On the noise generation mechanisms of overlapping propellers

Chaitanya, Chaitanya C., Joseph, Phillip, Akiwate, Deepak C., Parry, Anthony B. and Prior, Stephen D. (2021) On the noise generation mechanisms of overlapping propellers. In AIAA Aviation and Aeronautics Forum and Exposition, AIAA AVIATION Forum 2021. American Institute of Aeronautics and Astronautics.. (doi:10.2514/6.2021-2281.vid).

Abstract

Overlapping propeller systems are currently used on some urban flight vehicles and will become increasingly common in future electric propulsion, since they provide one of the most compact platform volumes per unit thrust. This paper presents the results of a detailed experimental investigation of the dominant noise mechanisms due to two overlapping propellers at hover conditions. A detailed parametric study is performed to investigate the sensitivity of the radiated noise to the rotor axial separation distance and the rotor radial separation distance An optimum separation distance to rotor diameter ratio of of 0.25 is identified in this paper for co-axial rotor configurations, whereby minimum radiated noise is obtained. The reason for this optimum distance is explored and the balance between the various interaction noise sources in co-axial configuration is discussed in the paper. Noise predictions from an analytical model for co-axial rotors are compared against measured data, where agreement to within 8dB is obtained for most sources. A significant increase in noise is observed for partial overlapping configurations of = 0.5. The reason behind this is attributed to the induced distorted mean flow onto the downstream propeller. A simple Fourier decomposition of the mean flow profile around the propeller disc is performed to explain the noise characteristics of overlapping propellers at the blade passing frequencies. It is also shown that the minimum radiated noise is obtained at the point of maximum aerodynamic efficiency.

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Contributors

Author: Deepak C. Akiwate

Author: Stephen D. Prior

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