The effect of blade crop and skew on the interaction tones produced by a contra-rotating unmanned aerial vehicle rotor system

Contra-rotating rotor systems are commonly used on unmanned aerial vehicles (UAVs). The periodic unsteady loading on the blades of these rotor systems due to their interaction with the flow field of the adjacent rotor produces a multitude of interaction tones. Skewing the blades has been shown to reduce the amplitude of these interaction tones in part due to the de-phasing of the unsteady loading along the blade span which results from the skewed geometry. Cropping, or reducing the diameter of the downstream rotor, has also been shown to reduce the amplitude of the interaction tones due to the cropped downstream rotor having minimal interaction with the tip vortices shed from the top rotor. This paper investigates the effect of combining both features to reduce the level of the interaction tones. Computational fluid dynamics simulations and semi-analytical models were used to determine the flow through a number of different rotor systems and the resulting unsteady loading on the rotor blades. Acoustic analogy methods were then used to calculate the level of the interaction tones at various observer locations. Predictions of the sound pressure levels of interaction tones at different observer locations are compared with experimental data and show generally good agreement. The overall A-weighted sound power level is also calculated for each rotor system. It is observed that cropping and skewing the rotor blades both produce reductions in the noise level. The physical mechanisms which cause this reduction are explored.