Do not panic if all of a sudden the helicopters won't fly when the blade rotates, it could happen due to the hard crash. Try checking the inner gear shaft by holding your gear and then manually rotate the upper blade slowly, if the blade rotating means inner shaft tucked. There are two possible location of its slip, could be in balancer holder or gear.
Remove the balancer holder of the shaft, try again to hold the gear and then manually rotate the inner shaft, if slip is a problem in gear, if not slip means that the problem occurs at the balancer holder.
If slip occurs on the balancer holder, replace the balancer holder, note the sign on the shaft should be in line with the bolt. If slip occurs on the gear is actually only need to lower the gear down by pulling gear while holding the balancer, but if these events are often recurrent, gear position should be strengthened. There are two ways to strengthen the position of gear on shaft
- Strangling gear holder with thread or fine wire
- Using super glue, slide the gear approximately 5mm from the initial position, apply glue to the shaft, then slide the gear back into position, you should leave on 6 hours