[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.

A comparison was made between computer model predictions of gear dynamic behaviour and experimental results. The experimental data were derived from the NASA gear noise rig, which was used to record dynamic tooth loads and vibration. The experimental results were compared with predictions from the Australian Defence Science and Technology Organisation Aeronautical Research Laboratory’s gear dynamics code, for a matrix of 28 load-speed points. At high torque the peak dynamic load predictions agree with experimental results with an average error of 5 percent in the speed range 800 to 6000 rpm. Tooth separation (or bounce), which was observed in the experimental data for light-torque, high-speed conditions, was simulated by the computer model. The model was also successful in simulating the degree of load sharing between gear teeth in the multiple-tooth-contact region.

Low-contact-ratio spur gears were tested in the NASA gear-noise rig to study gear dynamics including dynamic load, tooth bending stress, vibration, and noise. The experimental results were compared with a NASA gear dynamics code to validate the code as a design tool for predicting transmission vibration and noise. Analytical predictions and experimental data for gear-tooth dynamic loads and tooth-root bending stress were compared at 28 operating conditions. Strain gage data were used to compute the normal load between meshing teeth and the bending stress at the tooth root for direct comparison with the analysis. The computed and measured waveforms for dynamic load and stress were compared for several test conditions. These are very similar in shape, which means the analysis successfully simulates the physical behavior of the test gears. The predicted peak value of the dynamic load agrees with the measurement results within an average error of 4.9 percent except at low-torque, high-speed conditions. Predictions of peak dynamic root stress are generally within 10 to 15 percent of the measured values.