Engine driving blades

I fitted the blades to the hub. I set them both to the same angle using a pole pushed against the 750mm radius part of the underside of the blade.
I ran the engine and gave it some revs, but it seemed out of balance. The drive came disconnected from the reduction parts. This bent the drive shaft which hit my hand. My hand seems to work ok for typing. I need to work out some good ways to balance the blades. Also I need to fix the drive shaft more securely, next time.
The blades were going at 4.1 revs per second when it broke ( I looked at the video frame by frame to measure the speed ).