Acoustic theory of the many-bladed contra-rotating propeller: the effects of sweep on noise enhancement and reduction

Abstract

In the asymptotic theory of propeller noise the far-field noise is dominated by small regions of the propeller representing interior stationary points or boundary critical points. For contra-rotating propellers, the wake-interaction solutions can thereby be written in closed form as simple algebraic expressions which still retain the effects of swept blade-gust interaction. In this paper, these asymptotic solutions are used to investigate how blade sweep can lead to enhanced or reduced noise radiation. In particular, it is shown how the locations of the interior stationary points vary with sweep and, consequently, how sweep can be used to control noise. Results are shown for a family of critical designs that have enhanced noise radiation arising from a continuum of interior stationary points. The dominant interior stationary points can be completely removed for particular tones and directions (sub-critical designs), even in cases for which the wake-rear propeller aerodynamic interaction remains super-critical. Furthermore, the interior stationary points can be completely removed for all tones and directions (universally sub-critical designs); in these cases the wake interactions are sub-critical across the entire span of the blade, the resultant tones have low radiation efficiency and further increases in sweep lead to significant reductions in noise levels. Designs are presented, with corresponding noise calculations, that demonstrate these substantial noise benefits.