A .40 may be pushing it... and TURBOPROP I am currently running a .46fx, and have not have any overheating issues. I did however cut a half circle in the bottom of the cowel to allow for optimum airflow. I'm using a 12-4 prop, which has proven to be perfect for 3D. At first harriers and hovers were very hard, but i recently moved the battery back, and it's really locked in. Hovers at about 90-95% throttle depending on the fuel and wind, and I can harrier it a few feet off the ground for the length of the runway... This plane is absolutely fantastic and makes me look like a good pilot too