The tonal noise produced by a contrarotating unmanned aerial vehicle rotor system

Abstract

Contra-rotating coaxial rotor systems are sometimes used as propulsors for small unmanned aerial vehicles (UAVs) due to their small planform area, high thrust output, and the low torque they impart on the airframe. However, these propulsion systems generate significant levels of noise which is primarily due to the multitude of ‘interaction tones’ which they produce. These interaction tones occur at the sum of harmonics of the blade passing frequencies of each rotor and are produced by the periodic unsteady loading on the rotor blades caused by their interaction with the periodic unsteady flow produced by the adjacent rotor. This thesis describes a detailed investigation into these interaction tones. This thesis presents two methods for predicting the unsteady periodic loading on the rotor blades. The first method uses loading data directly taken from computational fluid dynamics (CFD) simulations, whilst the second method is semi-analytic and predicts the unsteady loading due to the bound potential field produced by the adjacent rotor interacting with the rotor. A novel frequency-domain method is also presented for directing calculating the far-field sound radiated from the rotor system. Predictions made using these methods agree well with experimental measurements made in an anechoic chamber. The agreement of the predictions made using the CFD and the semi-analytical model suggests that bound potential field interactions are a significant cause of the interaction tones. Several methods for attenuating these interaction tones are explored. The effect of skew angle on the interaction tones is investigated using the prediction methods presented in this thesis. The prediction methods also correlate well with experimental data for various skew angled blades. It is shown that skew angle can reduce the level of the interaction tones significantly without compromising aerodynamic performance. Secondly, the effect of cropping the bottom rotors on the interaction tones is explored. Cropping the bottom blades can have a minor effect on the radiated noise. Lastly, the effect of changing the pitch angle of the bottom blades on the interaction tones is explored.