Airplane and engine configurations that yield reduced fuel burn per seat-mile, which is a metric of airplane fuel efficiency and carbon dioxide emissions, are highly desirable for commercial aircraft. This need for reduced fuel burn per seat-mile may be in conjunction with anticipated near-term increases in stringency of community noise certification regulations. Current European workplace noise exposure regulations that affect allowable aircraft cabin noise work together with local airport environmental policies to also pose significant challenges to advanced propulsion design.
The emissions-based requirements motivate extremely high bypass ratio engines that can most easily be accomplished with un-shrouded engines. Some such un-shrouded engines, however, may generate high amounts of noise and may be perceived of by passengers as low tech.
One existing approach to providing improved fuel efficiency or reduced fuel burn is to utilize turbofan engines with higher bypass ratios. However, very high bypass ratio turbofans suffer from large weight and drag penalties associated with their very large nacelles. Very high bypass ratio turbofans also suffer from difficulties associated with achieving under-wing installations in low wing airplanes and difficulties in achieving simple lightweight thrust reversers due to their large bypass ratios.
Another existing approach to providing improved fuel efficiency or reduced fuel burn is to utilize a turboprop, propfan, or other “open rotor” types of propulsor. An open rotor propulsor is effectively a propeller with a plurality (e.g., six to twelve) of discrete individual blades exposed at their tips, with a gas turbine core engine driving the propeller through a gearbox. Open rotor propulsors provide substantially better fuel burn through a higher effective bypass ratio and elimination of fan duct drag and weight, but entail airplane integration challenges, non-optimal community noise levels, and non-optimal cabin noise and vibration.
Thus, there is a need for improvements in turbine engine technology to provide high fuel efficiency, low emissions, low noise, and overall improved integration.