Riff_raff,
(if that is your real name
)
I imagine helicopter gearboxes use straight cut gears for improved power to weight. Helical gears were invented to reduce geartrain NVH, and might help with the noise concerns (it was mentioned somewhere). To avoid the side thrust the norm would be to use two mirror image helical teeth (herringbone). If hobbing machines required an excessive central gap then two seperate gears could be bolted or friction welded together.
Load sharing between multiple planets is an interesting problem. The first solution which springs to mind is the idea of a compliant mount for each bearing housing, restrained by a guide plate to limit radial movement - the problem would likely be how to avoid transmission resonances excited by the rotors. If a hydraulic system is allowed on the planet carrier then each planet could be postioned by a small oil hydraulic ram, so that all planets see equal load. The next problem is then how to stop the rams all bottoming out over time if the oil in the hydraulic circuit is not replenished. This implies the need for a complex mechanical load sharing arrangement. Either way the clever stuff needs to happen in the planet carrier(s).
Of course all this assumes that tail rotor drive requires the one-way clutch to act on the high rpm side of the transmission (otherwise worm gears would get a mention). Are electric motor driven tail rotors outside the scope of the project?
Mart