Design and evaluation of an acoustic liner applied to a UAV ducted rotor in hover

Unmanned aerial vehicles (UAVs) are currently being used for reconnaissance missions, tactical surveillance, and infrastructure inspection. These missions and operations could be performed near inhabited areas, possibly leading to noise complaints. For most UAVs, the rotor blades are the dominant source of noise. While some UAV concepts are equipped with rotor ducts for aerodynamic and safety reasons, few studies focus on the acoustic benefit of rotor ducts with integrated acoustic liners, similar to implementations on turbofan aircraft. The liner concept used in this study, called a Long Elastic Open Neck Acoustic Resonator (LEONAR), achieves good low frequency performance in a limited available space. The LEONAR concept extends the holes of the facesheet perforations into the resonator cavities with hollow tubes. As a result, the columns of air in the facesheet holes are prolongated without increasing facesheet thickness, thereby shifting the resonant frequencies of the liner lower. The objectives of this paper are to describe the simulation of noise radiated from a rotor in a duct with a LEONAR liner, the design of a liner integrated along the duct inner surface to mitigate radiated noise between 1000-5000 Hz, and impedance tube tests of liner samples to validate the predicted impedances.