Acoustic Analysis of Compound Helicopters with Trim Variations

This study presents a prediction method that analyzes the acoustic radiation of compound helicopters when trim is varied. The goal of this project is to develop a strategy which uses the extra degrees of freedom available in a compound rotorcraft to reduce the noise. Noise radiation is an important factor for both military and civilian rotorcraft. There has been limited research on the specific topic of compound rotorcraft acoustics. A notional vehicle with a main rotor, tail rotor, wing, and auxiliary propeller is used in this investigation. A flight simulation model and noise prediction software are coupled together to calculate all of the results. Predictions indicate that with the addition of degrees of freedom, the noise from a compound helicopter can be vastly changed. For example, reducing rotor advancing-tip Mach number will results in a substantial noise reduction in high-speed flight, this can be useful when finding an optimal-noise flight condition. Results also show that changing the thrust produced from an auxiliary propeller does not have a dramatic effect on the overall sound for the vehicle considered, but can be crucial in changing the performance of the aircraft. Combining both advancing-tip Mach number changes along with thrust changes produced the best low noise results.