Forced response analysis of a shaft-driven lift fan

Abstract The aim of this paper is to give an overview of the multi-bladerow forced response analyses carried out on a shaft-driven lift fan. The lift fan, used for vertical landing and take-off, is situated behind the cockpit and contains seven bladerows, of which two are counter-rotating rotors. The aim of the analysis is to determine the maximum vibration amplitudes of the two rotor bladerows for a range of configurations and speeds. Unlike typical axial-flow compressors, a significant part of the unsteady aerodynamic excitation is due to the distortion of the inlet flow over the cockpit and fuselage, a situation that creates several low engine-order harmonics. In addition, the main blade passing harmonics, arising from the bladerows immediately upstream, also need to be considered. Both the blading and the inlet geometry are difficult to discretize, the former due to overlapping bladerows and the latter due to complexity of the aircraft geometry including the louvred doors. The forced response analysis methodology is based on using an integrated aeroelasticity model which combines a non-linear, time-accurate, viscous unsteady flow representation with a modal model of the structure. The rotor vibration response was assessed at various shaft speeds for both stationary aircraft and at a number of flight speeds and yaw angles. Wherever possible, the findings were compared against measured experimental data and good agreement was obtained in most cases. The main conclusion is the feasibility of being able to use a numerical tool as an integral part of the design process, a route that allows a much more efficient coverage of the flight envelope compared to actual rig and engine tests.

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