Numerical Investigation of Inlet Distortion for Different Rear Mounted Engine Installations at Taking-Off Conditions

Abstract

Rear mounted engines are widely used in business jet and regional jet applications. It is a “clean wing” design. The engine is mounted behind the wing, so that the inlet/exhaust of nacelle has a minor influence on the flow over the wing. The engine thrust line is close to the fuselage axis. As a result, the asymmetric yaw moment will be smaller when single engine stall occurs. Field experience and historical data have revealed that engine aerodynamic stability and fan aeromechanics are extremely sensitive to the uniformity and steadiness of inlet flow. Engine inlet flow could be distorted and separated at various operating conditions, such as, high ground crosswind, take off, and other high angle of attack (AOA) maneuvers. As a result, strict regulations and requirements were set by certification agencies to assess: i) aircraft maneuver capability, ii) engine operating characteristics, as well as iii) aerodynamics/aeromechanics behaviors and capability, with respect to flow field surrounding the entire propulsion system. Due to the nature of complexity of the flow field at air intake, the inlet compatibility of fuselage mounted engines becomes one of the most complicated & challenged item to meet the FAR33 as well as the FAR25 certification requirements. This research paper discusses the inlet compatibility of rear-mounted aircraft engine with respect to AOA and crosswind under various operating conditions. Models of two installed configurations, which set by relative position of engine to fuselage and wing, were created. At each case, the engine inlet flow field was calculated at various AOA and crosswind conditions. Comparisons of total pressure contours at air intake were made to assess the likelihood of flow separation. The radial and circumferential inlet distortion levels were calculated at the assumed inlet AIP location for each operating condition and installed configuration. Assessments are made based on intensive usage of CFD analysis, and substantiated by test results. The flow field information obtained by CFD calculation reveals a close coupling phenomenon exist among engine installation, AOA and inlet capability. Analytical results were also checked, and the results agreed well with that of the compliant flight tests.