Dave, I am continually amazed at the power inflation that has occurred in this hobby. People are constantly being advised that kit recommendations will be marginal. For sport planes, name your poison, but for scale models it just takes the magic out of the hobby. Not everything has to be able to do knife-edge loops.

The Sig Cub is a totally different animal from the Goldberg and GP Cubs even though the wingspan is not very different. It is a very old design originally powered with a .15 to a .19. I'm not kidding--the 71 inch Cub flew on 15s and 19s that had much less power than comparable engines today. Sig updated the kit and over the years they upped the power recommendations to a more popular range of sizes, but it is still light construction by sport model standards. The .48 is more power than it needs and it may be hard to fly it realistically with that much power and weight--it will tend to be a sport model in the shape of a Cub. Unless built very heavy, I'm quite sure you will need tail weight to balance. I've considered building this plane for my OS .26 FS, which would be plenty--I build light. My Champ was almost as large at 600 sp in, weighed 3 lbs, and flew beautifully on an OS .15 FP, including take-offs from grass. If not beefed up, your Cub will weigh around 4 - 4 1/2 lbs and fly very nicely on half the power you have. One guy on another newsgroup reported that his FIFTH scale Cub flew well on his OS .48 FS. That's got an 84 inch span and weighs around 7 lbs.

If you go through with the .48, then I think you should bite the bullet and move the firewall back. You're right--it would be disfigured by extending the nose and you'd need even more tail weight. If your OS 40 is an FP or LA, maybe you should use that. The power and weight would be more appropriate. Use an 11x5 or 11x4 prop on a 40 FP. Good luck, keep us posted. It should be a pretty plane. Jim