Propeller Synchrophase Angle Optimisation Study

[Abstract] Interior noise and vibration can be a serious problem in military propeller aircraft. Noise levels often exceed 100 dB, and typical vibration levels can be hazardous to sensitive cargo. This noise and vibration is dominated by the propeller blade-pass frequency and its low-order harmonics. It is generally accepted that propeller synchrophasing is a way of minimising this noise and vibration, however synchrophasing has only achieved limited success in practice. It is thought that the reasons for this are twofold: firstly, the synchrophase angles may be poorly optimised, and secondly, the optimal synchrophase angles may be influenced by flight conditions such as airspeed and altitude. This paper outlines an investigation into these effects for the Royal Australian Air Force AP-3C Orion and C-130J-30 Hercules aircraft. Comprehensive flight trials to examine and quantify these effects were conducted in November 2006. The C-130J-30 trial also included different cargo configurations. Twenty one microphones and seven accelerometers were used in the AP-3C trial, and thirty three microphones, eighteen floor accelerometers and fifteen cargo accelerometers were used in the C-130J-30 trial. Preliminary results are presented for the AP-3C trial. These show that the optimum synchrophase angles do change with altitude and airspeed, and compromises must be made to accommodate these effects.