An Assessment of Relative Technology Benefits of a Variable Pitch Fan and Variable Area Nozzle

Abstract

The progression towards ever higher bypass ratios in commercial gas turbine engines has increased the operating disparity between the fan flow conditions at different flight speeds, requiring the use of variable geometry to maintain the surge margin throughout the flight envelope. Mechanisms such as a variable pitch fan and a variable area nozzle help maintain the surge margin while providing greater fuel burn reduction, especially as engine bypass ratio increases. The objective of the current work is to assess the boundary between the aircraft performance gain from implementing a variable pitch fan or variable area nozzle on a commercial aircraft engine as bypass ratio is varied, while accounting for the negative impact from increased weight due to the both mechanisms. To perform this study, a new variable pitch fan model was implemented within the Environmental Design Space (EDS). The new model uses NASA’s Flow Modulating Fan (MODFAN) program to generate fan performance data. The variable pitch fan model effectively varies fan blade pitch angles within EDS to meet peak efficiency conditions throughout the operating regime with a constraint on the minimum allowable surge margin. To complete the EDS variable pitch fan model, a component weight was also associated to the variable pitch mechanism. The variable pitch fan model was then used to perform the tradeoff study to obtain the integrated, relative performance impacts of the technologies on the complete aircraft/engine system. For the variable pitch fan and variable area nozzle, the engine bypass ratio was changed by a fan pressure ratio sweep at three overall pressure ratios. The aircraft chosen for analysis is representative of a 150 passenger, Boeing 737-800. The analysis is performed on advanced notional geared turbofan (GTF) engines. The analysis concluded that the benefits of the variable pitch fan start to outweigh that of the variable area nozzle at a bypass ratio of approximately 20 to 27, dependent on OPR.