Coaxial transmission/center hub subassembly for a rotor assembly having ducted, coaxial counter-rotating rotors

One embodiment of a coaxial transmission/center hub subassembly for a rotor assembly having ducted, coaxial counter-rotating rotors includes a single stage transmission, a transmission housing, and a center hub support structure that are structurally and functionally interactive. The transmission housing includes upper and lower standpipe housings secured in combination with a middle housing. The single stage transmission includes an input pinion gear rotatably mounted in combination with the middle housing, and upper and lower spiral bevel gears rotatably coupled in combination with the input pinion gear to provide counter-rotation thereof. The spiral bevel gears include integral rotor shafts, respectively, rotatably mounted in the standpipe housings. The hub support structure is configured for securement of the middle housing internally in combination therewith, with respective surfaces thereof in abutting engagement so that dynamic loads of the rotors and thermal loads are directly coupled into the hub support structure via the middle housing while bending moments of the rotors are canceled in the middle housing. The hub support structure has radially extending arms for mounting the coaxial transmission/center hub subassembly in fixed coaxial relation to an airframe structure and for coupling the dynamic and thermal loads to the airframe structure. External surfaces of the standpipe housings function as sliding surfaces for linear motion of swashplate subassemblies to minimize the separation between the rotors so that the airframe structure has a compact aerodynamic and structural envelope. The transmission housing is internally configured to provide a splash lubrication subsystem.