Interior and exterior noise generated by multi-engine, turboprop aircraft remains a serious problem. In this paper, two phase locked loop schemes are used for engine synchro-phasing to effect noise cancellation in a twin engine OV-ZOA aircraft. For interior noise, a sampled microphone technique serves as the phase detector. For exterior noise, a digital timer/wunter board monitoring engine pulse tachometers is used Both schemes use high bandwidth, inner frequency lock loops to improve capture range. Principle of Noise CancellationThe noise generated by turboprop aircraft engmes poses problems to crew and passengers in the cabin during flight, and to surrounding communities in the far field during take-off and landing. Multi-engine turboprop aircraft noise can be minimized if the proper speed and phase relationship are maintained between propellers or engine shafts [ 1-51. This approach is illustrated in Figure 1 for a twin-engine aircraft. Unlike conventional Power lever (Cabin) ave engine l/rev si pnal Figure 1. Nobe cancella~on co~fig~ratioR for OV-10A. active noise control metho$s which require adding canceling sources, the phase lock oach &criM in this paper is simple and relatively heTraditional engine synchro phase angle between the propeller shafts thus avoiding the acoustic beats arising from varying speed and phase differences between them. A conventional phase locked loop consists of a phase detector, a loop compensator and a voltage controlled oscillator (the engine and propeller, in our case)[ 6 , q . The reference frequency is the master engine speed while the output frequency is the slave engine speed. In the approach presented in this paper, an inner frequency locked loop is used to extend capture range, while an the phase difference trated in Figure 2. The frequency frequency while the phase locked loop uses feedback of phase. The frequency difference detector and phase detector can be realized through analog or digital means [6,7].yields a fixed angle that may not m m t for varying ~~~ In this paper, the approach for in that " h s the cabin noise at a desired ~~a t i Q n by cons monitoring noise at that location using a ~~~h o~.Once per revolution pulses from m a s t e r and slave engine shafts are used for the phase locked li troller is used with erence angle is de interior and exterior c~e~~t i Q n 1104

This report describes the Active Noise Cancellation System designed by General Electric and tested in the NASA Lewis Research Center's 48 inch Active Noise Control Fan. The goal of this study was to assess the feasibility of using wall mounted secondary acoustic sources and sensors within the duct of a high bypass turbofan aircraft engine for active noise cancellation of fan tones. The control system is based on a modal control approach. A known acoustic mode propagating in the fan duct is canceled using an array of flush-mounted compact sound sources. Controller inputs are signals from a shaft encoder and a microphone array which senses the residual acoustic mode in the duct. The canceling modal signal is generated by a modal controller.The key results are that the (6,0) mode was completely eliminated at 920 Hz and substantially reduced elsewhere. The total tone power was reduced 9.4 dB. Farfield 2BPF SPL reductions of 13 dB were obtained.The (4,0) and (4,1) modes were reduced simultaneously yielding a 15 dB modal PWL decrease. Global attenuation of PWL was obtained using an actuator and sensor system totally contained within the duct.

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