I am partway through this build and at step 565. I have found the drive mechanism to be very tight and have tried to loosen the drive elements as much as possible. I have found what I think is a weakness in the design at step 515. Basically, a key part of the drive is this linkage where the white connector sleeve is linked to the other elements with smooth-ended connection pieces. When at stage 565 I found this simply slips as there is no positive drive link between the sleeve and two blue connectors. My solution has been to replace the white sleeve and two blue connection pieces with a solid splined connector that I happened to have spare. I hope it now works when connected. I have attached two photos to show the original detail and my "fix". Am I missing something here ?
The "white connector sleeve linked to the other elements with smooth ended connection pieces" is certainly the intended way to build the model. The lack of rigid connection there acts as a torque-limiting clutch, preventing damage or injury if anything gets caught in the tail rotor while it's spinning.