Angle of Attack Effects on Open Fan Propulsion Systems at Take-Off Conditions

Abstract

Open Fan Propulsion Systems, often referred to as Open Rotor and Stator (ORAS) or Unducted Single Fan (USF) engines, promise a significant improvement in specific fuel consumption over present turbofan engines, which they derive from their ultra-high bypass ratio, made possible through the omission of a shroud or nacelle around rotor and stator. This type of propulsion system aims to retain many of the efficiency benefits of the Contra-Rotating Open Rotor (CROR) studied intensively in many research efforts in the past decade while eliminating the main drawbacks of weight and complexity of that configuration. As for any open rotor or propeller, angle of attack effects are an important consideration for both the aerodynamic performance as well as the noise emission characteristics of this engine. In numerical studies of a generic ORAS engine in the frame of the Clean Sky 2 project, notable angle of attack effects were found to affect the aerodynamic performance of the swirl recovery vanes. A detailed aerodynamic analysis in this paper highlights these adverse impacts and investigates the impact on performance that SRV blade setting angle adjustments have to mediate the unfavorable flow conditions for the SRVs at angle of attack.