i have a GP cub it is a 60-90 size with 96" wing it is in about the 12 lb range will a ryobi 31cc be to big for this plane . it is a 1:4:7 scale size kind of a odd scale i think .

rik1024 :
i have the same plane that i got used , they said it has had a 90cc 4 stroke , and a 25cc homelite on it and they did not have enough power to do the job so i am putting togeather a Ryobi 31cc with CH ignition on it to try, if that wont work right i will go to a 33cc MACC , it is the same weight as the Ryobi. We will see if it will stay in the air with one of them.

J3_

umm... 0.90ci 4-stroke not 90cc....we'll cornfuze some newbies here
The ryobi with C.H. would be great it the GP cub. The flywheel might not fit under the cowl, but if you dont care fly it with out it just to see how it does and add the C.H. later if you want. Piper

sorry about that Pipercub, that is what hapends whe you get to be 66 years old and still building these rockets

I think you'll be pushing it. A Ryobi 31cc would be good for a true 1/4 scale Cub, but even then you'll need to plan extensively on how to mount the engine. with a motor mount, the Ryobi is more than 7" long from back of mount to prop hub. Another thing to consider before you modify the kit to fit the engine is prop clearance. The Ryobi likes an 18x8. You can go down to 16x10 but that's about the smallest prop you'd want on the Ryobi. Make sure the GP Cub kit can clear a 16" prop with ease.

If you've got the kit NIB and the Ryobi already, give it a try. Otherwise, I would recommend against it. I've got a couple of WM 1/4 Cubs - one full wing and one clipped. My Ryobi with CH Ignition will fit within the cowl, but I'll have to mount the engine such that the carb sticks through the firewall. This means that I'll have to cut a hatch cover to get access to the carb adj and throttle linkage - not to mention another firewall behind the carb to keep the gas sprays and spillage off the radio compartment. I am going to use one of my smaller Echo 25.4cc engines instead. Sure the carb and muffler might poke through the cowl a little, but it'll be a direct bolt on to the existing firewall.

For your reference, I just finished up a Sportsman Aviation Super Decathlon (86" span 1.245 sq.in. wing area) with a Homelite 30cc. I've also got CH Ignition on this one and it balances out the 15lbs plane with no lead needed. The weather here lately has been crappy, so I haven't had a chance to test fly it yet. I expect it to have just the right amount of power for scale aerobatics.

Volfy makes very good points. Some might say that there is no such thing as too much motor....but.....as far as coverted trimmer engines go, the ryobi as a lot going for it. Muffler and carb wont poke out the cowl and mees up the scale look but with every give there is a take. The wing area of the GP Cub looks to be able to be a good flier on the ryobi but there will may be more work in getting it to fit than some want. I'm building a Balsa USA 1/4 Cub and will hang a ryobi with C.H. on the nose. It would fit with the flywheel and coil but would be very tight. And as Volfy said, I'm going to poke a hole in the firewall to stick the carb through. I'll also need to hack up the lower part of the firewall and build a box to make room for the muffler. None of the mods to the airframe makes this impossable to do, just challanging and needs planing. The pay-off will be great when done and done right. Nice sound, be able to turn a scale prop with great thrust, runs on gas not glow and most of all will fly on a motor that I got out'a the trash Piper

I know it will take some mods but i like doing things like that . I am going to use the c&h ignition and one of there mufflers i am not worried to much about scale look just a every day flyer and we all know how beat up they can get . I think this will be a lot of fun to try and make work . thanks for all your help and suggestions . I will post more about this when i get some more done on it thanks again.
rik1024