Preliminary Design Framework for the Power Gearbox in a Contra-Rotating Open Rotor

Geared contra-rotating open rotors have the potential to further reduce fuel consumption relative to geared turbofans, but require a more-complex speed-reducing transmission system to drive the propellers. Hitherto, the preliminary design for such transmission systems has been reported independently of the overall engine modeling or has been limited by many predetermined engine constraints. This has restricted the feasible design space of the transmission systems. A simple transmissions preliminary design approach is needed that does not involve complicated mechanical assessments and can be integrated with engine preliminary design studies. This paper presents a novel design framework for sizing a double-helical differential planetary gearbox for a contra-rotating open rotor. An up-to-date methodology is proposed for the design of maximum load capacity gears for this application based on the power-transmitted, durability, and space envelope requirements. The methodology was validated with published data, demonstrating only very minor differences in geometry dimensions. Parametric analyses have been made to assess the impact of the design assumptions on gearbox dimensions. The framework also enables the identification of feasible torque ratios between the output shafts of the speed-reducer and the contra-rotating propellers driven by them. The impact of the torque ratio on the size of the gearbox has been analyzed for equal propeller rotational speeds and different speed ratios between the output shafts. This study shows that potential torque ratios lie between 1.1 and 1.33, with the higher ratios enabling more compact gearboxes having four or five planet gears.