautiful engine and the model has been designed around it. The reason I'm wondering about the ability to swap it is that I have these 2 engines I haven't used yet, which would probably also work in this model. The 182 is likely much more power than it needs and the YS could be just a little less, but that isn't a big deal, it's just about a temporary swap so I get to use those engines in the one model I'll have that's big enough to fly them, until I can build something better matched. For the YS maybe a Kaos 90, but that FA-130TD would be a nice engine for that model too (another reason it'd be good to know a way to include a swappable firewall or mount). The 182, possibly a pattern ship, or a scale model of something using a flat 4 or 6 engine, there are all sorts of options ; a Cessna 182 (I happen to think the FA-182 and 182 Skylane could work together well) or similar GA plane, Super Cub, or even something like an Extra. I also had in mind a scaled up version of a sports model like a BTE Venture 60. Whatever it is it should be fun.

Whatever it is, a convenient way to use different engines in the same model could be good. The best contender I've got so far seems to be the motor box idea, see sketches at the link above. But I'd appreciate any ideas, or any experiences you've had with the same idea. Even if it's just that it didn't work I'd be glad to hear about it. Thanks in advance
Last edited by BernardW; Jul 03, 2019 at 10:10 AM.
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Jul 04, 2019, 02:59 AM
What could possibly go wrong?
Thread OP
<sigh> I guess it's not really a workable idea or it would have been well known already. With a universal type mount you can use many different single cylinder engines, often without drilling new holes in the firewall, but that doesn't work for these engines, the 130 and 182 twins have their own mounts built in. They need to have the firewall drilled differently for each, which is why I was hoping I could swap firewalls on the same model. I made an engine test stand with that feature, you can just bolt a new piece of 1/2" ply onto it drilled for a different engine, but I'm not confident in that idea for a model. I had 4 bolts with blind nuts on the frame of the test stand but on the model I think it has to be more solidly fixed. I might consider it for much smaller, lower powered engines.

Oh well I guess I just build it for the 130 and that's its power plant, a perfectly fine option. In future when I want that engine for another model I might want to use something else for this one, so I could do a conversion then, but it would likely be a permanent change. That will either be to a universal mount or another engine with a similar mount, so in reality either the 182 or another twin, or even a radial if I own one at that stage. And that would have to mean basically a Saito 120r3, similar capacity to the 130, I don't know the weight.
Still if anyone has made a swappable firewall system, please share it! That 3rd one was about my best pencil sketch too but I guess no one thinks it's a real solution

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