I've been flying a Butterfly for quite a while now and in my opinion a .20 to .25 is more than adequate. I've had both on mine. A .15 would probably do it but in the wind it's nice to have some reserve!! The Butterfly isn't a sport or pattern plane thus you don't ever run it full throttle very often or for very long duration. I wrecked mine once and tore the firewall off and simply glued it back together - I have no idea if it's got the recommended thrust angle, I'm not sure if I care, what I'm trying to say is on this model is dosen't make a lot of difference. Once your airborne just set your trims, fly figure 8's and relax.