John-
Here's a couple good design handbooks for helicopter drives. One is from Sikorsky and one is from the US Army. They were published back in the 70's, but the fundamental design practices described are still in use. The major changes in the past few years are improvements in gear and bearing materials/heat treatment/manufacturing techniques, design/analysis tools, and techniques/instrumentation for monitoring condition of the drivetrain.
www.dtic.mil/dtic/tr/fulltext/u2/a069835.pdf
http://everyspec.com/ARMY/ARMY-Gener...974.050855.pdf
Here's a fairly recent (2010) NASA study for a large civil tiltrotor drivetrain:
https://mdao.grc.nasa.gov/publicatio...W-IPLC2010.pdf
I also forgot to mention one of the best resources for keeping abreast of the latest developments in rotorcraft drive systems. The US and other national patent office websites. I'd suggest searching both issued patents and patent applications.
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