Exploring the Operational Strategy of an Electrically-Driven Variable Pitch BLI-Fan

Abstract

Future commercial aircraft, from short range up to medium range, are expected to make use of hybrid-electric propulsion units to reach the ambitious emission targets. The design and performance evaluation of hybrid-electric propulsion systems is a challenging multidisciplinary task due to the necessity of matching the air breathing and the non-air breathing components. The addressed hybridelectric propulsion system consists of two conventional under the wing mounted gas turbines and an electrically driven, boundary layer ingesting aft-fan powering a medium range aircraft. This study investigates the low altitude and low speed operation of the aft-fan at end of field (EoF) conditions, which is challenging due to the limited power/torque supply of the electric motor in conjunction with challenging fan aerodynamics. The aft-fan is designed for the cruise boundary layer. Due to changes in the boundary layer profile at EoF compared to cruise, the aftfan is subjected to a radial incidence distribution transitioning from negative to positive incidence along the span. This distribution potentially diminishes the isentropic fan efficiency. This study also depicts a very small margin to the stability limit at take-off conditions. So, the aft-fan requires a variability to assure stable aft-fan operation. Three measures of improving the operability at EoF are investigated: Oversizing the electric motor with respect to cruise power, a variable area nozzle (VAN) and a variable pitch fan (VPF). The authors focus on the VPF as improvements in aerodynamic fan performance and electric motor sizing are expected. To give a clear picture of the underlying effects, the study on VPF takes advantage of detailed CFD analysis and the resultant benefit on propulsion system level is evaluated. To put the gains into perspective, the VPF is compared towards variable area nozzles and oversizing of the electric motor. In a final step, the synthesis of the findings assesses synergies of oversizing and introducing a VPF to reveal prosperous paths for future research.