On the influence of angle-of-attack or sideslip on far-field propeller noise

The effect of angle-of-attack or sideslip on noise radiation by a propeller is considered, (Fig. 3). For a single propeller, neglecting installation effects, the angles of attack and sideslip can be combined (Fig. 2). The simplest model of the propeller, as a point multipole source, leads to an effect of inflow angle on acoustic radiation intensity, which is specified by the Doppler factor (Fig. 1). This simple model gives results consistent with experiments (Zandbergen, Sarin & Donnely, 1984, 1990), with a scaled propeller in a wind tunnel; the simple alberaic formula (Section.2), gives better results than a more elaborate numerical method (Succi, 1979). Proceeding to consider finite source distributions along the blades, leads to a less simple analytical theory (Section 3), based on, radiation integrals valid in the far-field. It allows the consideration of unsteady monopole, dipole and quadrupole sources. This analytical theory, allowing for a distribution of acoustic sources along the span of the blades, gives a non-negligible correction to the algebraic theory. Thus the latter is adequate only as a simple order-of-magnitude estimate of noise level; a more acurate calculation noise levels requires evaluation of the radiation integrals, based on a knowledge of the acoustic source distribution.