I had an OS .50 SX on my Avistar... it flew very nicely. I only put a .40 LA on .25 sized planes... power output and weight of a .40 LA is right in between an OS .25 FX and a .32 SX. I had a .32 SX and a .40 LA in a .25 sized SPAD awhile back, the .32 definitely had more power, but the .40 wasn't too far behind, and it was less than half the price!