Choose "user". Load "Broadcast"(.urs). Near "Enable motion blur" click Edit. Change Number of Passes. Default is 1. Try 3 for a test. 5 to 10 should be cool.
And, the propellors should turn 90 during 3 frames.

The ultimate trick is to use alpha planes with a picture of fade propellors (motion blur in/on the image)... with slow rotation.