Hi Brad,

I built the Sig Spacewalker 2 (1/4 scale version), and power it with a stock zenoah G-38. The prop I use is an 18x6-10. I fly at 4100 ft, and this is a good combination. To accomodate the engine, I set the firewall back 1 inch. Additionally, I had to add a heck of a chunk of lead to the tail to get it to balance. The G-38 is not light. However, even with this added weight, she still flys very nicely. I typically fly at 1/2 to 3/4 throttle, and landing is fairly slow. The engine fits perfectly inside the cowl though I did have to put a hole in the side for the carb to protrude.

The 18x6x10 prop has good ground clearance.... I would be hesitant to use a larger prop, if you plop down a little hard on landing, the gear flexing could let the prop strike.

If I lived at a lower altitude, I would be tempeted to use the newer generation G-26 engine instead. Whenever I go to a meet that is close to sea level, I can see that the G-38 runs with so much more power. But on the other hand, if you already have a 38cc ready to use.... that is a bird in the hand. I would work on ways to lighten it as much as you can to reduce the amount of tail weight needed.