Re: Going to a .61 on a .50 sized airplane, there is also something to be said for being able to run a bigger prop at lower rpm for better performance. I had a 40 size engine on my LT40 trainer. It worked but when the grass hadn't been cut in 2 weeks because it rained everyday it made for some pretty slow and scary takeoffs. After I started teaching and a new guy came out with an LT40 with a 46 engine on the front the extra oomph really helped both on the field and in the air when the wind whipped up which it did quite often at our field. The plane didn't really fly any faster but the extra power was there when needed. [8D]