| Summary: | Rotorcraft transmission design is limited by empirical weight trends that are proportional to the
power/torque raised to the two-thirds coupled with the relative inexperience industry has with the
employment of variable speed transmission to heavy lift helicopters of the order of 100,000 lbs
gross weight and 30,000 installed horsepower. The advanced rotorcraft transmission program
objectives are to reduce transmission weight by at least 25%, reduce sound pressure levels by at
least 10 dB, have a 5000 hr mean time between removal, and also incorporate the use of split
torque technology in rotorcraft drivetrains of the future. The major obstacle that challenges
rotorcraft drivetrain design is the selection, design, and optimization of a variable speed
transmission in the goal of achieving a 50% reduction in rotor speed and its ability to handle high
torque with light weight gears, as opposed to using a two-speed transmission which has inherent
structural problems and is highly unreliable due to the embodiment of the traction type
transmission, complex clutch and brake system. This thesis selects a nontraction pericyclic continuously variable transmission (P-CVT) as the best approach for a single main rotor heavy
lift helicopter to target the above mentioned obstacle for drivetrain design and provides
advancement in the state of the art of drivetrain design over existing planetary and split torque
transmissions currently used in helicopters. The goal of the optimization process was to decrease
weight, decrease noise, increase efficiency, and increase safety and reliability. The objective
function utilized the minimization of the weight and the constraint is the tooth bending stress of
the facegears. The most important parameters of the optimization process are weight,
maintainability, and reliability which are cross-functionally related to each other, and these
parameters are related to the torques and operating speeds. The analysis of the split torque type
P-CVT achieved a weight reduction of 42.5% and 40.7% over planetary and split torque
transmissions respectively. In addition, a 19.5 dB sound pressure level reduction was achieved
using active gear struts, and also the use of fabricated steel truss like housing provided a higher
maintainability and reliability, low cost, and low weight over cast magnesium housing currently
employed in helicopters. The static finite element analysis of the split torque type P-CVT, both
2-D and 3-D, yielded stresses below the allowable bending stress of the material. The goal of the
finite element analysis is to see if the designed product has met its functional requirements. The
safety assessment of the split torque type P-CVT yielded a 99% probability of mission success
based on a Monte Carlo simulation using stochastic- petri net analysis and a failure hazard
analysis. This was followed by an FTA/RBD analysis which yielded an overall system failure
rate of 140.35 failures per million hours, and a preliminary certification and time line of
certification was performed. The use of spherical facegears and pericyclic kinematics has advanced the state of the art in
drivetrain design primarily in the reduction of weight and noise coupled with high safety,
reliability, and efficiency.
|
|---|
