An ultra--safe, high ratio compound planetary transmission, for application as a helicopter main rotor drive,
has been designed under the sponsorship of the National Rotorcraft Technology Center-- Rotorcraft Industry
Technology Association (NRTC/RITA). It is anticipated that this new planetary transmission offers
improvements relative to the current state--of--the--art including, reduced weight, reduced transmitted noise
and improved fail--safety. This paper discusses the analysis and design results for the subject planetary
transmission. Fabrication and testing of the transmission will be conducted in subsequent phases of the
project.
Typically, the final stage in helicopter main rotor drives consists of one or two simple planetary stages which
themselves are composed of a sun gear,multiple planet gears (typically between 3 and 6) and an internal gear
which is generally the fixed member of this epicyclic system. The main rotor transmission is the most critical
and usually the heaviest assembly in the drive system for any rotary wing aircraft, be it a single or tandem rotor
helicopter or a tilt rotor vehicle.
The new ultra--safe, high ratio planetary transmission design utilizes a compound planetary configurationwith
a 17.5:1 reduction ratiowhich would replace a conventional two stage simple planetary transmission. The new
design uses ultra--safe principles such as split--torque paths and high combined contact ratio gearing. Double
helical gears in the planet/ ring meshes balance axial tooth forces so that axial bearing reactions are not
required. The spur gear sun/planet meshes are staggered to achieve a compact spatial arrangement.
In order to provide major, simultaneous improvements in weight, noise, power density, and reliability, a
paradigm shift must occur in the basic design of the gear system in this component. This program brings
together a large number of individual gear system innovations which, taken together, are expected to yield a
large improvement in noise and performance combined with improved reliability and fail safety.