This is the Evolution Power System Propeller: A. The Evolution Engines Power System Propeller was designed to fly at slow speed while maintaining excellent thrust for climb and aerobatic maneuvers. Engine sizes from 36 to 46 are comfortable with the 10.5x4 three blade propeller. You will find that the top speed of your model will be controlled within the 52 to 70 mph range. Of course this is very practical for training purposes however the high thrust at low speed is perfect for current 3D flying in models designed for maneuvers at slow speed. Features: Produces great thrust Controlled 52 - 70 MPH top speed Perfect for engines from .36 to .46 1/4" Prop shaft hole Great for training, as well as 3D flight in models designed for slow speed maneuvers