I'm thinking of building a J3 cub powered by a 31cc ryobi. Can anyone tell me which would be a better fit, the long shaft or short shaft? The cub will most likely be Sig's 1/5 scale cub.

That engine is way too big for the 1/5 scale, if you insist on that engine I would recommend building a 1/4 scale.
Anthony

i agree with cubnut the 1/5 is too small for that motor a sig or balsa u.s.a 1/4 cub would be better

Cubnut, your right I misposted the message I should have said 1/4 scale. I must have cabin fever.

I'm also about to build the BUSA 1/4 Cub with Ryobi. I've laid the motor on the plans and it of course will require moving the firewall back two inches to accommodate the mount which encloses the rear carb. As to which shaft, I've the short shaft and I'm not sure there is an advantage either way. The short will work.

Even with the magneto, the only thing extending from the cowl looks to be the last half inch of the spark plug and of course the plug wire but on the bottom it won't be very offensive. The spark plug is not a large one and there is a little more than a half inch of porcelain between the wire boot and the plug base so if a shorter plug is available, it might be that even the plug would be within the cowl. It wouldn't take a very big fairing added to the cowl to cover the plug.

The exhaust stack will exit the bottom of the plane aft of the cowl and if too much exit ventilation existed at the spark plug... it might not ram enough air back to ventilate around the exhaust stack. This of course can be dealt with.

I went with the fiberglass cowl with motor appendages.

I"ve dealt with the argument about 4 stroke sound with the Cub... but I'm wanting both fuel economy and far less exhaust oil.... plus, the Ryobi was on hand and cheap.

Can you get a prop on the short shaft? I've only done the long shaft conversion but my impression was a prop hub was screwed onto the short shaft and a prop bolt was used to secure the prop. I would think the long shaft would be preferable. Just my opinion.
Edwin

Hi Edwin,

I think you are correct that the long shaft is generally preferable as it likely allows for a range of fore/aft adjustment of the prop. The short shaft uses a prop hub and thus the prop location is fixed. Fortunately for the Cub, the prop hub is an ideal length. There may be other advantages to the long shaft that I don't know about.

I did the conversion for my first trimmer motor making the hub, hub nut, velocity stack and even the exhaust stack from solid aluminum but the Ryobi I bought from another flyer and he'd all ready bought the conversion parts. The motor has never been run... but I don't know where he got it from.

The one other thing that I might note... the mount that I have has mounting flanges that turn out where many of the mounts are made from rectangular tubing and therefore have no outward flanges and bolt through the area aft of the carb. With the set back firewall, the longerons and deck will be 2" forward of the firewall making it impossible to get at all four mounting screws. Studs would need be fixed to such a mount and nuts used aft of the firewall and as I can't yet visualize the interior after the firwall move, I'm not sure that would be fun. I'd think that an outward flanged mount would be a better choice. The flanges will need to be on the sides... as the mount will set fairly close to the upper deck.

I'm building a busa 1/4 scale J-3 right now. The front end is beefy enough that you should be able to move the firewall back with no problems. Theres enough room inside (right behind the firewall) that you should easily be able to get your hands in to mount standoffs from the firewall to a backplate on the engine (with the fuel tank removed). I was thinking of doing this in a future mount and running some 8-32 or 10-32 all-thread from the firewall to the engine plate. That ryobi is perfect for narrow cowled designs.
Edwin

I'm also working on a SIG 1/4 scale CUB with a Ryobi engine. I moved the firewall back a few inches and it looks like a good fit. One thing I'm concerned about is getting air into the carb. Here are some pictures.

SMK... looking very good. Wow, a very neat building area. Will the Ryobi hand crank with the electronic ignition?

Thanks AA5BY. My work area is very nice it’s about 20ft x 20ft and could be bigger but I don't need the space yet. I have not run the engine yet but I was told its best to use an electric starter. A friend of mine does these conversations. I hope to start doing them myself.

Steve... Checking the progress on your cub?

I had a bad experience recently after building a plane and fitting a YS-120 to it without firming up that the motor would run to suit. I'm now backtracking to a different engine for that plane. Not wanting that to happen to my cub build, I've been breaking in the Ryobi and am not fully satisfied with it yet but getting closer.

Other projects are finishing up and the cub build is next so want the engine nailed before starting especially because of the firewall setback.

I am in the process of building a 1/4 scale Balsa USA Cub. I'm planning on installing a 31cc Ryobi also. What is the exact measurement that you moved the firewall back. Is there anymore pictures of your build. Can't seem to find a good thread with any details.