Don't ever buy an inrunner motor for a helicopter. They are fine if used in airplanes or ducted fan jets. But in my experience, they just don't get enough airflow in helicopter applications. The brushless inrunner motor that came with my falcon 3d burned up pretty quickly. I replaced it with the align 430L outrunner and dont have any cooling problems, and it appears to be more efficient. Kv rating is one of the most important factors. It tells you how many RPM the motor will deliver for each volt applied. That will help you choose the right motor.