Experimental noise characterisation of phase-locked tandem-rotor in edgewise flight

An experimental study is conducted on a tandem-rotor in edgewise flight to study its noise emissions under phase-locked operation. Acoustic results captured at both in-plane and out-of-plane observer locations revealed substantial noise reduction concentrated at the blade-passing frequency, which is attributed to the destructive interference between acoustic wavefronts generated by each rotor. The relative phase angle that promotes destructive acoustic interference most effectively is different for the in-plane and out-of-plane observers, which appears to be caused by the unsteady loading intrinsic to propeller in edgewise flight. For in-plane observers, the noise reduction is greater in amplitude but covers a smaller range of observers, whereas for out-of-plane observers, the reduction is more modest but its effect is experienced over a broader range of observer locations. Under edgewise inflow, the reduction in blade-passing frequency noise facilitated by phase-locking caused amplification in some higher blade-passing harmonics. These side effects are most prominent in the out-of-plane locations, and unlike the noise reduction at the blade-passing frequency, they have been shown to be localised in the temporal domain.