Investigation of Flutter Mechanisms of a Contra-Rotating Open Rotor

Abstract

The growing pressure to reduce fuel consumption and cut emissions has triggered renewed interest in contra-rotating open rotor technologies. One of their potential issues is self-excited or forced vibration of the unducted, light-weight, highly swept blades. This paper presents a numerical study into the flutter behaviour of a contra-rotating open rotor rig at take-off conditions. The study presented in this paper aimed to validate the numerical approach and provide insights into the flutter mechanisms of the open rotor under investigation. For the initial validation, pressure profiles and thrust coefficients from steady state mixing plane calculations were compared against rig measurements. A full domain unsteady analysis predicted front rotor instability at low advance ratios. Flutter occured in the first torsional mode in 0ND and 1ND which agreed with experimental observations. Subsequent unsteady computations focussed on the isolated front rotor and first torsional mode. The flow field and aerodynamic damping over a range of advance ratios was studied. It was found that minimum aerodynamic damping occured at low advance ratios when the flow was highly three-dimensional on the suction side. A correlation between the quasi-steady loading on the blade and aeroelastic stability was made and related to the numerical results. The effects of variations in frequency were then investigated by linking local aerodynamic damping to the unsteady pressure on the blade surface.