Experimental Study of Flight Effect on Fan Noise : 2nd, Report, Fan Noise Variations due to Flight Effect

Clarification of fan noise source variation due to a flight effect is important for a turbofan noise reduction and a noise estimation. A fan noise spectral extent and a range of fan operation influenced by the flight effect were studied experimentally using a large turbofan engine and a geodesic 4m diameter semi-sphere inflow control device that can simulate the flight effect on fan noise in static engine tests. The fan noise reduction by the flight effect in a fan approach condition was higher than that in a fan take-off condition, because the fan rotor generates BPF tones by itself near the take-off condition. The highest reduced component in fan noise spectra was a fundamental fan blade passing frequency (BPF) tone, whose reduced level was 20 dB at maximum. Higher harmonic BPF tones and a buzz saw noise were also reduced by the flight effect, but a reduction of broadband noise was not evaluated in the whole range of fan operation.