Aerodynamic damping of composite UHBR fans under the consideration of acoustic intake reflections

Abstract

Steadily rising regulations and demands on aeroengines require the continuous reduction of CO2 emissions. Among other factors, particularly the weight reduction of the engine as well as synergetic engine-airframe integration are of major concern. Both lead to the application of shorter and more sensitive intakes as well as new materials, such as fiber composites. Fiber composites generally have a considerably lower density but a similar stiffness compared to common fan blade materials resulting in a reduction of the engines' weight. A further difference from conventional materials is the anisotropic behavior of the stiffness, imposed by the ply orientation. In this paper, the impact of the use of fiber composites for the scaled rotor of an ultra-high bypass ratio (UHBR) fan on the aeroelasticity is investigated numerically. In order to influence the eigenfrequency and mode shape, the ply orientation of the blade lay-up is varied. The influence on the resulting aerodynamic damping is analyzed numerically, using a harmonic balance approach. For an accurate prediction, the aeroacoustic reflection at the intake highlight plane is incorporated in the numerical model and its impact is quantified for different intake lengths. The results are compared to a titanium alloy blade design (Ti-6Al-4V) and show the capability of varying the eigenfrequency with a coupled change in twist-to-plunge ratio due to lay-up variations. This change of the structural dynamics of the rotor blade influences the aerodynamic damping. Additionally, acoustic reflections are found to affect the stability, depending on the lay-up, operating condition, and intake length. A lay-up was found, which stabilizes the fan blade for all investigated conditions operating with a typical short ultra-high bypass ratio (UHBR) intake. A special lay-up generates negative aerodynamic damping of -6.6% (logarithmic decrement) when operated close to stall. This fulfills the present project's particular need to measure flutter in a wind tunnel.