A Chinook Weather Vane with synchronized rotors. This is a large model the fuselage is about 470mm long and the rotor discs are 580mm diameter. I would not recommend printing the blades in PLA as they will just droop down in the sun.

I have catered for 22x8x7, 19x8x6, 16x8x5 ball bearings print only the ones you need, you can mix and match if you don't have enough as the centers are all aligned. Each plate of bearing has spacers to use with that width bearing. The spacers are for aligning the crown gears on the main rotor shafts.

The attached profile is for Bambu ABS, the parts fit tightly and no glue is needed. I tried printing some parts in PLA Basic, the only parts that wouldn't fit were the hex shafts if you do use PLA then set the “XY hole compensation” to 0.1 and print just the gears and rotor hubs like that.

Assembly

1. Press all the bearings into the bearing blocks.
2. Press the 3 bearing blocks into front part of the fuselage. The two main rotor blocks need to have the bearings face up
3. Push the shorter main shaft in into the topmost bearing and thread the spacers and crown gear onto the shaft. Use the longer of the two short spacers and the shorter of the two long spacers for this.
4. Press the rotor hubs onto the main shafts, I put the CCW rotor on the front. I still wonder if it will work better the other way around Leave the blades for last!
5. Assemble the back bearing blocks the same way but leave the bottom rotor shaft block out or just halfway add the long spacer between the blocks and then press the bottom block into the frame fully..
   
6. Push the long drive shaft into the front fuselage pinion bearing block, add the 0.8mm washer and the pinion gear and press the shaft into the pinion until it flush with gear front face.
7. Slide the two halves of the fuselage halfway into each other and sort out the back pinion the same as the front. Before you push them together align the rotor shafts as best you can so that both heads have a grip facing directly forward
8. Stick a piece of filament into the frames to keep them together.
9. Work the gears, they should have the right clearances already. I added loads of WD40 and it seemed to wash out the grease and they run much easier.
10. Push in the blades with the printed side facing down and secure with a piece of filament, glue it it's too loose, mine were tight as! Clean the holes with a drill if needed.

This was a rushed project for the flying things contest, please let me know if there's anything I need to fix or adjust.