Whether a prop has less drag freewheeling or braked has a lot to do with its pitch to diameter ratio. A plane gliding at 30 mph with a 3.75x12 3-D prop would have the prop windmilling at about 8400 rpm with the tip speeds at about 300 mph. At 300 mph, there is a lot of air drag, even if the tip is lined up with the airflow. If the prop is stationary, it sees an airspeed of 30 mph. Prop blades that see 30 mph wind have a lot less drag than at 300 mph, even with the prop's blade broadside to the airflow.

Rubberband powered freeflight planes have slow turning props with a very coarse pitch. Such a prop might only be windmilling at 1000 rpm at 30 mph. On this kind of prop, there may be less drag if the prop is allowed to freewheel.