Broadband noise modulation of multirotor aircrafta)

Area of Science:

• Acoustics
• Aerospace Engineering
• Vibration Analysis

Background:

• Rotor broadband noise is typically analyzed over long time scales, overlooking modulation effects.
• Modulation of broadband noise with blade passage frequency is crucial for perceived noise in helicopters and wind turbines.
• Broadband noise modulation in multi-rotor aircraft like UAVs remains understudied.

Purpose of the Study:

• To investigate broadband noise modulation in hexacopter unmanned aerial vehicles (UAVs).
• To determine the influence of inter-rotor phase relationships on broadband noise modulation.
• To explore the potential of synchrophasing control for mitigating broadband noise modulation in multi-rotor aircraft.

Main Methods:

• Conducted flight tests and anechoic chamber experiments on hexacopter UAVs.
• Measured broadband noise levels and analyzed modulation characteristics.
• Investigated the impact of varying azimuthal phase offsets between rotors.
• Computed the probability distribution of modulation depth for unsynchronized rotors.

Main Results:

• Significant broadband noise modulation was observed in hexacopter UAVs during tests.
• The amplitude of broadband noise modulation was directly dependent on azimuthal phase offsets between rotors.
• Unsynchronized rotors exhibited time-varying phase offsets, leading to random modulation depth.
• The probability distribution of modulation depth was successfully computed.

Conclusions:

• Broadband noise modulation is a significant factor in multi-rotor aircraft acoustics.
• Rotor synchrophasing presents a viable strategy for reducing broadband noise modulation, analogous to tonal noise control.
• Understanding the summation of broadband noise modulation from individual rotors is crucial for predicting overall noise characteristics.