Inlet Compatibility and Fan Aeromechanics of HBP Turbofan Engine

As an integrated system, turbofan engine airworthiness certification is a complex network because design, operating conditions and multi-disciplines are interlaced. Inlet compatibility specified in FAR regulations is to demonstrate satisfactory of engine operating characteristics throughout the flight envelope, which can be affected by engine installation and operation conditions. One limited operating condition is the high crosswind on the ground. Flow separated at engine inlet, unsteady and non-uniform, passing through the diffuser to the fan face, stimulated the fan blade at a broad frequency range, which could lead to high cycle fatigue. A ground crosswind test was conducted by an airplane company to demonstrate the engine inlet compatibility with engine mounted on the rear of the aircraft under various crosswind conditions [1] including 90-degree crosswind, quarterly headwind (315-degree) and quarterly tailwind (225-degree). Results showed that among all tested ambient wind conditions, the engine was the least stable under quarterly tailwind (225-degree). To predict the fan blade response driven by inlet separation, a process of evaluating inlet separation induced stimulus was illustrated in this paper. The stimuli were classified in two parts, i) synchronous stimulus induced by inlet distortion, and ii) non-synchronous stimulus induced by turbulence. Vibration of a wide-chord fan blade was evaluated by modal analysis and Campbell diagram. Test data of total pressure distortion at fan face were analyzed by Fast Fourier Transform (FFT), and the excitations in frequency domain were applied to fan blade for harmonic analysis. Results revealed that the synchronous excitation caused the blade resonating at an elevated stress level, as expected. This study provided a preliminary assessment and a better understanding of fan aeromechanics, when the engine is operating at the unsteady, unstable, and non-uniform flow environment. Discussions of how to control and how to decrease the vibration level were given in the study.