Gentlemen, I would like to retract my earlier thumbs down on using a 15cc gas engine in a Kadet Senior. As 2walla points out, a 15cc gas is equal to a .60 size glow engine. My brain must have been "out of gear" at the time as I was thinking it was equal to a 1.20 glow engine. I had it wrong. The extra weight of any of the discussed oversized engines is not the problem. The Kadet Sr. is inherently tail heavy and needs lots of weight up front to balance, so the old theory of adding nose weight with a bigger engine works here except most of them won't fit in the cowl. If you're going "cowless" then it works. If you are like me and must have that finished look of a nicely done cowl, then fitting bigger engines into a cowl limits how big you can go. For sure, a stock Kadet Senior will fly very well with a good .46 two stroke. Just last month, I passed on my stock Sig Kadet Senior ARF, that was flown for years with an O.S. .46 FX, to one of my fellow club flyers that needed that airplane to match his flying level. He returned my .46 FX and is powering it with a .55AX. It still flys great and he is a happy camper. To replace my missing Kadet Senior, I went to the boneyard in my shop and dug out a rejected Nitro Planes Super Senior ARF (Kadet Sr. knockoff) that I got from another Club member after he striped off the covering and realized what a POS it was. He was smart, he dumped it ! I was stupid, I followed through and reinforced, repaired and modified it. Lots of time, glue and additional balsa and plywood. Anyway, I completed it this past weekend and will maiden it next Sunday. How does that tie into the Kadet engine size question? I have a large inventory of .60 to .90 two strokes and .52 to 1.20 four strokes to pick from building this project. Yea, I wanted something more powerful than the .46FX in this one as I knew it was going to be heavier and I will add floats in the future. So, the only engine in this group that would fit inside the cowl (length was the problem) was an old K&B .61 with a slimline pitts muffler that I had. It fit in over 90 degrees with the cylinder head sticking out the right side for cooling just as nice as the O.S. .46FX had in my old "real" Kadet. My old Sig kadet weighed somewhere around 8 lbs RTF. The knockoff with reinforcements and mods came in at a whopping 11.0 lbs RTF. It's a TANK ! It required 1 lb 3 oz of lead to balance at 30% of MAC. Wing loading is still only at about 21 oz/ft2, so it should still fly well with that big flat bottom wing. Yes, I could have benefited from a heavier/larger engine but the K&B .61 was the best physical size for the finished goal of the project. The maiden flight this coming Sunday will tell me if struck a good compromise on this plane or if I missed the mark and it is underpowered. I'll let you know.